Introduction: Tropical Diversity, Classification, Evolution1

I begin this essay with a short statement honoring tropical biology and tropical biologists. Then I summarize Darwin’s theory on evolution. At the end of the essay I include four special-topic sections on evolution. These are hard, and they are not directly relevant to our herpetology course. Still, in biological education, almost everything worthwhile is a bit difficult, so if by chance these special topics interest you, give ‘em a look. The varmints shown above are marbled reed frogs, Hyperolius marmoratus. I found all of these individuals within an area ten meters square. Presently herpetologists understand neither the genetic bases underlying the color variations nor the selective value that any particular variation might impart. Throughout tropical Africa many populations of reed frogs are in danger of extinction.

1 This essay is dedicated to Dr. Wendy Campbell and to her daughters. Once my lab assistant, Wendy became my hero and my conscience. When I start preaching about the protection of tropical biodiversity, Wendy does not fail to remind me that issues of poverty, justice, and conservation are inextricably linked. Lucy and Sally Campbell are the two best reasons that I’ll never give up trying to explain evolution right. (Hey, Wendy; thanks for the miles.)

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In the beginning: Tropical Biodiversity.

The business of a biologist is to study life, and therefore every good biologist needs to live for at least a few years in the tropics, where life is seen in so many forms. This is a theme that I will repeatedly emphasize in class, and at the beginning of this essay I want to give the subject a personal slant. I know lots of home-grown tropical biologists, most born under conditions of poverty that, logically, should have crippled their lives. But they triumphed, and now they are changing the world. Little Zilca’s English is terrible, but she rides a horse as if she’d been born in the saddle, and she can catch more crocodilians in a night than any man twice her size. She is also a master of radio telemetry. As a child, Mustafa Jean had few opportunities for formal education. After he lost his family in Rwanda’s genocide, he walked 2000 kilometers to enroll in a small Methodist college where he amazed his instructors by inventing enough calculus (he’d never heard of it) to study statistical theory. Jean is now an expert on pollution-biology. Hoang was escaping Vietnam among thousands of “boat people” —but his boat sank, and he spent his childhood as a refugee without a country. Trained in art and biology, he paints enormous, true-to-life murals of tropical ecosystems. These murals decorate the walls of Vietnam’s children’s hospitals. I could list tropical biologists for page after page, but three examples should be enough. I have tried to make these folks sound like heroes, and they are, but that’s not the best part of their stories. These are fun folks to be around: you’d like them, and they could show you a world that actually merits the adjective awesome.

My point is that tropical biologists deserve our particular respect. Indeed, I believe that they have the second-most difficult and the second-most important job in the world.2 Without the best efforts of these scientists, this planet is likely to lose an important, beautiful, and irreplaceable bio-heritage. Today’s native-born tropical biologists work under social and economic challenges that would daunt all except the bold. And because tropical biodiversity is the heritage of all people, home-grown tropical biologists should receive bountiful assistance from the developed world—while in fact they are unlikely to get more than the crumbs from our rich-folks’ tables. Nevertheless, in South Carolina we say that God favors steadfast saints, brave fools, and curious little children. A good tropical biologist is all three of these things, so I fully expect that someday Zilca and Jean and Hoang and a multitude of other saints will succeed in their impossible missions!

Now, before I start reviewing the evolution stuff for you, let me write what’s probably the most important sentence in this long essay. I challenge every of you herpetology students to go to the tropics someday, to observe tropical biodiversity, to befriend an impoverished tropical biologist, and to help design ways to conserve the world’s biological wonders.

Evolution.2 I believe the most difficult and most important job in the world is parenthood. Without good parents, the world is unlikely to produce good children, and without good children, there is no hope at all for our species. I am undecided about the world’s third-most important and difficult job. Candidates might include the Pope, the Secretary General of the UN, and the political leaders of a few big and difficult nations (e.g., India, Russia, China, the DRC, Brazil, the USA, Mexico, and Indonesia.)

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Evolutionary theory is the cornerstone of modern biology. Expressed overtly or not, it underlies every significant program of biological research, and it provides the lens through which every competent biologist views the natural world. In the body of this chapter I offer a bare-bones sketch of Darwin’s ideas on how it works. That will provide sufficient background for understanding the material I present in Biology 150 / 104. In three Special Topics at the essay’s conclusion, I consider (1) the bio-philosophical context in which Darwin worked, (2) the theory of evolution in a socio-religious context, and (3) the current efforts of theoretical biologists to refine Darwin’s theory. Earlier in my Wofford career I took on the task of converting any “creationist” students from their counter-evolutionist beliefs. I now realize that my attitude was arrogant. My legitimate mission is just to ensure that students understand the ways that evolutionists apprehend, organize, and express the long history of Life. If Wofford alumni—whatever their personal beliefs—can construct coherent intellectual arguments within the overall paradigm of evolutionary theory, then Wofford’s biology teachers will have done a job worthy of their pay. If a graduate should, in his or her heart of hearts, still believe in “creationism,” then any instructional blame might be laid at the feet of Wofford’s Department of Religion.

A. Introduction. Four lines of “suggestive evidence” for Darwinism. Evolution by natural selection is a theory credited to Charles Darwin3 that explains (a) how life-forms change over (many) generations and (b) why so many different types of living organisms exist. Evolutionary theory also provides a framework for organizing the diversity of life-forms. By the middle of the nineteenth century most academic biologists had begun to believe that biological lineages changed over time, and some thought that such changes might be the root of earth’s biodiversity. For Darwin, four lines of evidence suggested that both phenomena were largely driven by the force of natural selection. Here are some of the thoughts that were rolling around in Darwin’s head 150 years ago.

1. The earth is extremely old. Furthermore, life itself has existed for a very long time (as demonstrated by the fossil record). In other words, living things have been on earth long enough for very slow processes to make very large changes.

Geologist Charles Lyell was one of Darwin’s intellectual heroes. In his Principles of Geology (3 volumes, 1830-1833, London) Lyell argued persuasively that geological features were in general created by observable geological processes (e.g., erosion) operating at approximately their presently observable rates (for most processes, very slow). The late Stephen J. Gould interprets Lyell as saying “…that all scales of history must be explained by currently

3 Some textbooks suggest that Darwin invented the ideas of evolution and natural selection. This is incorrect, and at the conclusion of this essay I offer a Special Topic on pre-Darwinian concepts of evolution and natural selection. Darwin’s major contribution was his argument that natural selection, acting on practically random intra-specific variation, was the driving force behind evolution. For fairness’ sake we should also note that while Darwin was back in England, writing the Origin…, Alfred Russel Wallace was developing similar ideas in the forests of Southeast Asia.

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observable causes acting within their current ranges of magnitude and intensity (Gould, 2002, p. 143).” This doctrine, called uniformitarianism, was accepted by Darwin and underlies his theory of evolution. Thus Darwin did not believe that evolution made great jumps. He did not believe evolution operated in different ways during different epochs. And he did not posit past episodes during which evolutionary processes moved at aberrant speeds. Today many biologists criticize Darwin’s theory for over-strict adherence to uniformitarianism. (See, for example, Gould and Eldredge, 1993, “Punctuated Equilibrium Comes of Age.” Nature 366:223-227.) Nevertheless, at the time of Origin’s publication, uniformitarian assumptions helped make Darwinism stark, simple—and as nearly testable as a history-based theory can be.

2. Despite their diversity, all living forms are essentially similar. They are all carbon-based, they run by the same biochemical processes, and they are composed of cells that are fundamentally alike in plan.4 These basic similarities at least suggest a unitary origin (rather than a set of separate de novo creations as posited by creationists) for all life.

3. Living species are diverse, but their diversity is not random or disconnected. Some species-pairs look lots more alike than other species-pairs. For example when Darwin hopped up the Pacific Coast of South America in the Beagle, he observed a gradation of changes in several plants and animals.5 This gradation suggests historical relationship, not special creation.

4. Evolutionary theory and creationism both affirm that an organism’s observable traits will show “fit” with environment. But sometimes a bio-structure or the fossil record of a trait suggests historical development rather than designed, de novo, creation.6

4 Consider the genetic code, in which triads of base-pairs (“codons”) call for the use of specific amino acids to make proteins. If a maker-god were being truly “creative,” then why did she or he have all cells communicate amino-acid requirements in almost exactly the same code? (And if the maker-god were being precisely systematic, why are a few organisms programmed according to a slightly different version of the genetic code?) Of course Darwin had no knowledge of genetics.5 If organisms were “specially created,” then why do some varieties seem to flow seamlessly into one another, both over space (like the west coast of South America) and time (like a few fossil records)? One should also note that some pre-Darwinian evolutionary theories accepted the idea that, in the geological sense, some species were more closely related than others.6 Gould argues that some anatomical structures appear “jack-legged” to fit; that is, their contemporary structure reflects their evolutionary history. Gould’s favorite example was the “thumb” of the giant panda. This structure looks like an extra finger on a bear’s front paw but is actually an extension of the radial sesamoid bone. If a maker-god wanted the panda to have a manipulating digit, why didn’t she or he give the animal a conventional thumb rather than modifying a pre-existing, very un-thumb-like structure? And why is growth of the radial sesamoid “thumb” controlled by a gene that also affects (to a lesser degree) the growth of a foot-bone in an absolutely useless manner? It all sounds very un-god-like and reminds me instead of a redneck mechanic friend who sawed apart an old Cadillac car to make a pickup truck.

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B. Four basic assumptions underlying Darwinism. The four points above suggest that, over enormous periods of time, one sort of organism may give rise to another—but the points provide few clues as to how such changes might occur. Darwin’s theory on how evolution works rested on four basic assumptions/observations.7

1. At least in the short run, the process of reproduction is taxonomically stable. That is to say, organisms beget like organisms. Dogs produce puppies, not kittens. Children tend to resemble their parents.8

2. But variations exist within any population, and at least some of those variations are heritable (by 1. above) from one generation to the next.9 Some variations are disadvantageous, some may be neutral, and some (perhaps relatively few) are advantageous.

3. The number of individuals surviving to reproductive age is generally much, much smaller than the number of individuals produced by reproduction.

Charles Darwin owed this insight in part to the Rev. Thomas Malthus, an 18th Century British philosopher-clergyman who pointed out that populations tended to increase geometrically —while the supply of resources upon which they depended could not increase so rapidly. Thus parents produce a “surplus” of offspring. The offspring that can obtain access to resources may survive; the others cannot. Darwin would ask, “If there is variability among the oversupply of offspring, should we not suspect that the most fit offspring would be the most likely to survive?”

4. Although luck also plays a significant role, the question of which individuals survive to reproductive age versus which do not is determined in part by advantageous heritable variations. Thus, inherited “favorable” variations should become relatively more common from one generation to the next. And “unfavorable” variations should become relatively less common. Over time, accumulation of these evolutionary changes can amount to vast taxonomic differences.10

Functionally the vehicle was an excellent truck, but there was no doubt about its passenger-car ancestry!7The basic Darwinian idea is modification with descent. Today the idea is universally accepted by biological scientists. A Special Topic at the end of this essay sketches some current reformulations of the way in which Darwin worked out the details. Some biologists think that these reformulations are a big deal. More and more, I tend to agree.8 Prior to the rediscovery of Mendelism in 1900, the mechanism of heredity was not known to science. Nevertheless, the basically replicative nature of reproduction was undeniable.9Obviously Darwin was unaware of genetic mutation as the original source of heritable variation.10 This last sentence reflects the most important and contentious element in Origin and was also the idea upon which Darwin expended the most intellectual effort. Many nineteenth century biologists admitted that, over generations, organisms developed modest adaptations to environments. (Please see my Special Topic on pre-Darwinian theories of evolution.) On the other hand, Darwin argued that these adaptations could accumulate and create taxonomic differences at the highest level—that indeed all living forms could share a common ancestor. Making Darwin’s full, uniformitarian, argument for the efficacy and scope of natural selection

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I’ve listed above the four most basic concepts of Darwin’s theory. In a pre-publication letter to Harvard’s Asa Gray, Darwin himself outlined in only six points the essential content of his forthcoming Origin:

1. As English animal-breeders have demonstrated, artificial selection is powerful. (In only a century or two, many different varieties of dogs, pigeons, cattle…have been developed.)

2. Natural selection can be even more powerful than artificial selection because natural selection “looks at” all organism-characteristics at once and because it operates over vastly longer periods of time.

3. A biological species typically produces far more newborn individuals than can survive to reproductive maturity. The fitness of these individuals determines at least in part which ones live and die. Because life and death are essentially processes of individuals, natural selection of the fittest operates at the level of individual organisms (not at the levels of populations or species).

4. Fossils are available that allow us to track the history of natural selection in at least a few specific cases.

5. Uniformitarian gradualism can be defended as the way in which evolution works. (That is, extrapolation of current, observable process-rates over vast periods of time can explain the characteristics of organisms. No observable biological phenomenon demonstrates the necessity of special evolutionary processes operating in special ways at special times.)

6. The formation of species is a function of divergence, of the splitting of lineages from a common ancestor (Darwin, 1857, revised from Gould, 2002).

C. Four notes about the socio-intellectual context of Darwin’s work.11

1. Darwin cleverly emphasized the well-understood phenomenon of artificial selection. He was sensitive to the political realities of his day, and he knew that for his theory to be widely accepted he needed to convert the upper class of Victorian England. He therefore began his argument with examples that these influential people could understand and appreciate. Here is the sort of point that Darwin could make: “You, dear audience, represent the educated, gentleman-farmers of Britain. You folks practice selective breeding of domestic animals such as cattle.12 Starting with one primitive stock, you have developed breeds of cows that are appropriate for the production of milk and different breeds appropriate for the production of meat—and you have accomplished this in perhaps 200 years. In some ways natural selection will be less efficient than your artificial selection, but it has been working for millions of years. And while you may be concerned with only a few traits in the species you breed, natural selection can operate simultaneously on every organism-characteristic.”

would require a recapitulation of the most important biology-debates in nineteenth century biology. I’m far too lazy for any such exercise.11 I’ll write much more about this in the first “special topic” below.12Above I write about cattle, and Darwin did give them substantial attention. Nevertheless, the great man’s favorite domestic-animal examples were pigeons.

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2. Many nineteenth-century popularizers of evolution assumed that the “favorable variations” were always changes that made animals bigger, stronger, and fiercer. This assumption is reflected in British literature. For example, Poet-Laureate Alfred Lord Tennyson wrote about “nature red in tooth and claw.” Actually, “favorable variations” can be gentler: a flower with brighter color may attract more pollinators and therefore reproduce more successfully. Nevertheless, Darwin remained certain that the process of evolution was driven by Malthusian economics: for every organism that reaches reproductive maturity, many will die young—or at least unreplicated. Social historians should note that some 19th Century capitalists saw Darwin’s theory as moral license for them (“the strong”) to exploit their workers (“the weak”). In the next box I’ll rant moralistically against this non-scientific philosophy. Meanwhile, remember that having lots of kids who grow up to have kids of their own gets you more “Darwin-points” than kicking lots of ass.

3. Some nineteenth-century writers tried to personify Darwinian evolution, to pretend that Evolution (which they usually spelled with the capital “E”) had thoughtful guidance and direction. This is not true. The variations upon which natural selection operates result from essentially random changes at the molecular level. Indeed, more variations are deleterious than helpful, and some may be downright neutral. Nor can the environment sort varieties with conscious intent. Some varieties just “work,” and others do not. Within a population some of the animals (or plants, etc.) are, by pure accident of their variations, more likely to attain reproductive maturity. They “win,” in the Darwinian sense—they are more likely to pass on their characteristics to more descendents—but this does not signify merit or purposeful guidance.

This directionality error was particularly popular with the 19th century progressivists and so-called Social Darwinists who overpopulated English and American sociology for too many years. These folks thought Darwin had proved that survival of the fittest inevitably makes things “better,” even in the social world. But like every other scientific theory, Darwinism says nothing about “good” or “bad.” Individuals that, by chance, fit their niches more exactly are more likely to leave descendents—and there’s no moral dimension to the process. Scholars might, I suppose, argue that history has a progressive direction. (Or it may not: I certainly don’t know, and your position on this issue probably depends on your definition of progress and whose history books you read.) But to the degree that Darwinism is scientific theory, it says nothing about this value-question, one way or the other!

The academic errors of “progressivist” sociology were addressed by Max Weber, who elucidated the philosophical difficulties involved in reasoning from “is” (the domain of science) to “ought” (the domain of ethics). The moral error of “justifying” human power and privilege by Social Darwinism—well that error persists, despite counter-arguments by Jesus, John Wesley,

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Karl Marx, and a whole lot of other heroes.It is true that Darwin wrote, somewhat ambiguously, about the wondrous beauty of life-

forms that had resulted from so many millennia of natural selection. And most biologists agree (as I certainly do!) that the development of earth’s present biodiversity from a planet populated by prokaryotes involved a sort of “progress.” But any such affirmation grows out of aesthetic sensibilities—and not scientific theory. And it certainly does not affirm progress in the terms of the social Darwinists.

4. “Progressivist” errors in interpreting Darwin are associated with the misconception that vertebrate evolution is appropriately represented by steps on a ladder. According to this view, fish ruled the earth for a time, until they were largely replaced by the more advanced amphibians, which were succeeded in turn by much better reptiles, which eventually gave way to the truly noble mammals.13 Nothing could be farther from the truth. Evaluated by number of species or number of individuals, fish are still the most successful vertebrates. Similarly, reptiles and even amphibians are substantially more speciose than mammals.14 From the beginning Darwin rejected the ladder-metaphor for evolution. Consider instead a very bushy tree of live. Many branches and twigs are produced. Some branches wither and disappear before they reach the tree’s present-day crown. Others prosper greatly and become new, bushy offshoots of the same tree.

D. Brief notes on genetics.15 The union of evolutionary theory with genetic theory is sometimes called “the great synthesis” and is a crowning intellectual achievement of twentieth century biology. Darwin had merely observed the fact that many traits were passed down from parents to offspring. He had no idea of the mechanism by which this inheritance occurred. We, on the other hand, know a great deal about this subject.

1. Thanks to the early work of Father Gregor Mendel and the more recent advances of molecular geneticists, we now know16 how heritable traits are transmitted.

2. We now understand why some traits are not slowly “blended out” of the population but rather can reappear; similarly we understand why evolution need not always be as gradual as Darwin had suspected. (And see Special Topic Number 3, below.)

3. We know in a general way how variations arise, how mutation provides the basic raw material for genetic recombinations eventually expressed as phenotypic variation.

For a taxonomist, genetics can be highly useful for reasons related only indirectly to

13In some versions of this heresy, human beings stood at the top of the mammalian heap. And, for many light-skinned progressivists, European males were the absolute pinnacle.14 If you want to know what life-form is really successful, probably you should forget all this flashy multi-cellular stuff! The majority of all living species are bacterial. Furthermore, the number of individual bacteria found in a single cow’s digestive tract may exceed the number of all living multi-cellular individuals in the whole world!15 In a Special Topic I shall consider in greater depth the relationship between modern genetics and the theory of evolution.16 I often write “we know,” but many mysteries remain. Also, I’m not as hot a geneticist as I should be, so the pronoun we assumes that you erudite folks will compensate for my ignorance.

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natural selection. In our class I shall occasionally talk about convergence—about unrelated plant or animal species that look alike because they have been “shaped” in similar ways by natural selection to fit similar environments. As you might imagine, taxonomists have difficulty in deciding whether similarity of appearance reflects true relationship or convergence. Fortunately for taxonomists, all advanced animals have a huge amount of genetic material that does nothing at all—and is therefore untouched by natural selection. (For example, in human beings only about 2% of our genetic material gives actual instructions on how proteins, etc., are to be constructed. Perhaps another 2% contains genetic “switches” that control the activation of protein-coding genes. The remaining 95%, plus or minus, is sometimes called “junk DNA.” (Some geneticists don’t much like the term, but it is often convenient, and I’ll use it freely.) Of course these “junk” sequences of nucleotides also mutate. And because such mutations do not affect the bearer’s phenotypic expression, they accumulate over time. Let us say that a taxonomist looks at long, homologous “junk” sequences in two different mammal species. The dissimilarity between the two sequences (which is admittedly difficult to determine) would be a measure of genetic distance (or distance of relationship) between the two species. Many taxonomists believe that mutations in “junk” regions of DNA accumulate at a rate that can be mathematically modeled (perhaps by a Poisson Distribution; ask me if the idea interests you). If so, then genetic distance can be used as a sort of “molecular clock” to estimate the date at which two organisms shared their most recent common ancestor.

E. How new species arise. (Or, “Why do we have so many different kinds of varmints in this wonderful world?”) As I have already suggested (and as Darwin’s famous title tells everybody anyhow), evolutionary theory addressed the question of how species originate. This will not be a major, explicit focus of our class, but it underlies all my subsequent discussions and is worth at least a mention right now. Many biologists agree that key events in the origin of new species involve reproductive isolation, usually accompanied by differential selective pressures. For almost two pages I’ll scribble on and on about reproductive isolation. Then I’ll return to the question of differential selective pressures. To make this organization a little bit clearer, I’ll write the two topic headings in bold type.

1. Here begins my scribbling about reproductive isolation. In very large, panmictic17

populations, new mutations are likely to be swamped by the overwhelming numbers of “old-style” genes being swapped around in near-random matings. On the other hand, if the number of interbreeding individuals is small, new mutations are more likely to survive dilution—and, if adaptive, to increase in relative frequency. Such alteration in gene-frequency is what a population-geneticist calls evolution, and clearly it is facilitated by the separation of a population into reproductively isolated fragments. Sub-sections of a population can become isolated in one of several ways.

a. One common mechanism of reproductive isolation is the intervention of

17If mating is random across an entire population, then a population is said to be panmictic. Of course pure panmyxia is a hypothetical concept that probably never occurs in perfect form. In practice we might say that a population is panmictic if there are no important breeding sub-groups with in it, that is, if almost any female is almost equally likely to mate with almost any male.

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environment between subgroups of a population.18 For the most dramatic example, think of continental drift (a phenomenon I should address in class) and how it might separate portions of a plant or animal population.

Here is a huge-scale example of reproductive isolation. South America and Australia, once connected, eventually broke apart. Interbreeding of mammals from the two continents thereafter became geographically impossible, and the original mammalian stock could follow separate evolutionary trajectories on the separate continents.

Or consider how climate-changes can produce “islands” in a formerly continuous habitat.

For a medium-scale example, we might think about rock rattlesnakes on the tops of mountains in the American southwest. At one time this part of the world was cooler and wetter, and habitat for these snakes extended for millions of square kilometers, across many mountains and the valleys in between. When the climate became hotter and drier; the snakes in question could survive only upon mountaintops—cool, moist “islands” that were separated by the hot, dry “sea” of the desert. Each isolated, mountaintop population could accumulate its own, particular set of heritable variations.

Or consider animals that are somehow moved as pioneers or founder stocks into geographic areas previously uninhabited by their species.

For a small-scale example, we might think about a hypothetical pregnant rat that is rafted (perhaps on a palm trunk, perhaps by a hurricane) to an island previously empty of her type. On this isolated piece of real estate the rat and her offspring could build a population that would have its own, unique evolutionary trajectory. (Here’s a side-thought of biological trivia. Should we ask how much genetic variation is available in one pregnant rat? Don’t we have to call upon lots of mutation in order to generate speciation? Not in the initial stages of speciation. The gene re-sorting associated with sexual

18 In his early writings Darwin apparently considered the spatial fragmentation of populations to be the most important isolation mechanism of species-creation. (This is called allopatric speciation, or the creation of species in different places.)

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reproduction can begin the process of differentiation. And when mutations do occur, they can become genetically “fixed” in a small, founder population. Eventually, under such conditions, phenotypic variation becomes quite visible. As an exercise, you folks could visit a number of large pet shops and observe the variety of common hamsters for sale. All these animals are the descendants of a single, pregnant female imported from Syria in the twentieth century.)

b. Subsets of a population can also become reproductively isolated without the intervention of habitat differences. This can lead to sympatric19 speciation. I shall offer three hypothetical examples of reproductive isolation “in place.”

(1) On very rare occasions genetic “catastrophes” can produce immediate reproductive isolation.20 Consider the phenomenon of polyploidy. At mitosis prior to meiosis of germ cells, say the chromosomes replicate as usual. But what if the cell does not subsequently divide—and an egg (or sperm) ends up with two sets of chromosomes instead of one? If fertilization can occur (it usually cannot), if the embryo can develop (it usually cannot) and if the offspring can survive (it usually cannot)…. Well, under those very rare conditions the resulting polyploid organism can sometimes produce polyploid offspring of its own. And these creatures may be reproductively isolated from their parent population.21

(2) As suggested in a footnote, Darwin believed that reproductive isolation could occur when natural selection for extreme values of a trait produced a divergence of subpopulations in a single locality. Here is a simplistic, made-up example. What if very small mice on Rattlesnake Mountain could squeeze into cracks too narrow to admit rock rattlers—and what if very large mice were too big for the snakes to eat? Natural selection might eliminate the middle-sized (= eating sized) mice. Eventually the big ones and the little ones might become unable to interbreed and therefore become actual species.22

19Remember, sympatric literally means “occurring in the same home-country.”20 Given Darwin’s ignorance of genetics and his commitment to uniformitarianism, we should not expect him to provide examples of exactly this type. Darwin did, however, believe that in many circumstances an environment might favor extreme values of a set of traits. Long-continued selection for extremes might cause subsets of a population to diverge sufficiently to preclude interbreeding. In some later writings Darwin seemed to prefer this sort of sympatric speciation (development of additional species in the same place) as an explanation for the origin of species.21 Herpetologists are familiar with polypoloidy as an explanation for the origin of some new species in ambystomatid salamanders. And speciation by polyploidy is in general much more common among plants than among animals.22 Yeah, I know; we got to let youngsters of the large-mouse variety grow rapidly through the intermediate, edible sizes. I told you it was a made-up example.

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(3) In most real populations, panmyxia is less than perfect. Given a mating distribution that is to some degree clumped, and given very strong selective pressures in favor of some adaptive mutation, that mutation can increase in frequency. Let me put that a little bit differently. Say that within a given piece or real estate a species has a mating system so structured that individuals from Subgroup A seldom mate with individuals from Subgroup B. Subgroups A and B would then be partially isolated, reproductively. And a mutation of sufficiently strong adaptive advantage, arising in either group, might become fixed within that group, even in the absence of complete reproductive isolation. That is, evolution may be a function of both degree of isolation and intensity of adaptive advantage.23

Some biologists believe that evolutionary change often occurs in the midst of a large population (rather than among reproductive isolates at the population’s fractured edges). If a strongly favorable mutation did gain a foothold at the center of a large population, then over time its bearers would probably become the most common phenotype in the population. Thus the species would change, or evolve, in place.* Gradual evolution of a given species, in place, is sometimes called anagenetic evolution. But this process would not be an answer to an ecologist’s question: “Why do we have so many different kinds of varmints in today’s wonderful world?”*Here are 3 notes. (1) If anagenetic change were profound, then a paleontologist, examining different fossil strata (same geographical place, different geological times), might describe a single evolving population as more than one species. (2) If the “center” of a population evolved anagenetically into a new species, the old phenotype might still be preserved among isolates at the population’s periphery. This could result in the “origin” of more species. (3) Many evolutionary theorists believe that anagenetic evolution in the strict sense is an uncommon phenomenon.

Here ends my scribbling about reproductive isolation; you’ll soon be reading about how separate evolutionary lineages can diverge into different adaptive directions.24 Of course reproductive isolation is not the same thing as the creation of new species. Actual speciation typically occurs after extended reproductive isolation, and by one (or both) of the following mechanisms.

2. Differential selective pressures. Let’s say that two mouse-population fragments become separated onto the tops of two different mountains as Pleistocene Arizona warms up and dries out. There is little in the mere isolation-event to make us think that these two isolated

23Of course, mechanisms of genotype expression—straight-out dominant/recessive expression, or blending expression, or…—are also involved, but that is beyond the scope of our present discussion.

24Remember, back a couple of pages ago I said that the question of “how many species?” had a two-part answer involving both reproductive isolation and differential selective pressures. I’ve discussed the former and must now consider the latter.

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population-fragments will become different species in the usual sense of the term.25 But suppose, hypothetically, that Mountain-System A supports a nocturnal, mouse-eating species of rattlesnake while on Mountain-System B, small, diurnal hawks are a more significant predator. Can you speculate on how the different selective pressures might “pull” the two mouse-populations in different evolutionary directions?

3. Evolution by genetic drift. If the founder stock for a population is not numerous, then sometimes purely random processes associated with small samples can “fix” a trait. Although differential selective pressures are almost certainly more important in evolution, an increasing number of biologists (including me) believe that genetic drift can also be significant under some circumstances. The concept is explained by mathematical example in a box below.

In the shadow of Mendel, we can consider drift-examples from genetics. If a rare, near-neutral allele (perhaps a new mutation) exists in a large population, then the rare allele is likely to remain rare or be lost through “swamping.” In a tiny founder-group, by contrast, a rare allele may comprise a significant percentage of the gene-pool, and it may eventually prevail by chance. Consider, for example, only one gene-locus in two hypothetical populations: Population 1: Half a million animals, with 999,999 normal alleles and 1 rare allele. Population 2: Two animals, with 3 normal alleles and 1 rare allele.

Remember, we’re questioning whether an allele might become fixed as a result of pure chance. So allow all genotypes to have the same Darwinian fitness. Now replicate each population several times by random combination of alleles. Clearly the chances are about zero that the neutral, rare allele will ever become fixed (= “become the only allele”) in the larger population. By contrast, under one reasonable demographic scenario, in Population 2 the chances are about 1/256 that the rare allele will become fixed (become the only allele) after two generations. Obviously 1/256 is not a large probability. But think about a pair of tree-mice living in the top of a coconut palm. Each mouse will have many thousands of genes, of which a tiny percentage but a large absolute number (let’s say a thousand) will exist as rare alleles. Let a hurricane blow the coconut palm (with our honeymoon couple of tree-mice) to a suitable, but previously mouse-free island. If each of the thousand rare alleles has an independent 1/256 probably of becoming fixed, then perhaps about 4 of the rare alleles will be fixed on the hypothetical island.

My favorite example of speciation (by isolation, differential selection, and probably drift) involves repetitive climatic changes in Africa. I shan’t inflict my thoughts about this process on you as an integral part of this review about evolution. However, if you ask me for it, I’ll email

25Genetic drift is briefly discussed next. Also, note that “junk” regions in the DNA of the isolated populations will accumulate mutations—which might be used to estimate an approximate time of isolation. If the reproductive isolation of the two separate populations persisted long enough, then many evolutionary theorists might designate the two reproductive isolates as different species, even if they still “looked” a whole lot alike.

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you a copy of my essay about Africa as a “species machine.”

F. Behavior. In addition to explaining anatomical structures, evolutionary theory also provides a framework for understanding the development of behavior patterns.

Here are four points about behavior that do not fit very well into the present chapter. Nevertheless, you should keep them in mind as we consider behavioral evolution in several of the following essays.

1. For years, academics debated whether behavior patterns were instinctive or learned. This “nature-or-nurture” argument is now largely out of fashion, and biologists insist that both processes are involved.

2. Complex organisms (like vertebrates) exhibit a large number of behavior patterns. Everybody knows that at least some behaviors for at least some species are almost purely learned while others are almost purely instinctive (hard-wired into the organism’s genetic makeup).

3. More important, most folks would agree that behavioral propensities can be genetically transmitted. For example, the cheetah’s hunting pattern—stalk, quick rush—is not instinctive in the strict sense. That is, the cat is not born knowing exactly how to hunt; however a cheetah is born so that it can easily learn the species’ particular hunting pattern.

4. It is also true that certain genetically set anatomical (or physiological) adaptations are advantageous only given particular behavior patterns. Note, as a trivial example, that a hunting dog’s acute olfactory senses are of no value unless the dog learns to sniff its quarry’s trail. (Of course, to re-emphasize point # 3, we might note that dogs learn very easily to sniff!)

Special Topic 1: Brief Notes on Pre-Darwinian Ideas on Evolution

This Special Topic is intended for liberal arts students who are particularly interested in the history of science. Pursue it at your own risk. Some introductory biology texts suggest that Charles Darwin invented the ideas of evolution and natural selection. This is incorrect. At least in the British Isles, most nineteenth-century biologists believed in some form of evolution—and “special creation” (the idea that God formed each species separately at the earth’s beginning) had probably been a minority position among biologists since the 1700’s. In part because of suggestions in the fossil record, most biologists knew that local biota changed over time, and many of these folks agreed that such changes involved alterations in (or, evolution of) biotypes. Some biologists believed that such changes were explicitly choreographed by God, and others believed they were directed by some impersonal force of Progress. Also, most observant biologists agreed that organisms were shaped to some degree by natural selection. After all, the inheritance of biological traits was universally accepted by observant farmers: to improve your crops, you saved your best seeds. And in a world of hungry foxes, what English gamekeeper would deny that slow rabbits were unlikely to reach reproductive adulthood? The work of Jean-Baptiste Lamark offers a sophisticated but not unrepresentative example of pre-Darwinian evolutionary theory. This French biologist posited the existence of two evolutionary forces. A strong, or progressive, force received feedback from an organism’s interaction with other species and with the world environmental system. The strong force

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incorporated such information into inheritance—and built better descendents.26 A weaker force, natural selection, played a secondary role. It weeded out less-fit individuals and thereby fine-tuned species to some particular requirements of their immediate surroundings. Lamark’s theories are remote from my biological understanding, and my summary is little more than a caricature. Nevertheless, a historian of science should note that Lamark, along with others, did present a pre-Darwinian theory of evolution, and that the theory even included a minor role for natural selection. Perhaps Darwin’s brilliance lay in his decision to demote the strong, or progressive, force to near non-existence and to allow a creative role for natural selection.

Some modern evolution texts treat Lamark even less fairly than I did, presenting his theory as a mere statement about the inheritance of acquired characteristics. In fact the man’s work was complex and interesting—I think. If your academic curiosity is finely honed, and if your French is much better than mine, you might see Lamark, 1809: Philosophie zoologique, ou exposition des considérations relatives à l’histoire naturelle des animaux; 2 volumes; Paris, Dentu. On the other hand, maybe you have better things to do with your time! Darwin and Lamark were not alone in offering an evolutionary role to natural selection. The results of artificial selection were evident to every intelligent animal breeder—and, with increasing information about the age of the earth, most European biologists accepted that natural selection could accomplish similar modifications. However, many of these biologists held a Platonic notion about the nature of species: types of animals could be superficially modified to fit environments, but such modifications could occur only within limits. Gould explains this by

…Fleeming Jenkin’s (1867) famous analogy: a species may be compared to a rigid sphere, with morphology of individuals at the center, and limits to variation defined by the surface. So long as individuals lie near the center, variation will be copious in all directions. But if selection brings the mode to the surface, then further variation in the same direction will cease—and evolution will be stymied by an intrinsic limitation upon raw material, even when selection would favor further movement (Gould, 2002, p. 142).

I have not written about A. Wallace, a contemporary of Darwin who worked mostly in Southeast Asia. This is because in most ways Wallace stands within the Darwinian tradition; indeed, viewed from a different perspective, Wallace might be considered almost as Darwin’s co-inventor of modern evolutionary theory. Certainly Darwin was aware that Wallace was working on the problem, and Darwin probably accelerated the publication of Origin… to ensure

26 Here is a rough example of Lamark’s incorrect idea on inheritance of acquired characteristics. Consider a hypothetical band of Native Americans who moved into the high Andes of South America. Within months they would develop greater lung capacities and denser concentrations of blood hemoglobin. (So would you, really!) These physiological changes would be incorporated in the body, and since the genes “remember” the structure of the body and pass that structure to descendents, children born to this Native American band would be ready for the high altitudes. (Of course Lamark would not have used the word genes, and you know that an individual’s genetic makeup does not “remember,” or directly reflect changes resulting from life-experiences.)

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that his work was not scooped by Mr. Wallace.

Special Topic 2: My Response to Creationism27

Many human cultures have powerful, pre-scientific explanations for the origins of the natural world. For example, a story in Genesis (the first book of the Christian Bible) tells how God created the earth and everything on it in 7 days. In 1859, when Charles Darwin published his famous book about the origin of species by natural selection, a substantial proportion of the European readership may have accepted the Genesis creation-story as literal fact. (Darwin was aware of this, and he framed some of his arguments to counter the biblical-literalist position.) On the other hand, vague ideas about evolution had floated around Europe for a very long time (see above), and we would be incorrect to assume that Victorian England was more religious than many contemporary societies.

Today, only a minority of American Christians take the creation-story literally—but some do. Indeed a handful of persons have established careers by attacking the idea that species originated by natural selection. In my experience, these “creation scientists” are often very nice people, and they’re almost always highly intelligent. They often have substantial mastery of selected paleontological facts. And, in debates, they hang on with a persistence that would be the envy of a snapping turtle.

The seemingly interminable discussions about “creationism” have doubtless had some good consequences. (Popular proponents of evolution have been forced to check their facts, sharpen their arguments, and moderate their arrogance.) On the other hand, the debate has also wasted a lot of time and has generated considerable confusion. Consider, for example, one common creationist statement. “Evolution is not a logical certainty; it is only a theory.” In a sense this statement is true. Darwin’s concepts of evolution are “only” a theory—in much the same sense that Newton’s ideas on gravitation are “only” a theory. Both theories are known to be incomplete. Both theories fit badly with some well-established data. Indeed, both theories, as originally stated, are now known to be wrong in many details. Furthermore, future research will probably show that some of our present ideas (about evolution and about gravity) are also incorrect. On the other hand, the “theory” of evolution is axiomatic in contemporary biology. It provides the groundwork for our understanding of taxonomy, ecology, natural history, and conservation biology. Without the “theory” of evolution, population genetics (perhaps genetics as a whole) makes no sense. Competent physicists work to resolve problems in gravitation-theory, but they do not jump out of airplanes without parachutes. Competent biologists argue about and tinker with the details of evolution-theory, but they no longer question the fundamentals.

27 This special topic is more concerned with religion than with science, so you might want to skim it or even skip it. (Besides, it may say more about me than it does about the topic.) Readers will note that I’ve written nothing about “intelligent design.” That’s because (1) I believe that the “intelligent design-ers” are just closet creationists—uh, but perhaps without strong convictions, and (2) after I learned about the spotted hyena’s birth-process, I could not say “intelligent design” without laughing.

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For the past several years I have avoided dealing with the “creation versus evolution question” in class. This is in small part because I consider the issue to be “as dead as a sack of ball-peen hammers on I-26 in the November traffic after the Clemson-Carolina football game.” But mostly I refuse to get involved because I dislike becoming emotional in class. And the so-called debate makes me angry—not as a would-be intellectual scientist, but as an old-time, left-wing, redneck, liberal, South Carolina Methodist. When people tell me they accept Genesis literally, I want to ask, “How literally do you take Mark 10:21?” (“… [S]ell whatsoever thou hast, and give to the poor, and thou shalt have treasure in heaven: and come, take up the cross, and follow me.”) Of course any such response would be hypocritical of me; by the standards of my friends in developing nations, I am very rich and need to sell a whole lot of material crap.

But, in addition to selective Biblical literalism, another religiously offensive problem bedevils the creationist position. For many contemporary Christians, the creationists rob the Genesis-narrative of its real power. As I understand it, the Creation Story is not meant to be a paleontology textbook—any more than Joshua’s attack at Jericho is intended to be an Infantry-School solution to a problem in military science.28 Instead, the story of Creation is a holy proclamation that life has the ultimate sanction of God, that life (in all its beauty and diversity) is not merely a “thing” that can be fabricated by human hands or disposed of like industrial trash. So here’s what I’m saying: if the Genesis narrative is a literal biology lesson, then it is absolutely wrong. On the other hand, if it is a religious lesson, then (I believe) it is absolutely right.

Despite (or because of?) his clerical connections, Darwin engaged in direct debate against the creationist position. His strategy was to select a specific phenomenon and amass suggestive evidence (against creationism) relating to that phenomenon. Then he would attempt to overwhelm potential opponents by sheer quantity of data. Stephen J. Gould (2002: The Structure of Evolutionary Theory, Cambridge, MA, Belknap) illustrates this technique of argument by a 10-point discussion of Darwin’s views on island biogeography (Origin…, 1859, pp. 388-406). I am impressed by Gould’s synopsis and shall abbreviate it (and alter it slightly) right here.

Point 1: In comparison to continents, most islands have relatively few endemic species, even per unit area. But why should a generous God place fewer species on islands?29

Point 2: When people introduce continental species of plants or animals to islands, the continental species often out-compete and replace the endemic island species. If God created the island endemics especially for islands, why could the continental species often be superior competitors, even on islands?

28 But I would like to contemplate the presentation of a Biblical-literalist scenario at Fort Benning, in an Infantry School classroom filled with senior captains and their instructors. Problem: The Infantry Battalion in Attack of a Fortified Area. “What would you do, Captain?” “Sir, I would requisition rams’ horns.”29Note: Darwin offers this and the following nine points as questions that a creationist could answer only with difficulty. On the other hand, Darwinian theory has a cogent, easy response to each question. Can you think up the Darwinian answers? (Or—harder assignment—can you come up with good creationist answers?)

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Point 3: Within taxonomic groups, similarity between island and non-island species is in part a function of colonization-ability. Consider, for example, hypothetical animal Families A and B. Now consider two similar species from A: Acontinental and Aisland. Also, consider two similar species from B: Bcontinental and Bisland. In general, if A is a better colonizing taxon (e.g., bats) than B (e.g., salamanders), then Acontinental and Aisland will be more similar than Bcontinental and Bisland. (I.e., continental bats resemble island bats more closely than continental salamanders resemble island salamanders.) If God created species especially for islands, why should this be the case?

Point 4: Ecological niches on islands are often occupied by members of taxonomic groups that fail to hold such niches in continental systems. (On the Galapagos Islands, for example, large tortoises occupy grazing niches held by mammals on most continental landmasses.) Why would God maintain different taxonomic-ecological plans on islands?

Point 5: Endemic island species sometimes show anatomical characteristics possessed by similar continental species—even though the anatomical characteristics are not useful to the island species. “For instance, on certain islands not tenanted by mammals, some of the endemic plants have beautifully hooked seeds; yet few relations are more striking than the adaptation of hooked seeds for transport by the wool and fur of quadrupeds (Darwin, 1859).” If God created these species especially for islands, then why are the useless hooks of similar species present?

Point 6: When island species occupy niches held continentally by different taxa (that did not reach the island), the island species often exhibit “strange” morphological characteristics not found elsewhere in their taxon. For example, a plant Family typically herbaceous on continents may have tree-sized species on otherwise treeless islands. Why would a special-creator God change morphological plans (and see 4 above)?

Point 7: Species suitable for island habitats are often missing from those habitats. Why would a generous, special-creator God build so much great snake and salamander habitat in Hawaii—and then deprive the archipelago of these wonderful creatures?

Point 8: The richness and diversity of an island’s biota are often correlated inversely with the distance of the island from a continent. “…[W]hy, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands (Origin…, 1859, p. 394)?”

Point 9: The richness and diversity of an island’s biota are often correlated with ease of access to the island. If God creates faunas especially for islands, why should God choose to put richer faunas on islands separated from continents by shallower intervening seas? (Note: Point 8 above can be considered as a special case of this Point 9.)

Point 10: An island’s endemic species are almost always most similar to species on the closest neighboring continent. If God created endemic species especially for islands,

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then why did God choose to copy, almost, nearby continental species?

Any creationist could point out—and Darwin would agree—that none of the above points disproves the case for special creation. To such an objection, Darwin would make two replies:

Reply 1: The above points fit smoothly and neatly with my theory of evolution. On the other hand, in order to make your creationism-theory fit these undeniable facts, you must posit interesting inconsistencies in the Creator’s mind. Most scientists (and many theologians) would consider my evolution-argument to be favored by the principle of least astonishment.

Reply 2: I’ve just been talking about islands. I have hundreds of other examples….

Special Topic 3: Current Ideas on Revising Darwin

Biologists have recently shown interest in revising 3 facets of Darwin’s theory. These are (1) Darwin’s absolute commitment to uniformitarianism, (2) Darwin’s insistence that natural selection operates almost exclusively at the organismal level, and (3) Darwin’s belief that, compared to natural selection, all other factors are unimportant in evolution. Note that although these three principles were fundamental to Darwin, modern challenges are largely matters of degree and do not threaten Darwin’s basic theoretical edifice.

1. Criticism against Darwin’s Uniformitarianism. Darwin believed that evolutionary changes were gradual, slow, and approximately

unvarying in rate. However, paleontologists are seldom able to document the gradual, “anagenetic,” origin of new species. Instead, the fossil record is more likely to indicate the persistence of a given species (perhaps with minor changes30) across several geological strata. Then, between one stratum and the next, the old species becomes so radically different that it can no longer be called the same species at all.

Darwin was familiar with this phenomenon, and he attributed it to gaps in the fossil record: given more strata, we’d document the gradual formation of a new species. All modern biologists admit that the fossil record is indeed incomplete. Many argue, however, that if evolution ran at an essentially steady pace, then even our limited fossil record would be statistically likely to record more intermediate forms than it does (and thereby to support a gradual, anagenetic nature for the speciation-process, which in general it does not).

Some biologists also point to specific geo-historical times during which evolution appears to have run at “non-standard” rates. For example, dinosaurs had dominated the large-vertebrate terrestrial niches for over a hundred million years. Then, about 65 million years ago,

30 Often these changes suggest modest adaptive adjustments to a changing environment. For example, the median size of the fossils may increase across a couple of strata and then decrease across the next few bedding planes; perhaps other modest increases or decreases follow…. The point is, although the fossil sequences often imply the operation of natural selection, seldom do they indicate an accumulation of gradual changes sufficient to imply the anagenetic formation of a new species.

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something happened, and the big dinosaurs became extinct.31 Clearly, many large-animal niches became vacant with the demise of the dinosaurs. And mammals, which had remained uniformly small for more than 100 million years (during the Jurassic and Cretaceous), suddenly evolved to fill many big-animal niches during the Paleocene, which lasted less than 10 million years.32 At least from the fossil record, it appears that mammalian evolution operated at an accelerated rate during the Paleocene.

This critique of a strictly uniformitarian Darwinism is sometimes called the theory of punctuated equilibrium (see Gould and Eldredge, 1993). One should note that Darwin did not argue that evolution should always run at exactly the same rate. Furthermore, proponents of punctuated equilibrium do not believe that the process of species-origins involves instantaneous jumps from one organism-type to another. (Evolution, the punctuators say, can occasionally be very rapid and yet always remain smoothly incremental.) The argument, then, is whether evolutionary rates vary enough to require significant patches on the old cloak of Darwin’s uniformitarianism. In a couple of long boxes below I’ll suggest a relationship between ideas of punctuated equilibrium and the modern understanding of how genes are turned on and off. You might read them first now; you’ll be seeing them again later, in connection with turtles and shells.

1. Acknowledging my ancient errors of understanding. In our modern genetics-age we have begun to learn ways in which evolution is not always Darwinian-smooth but rather can occur in small but distinct steps. To appreciate this new insight, we must consider the possible effects of mutations—and I need to confess an error in my former understanding of how mutation typically affects a biotype. Let me summarize my old assumptions in italics, and then I can point out how they were incorrect. Remember, don’t learn the stuff in italics! It describes my old, non-useful model: try to see how I was wrong, and together we can learn a better model for understanding genetic bases underlying evolutionary change. A gene consists of a long string of base-pairs [true; you remember: A, G, T, C] that codes for the construction of a specific protein that will comprise a particular structure or fulfill a particular function in the given organism [true]. Thus, metaphorically, a gene can be considered as a set of written instructions on how to build part of an organism [still not too bad]. When the coding sequence of a protein mutates, this is metaphorically like a typographical error in the set of written instructions [OK]. Sometimes, despite the typo, the old instructions can still be followed with the same old results [OK, some mutations are “silent”]; sometimes they cannot. If the old meaning is indeed lost, then the probability that the new meaning will be “better” [= more adaptive] is exceedingly slim [my reasoning is getting shaky here]. Metaphorically, then, a beneficial mutation is analogous to this: You are typing a copy of Shakespeare’s greatest sonnet; you

31 The explanation currently most popular is that a large meteorite struck earth near the Yucatan Peninsula. The resulting dust-storms and fires created a greatly prolonged, worldwide “winter” that the large, terrestrial dinosaurs did not survive.32 The phylogeny of mammals during the early Cenozoic is more complex than I have suggested above. Actually the large-mammal fauna was somewhat depauperate during the first several million years of the Paleocene, and many of the more obvious mammalian radiations occur closer to the Eocene boundary. Still, there is a new suddenness in the post-Cretaceous occupation of large-vertebrate niches by mammals.

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accidentally hit a bunch of incorrect keys, and your random transcription-errors actually improve the sonnet. I have now told you my “sloppy-typing // good-mutation” analogy, according to which I formerly (mis-)construed a genetic basis for evolution. The analogy is not exactly wrong, but it does not describe how most evolutionarily important mutations work. You see, the most important mutations do not occur in a gene’s protein-coding sequence. Rather, the mutations of greatest evolutionary significance take place in a control region that activates (or inactivates) the gene at a certain time and at a certain anatomical location within a developing embryo.

2. Building a better model for understanding genetic bases underlying evolution: the concept of switches. Many genes do their most important “construction work” during an organism’s embryological development. And, as you probably know already, not all genes are turned ON in all parts of the developing embryo all the time. During embryological development a gene does its work (is turned ON) when a long molecule of mRNA lies alongside it and makes a transcript of the gene’s protein-building instructions. In general, the mRNA molecule can approach and transcribe the gene only when a specific protein, called a promoter, binds to a non-coding region of DNA “upstream” from the instructions to be transcribed. Think of this upstream, non-coding region as a switch by which the gene’s work can be turned ON. (For a pictorial definition of a genetic switch, see my diagram on the next page. Also, in case you are interested, I’ll eventually describe a simple genetic switch, in a box cleverly entitled “An Example of a Switch.” If you demand a sequential, complete presentation of my present argument, then you might want to skip way down and look at that Switch box before you read any farther. But I think you’ll find my explanation easier if you keep on reading right here.) To reiterate, a switch is not exactly part of the gene (that is to say, it is not transcribed as part of the protein-making process); rather, a switch turns the gene ON or OFF. If a promoter-substance (usually itself a gene-created protein) binds to a gene’s switch, then the gene is set to ON; if a promoter does not bind to the switch, the gene is set to OFF. Therefore, if the switch-region of a gene mutates, then the cell does not produce a nonsense-protein whose presence in the developing organism would almost certainly be deleterious. Rather, the appropriate promoter-substance cannot bind, so the gene will not call for the production of any protein at all. And if thereby some part of an anatomical structure is neatly deleted (or neatly duplicated, when a switch usually OFF is turned to ON; see below) then the result is not necessarily disastrous to the developing organism. Shortly I shall illustrate this important point by discussing a hypothesis concerning the reduction of appendages during the evolution of insects. Because many switches work in sequence, mutation of a switch-region can also turn ON the construction of a protein. E.g., perhaps Switch A turns ON the construction of a protein that prevents the binding of a promoter to Switch B. (In other words, when A is turned ON, B is thereby turned OFF.) So, if the A-switch region mutates so that it cannot be turned ON by its promoter, then it can no longer turn B OFF—and B therefore builds its protein. During embryonic development, many complex structures are created through the operation of long sequences (“cascades”) of switches, and as you read what follows, please keep this idea of switches in mind.

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3. Understanding that different switches work at different times in different places. As an embryonic organism develops, regions become defined in the mass of multiplying cells. This is easy to understand if you think metaphorically about the embryonic cell-mass as a globe upon which lines of latitude and longitude are drawn. In an embryo, the “coordinate system” actually defines 4 dimensions: (1) “north-south,” (2) “east-west,” (3) “depth” beneath the surface of the cell mass, and (4) “time” within the schedule of embryonic development. The initial spatial demarcation of the embryo results from an asymmetry of molecular distribution in the egg, and the “geographical coordinate” system is increasingly refined throughout embryonic development. Specific switches are turned ON in some regions and OFF in others. For example, during the development of an insect embryo, a cascade of switches (…operating on other switches…) instructs the development of: (1) walking appendages (legs) upon 3 body-segments, (2) antennae and feeding appendages (mouthparts) on the composite head-segment and (3) no appendages on the remaining body-segments. Experimentally, biologists can transfer a turn-the-switch-ON promoter substance to the cells that will become an insect’s head—and legs will grow where we would normally expect to see antennae.

4. Examining a (partly hypothetical) model for the evolution of insects from ancestors with many branched appendages. Now let’s consider a simplified scenario for the origin of the Class Hexapoda (formerly called Class Insecta, the insects). As you follow this scenario, you’ll note that segmentation, duplication, and multi-functionality are important factors: they allow evolution to tinker with what’s already there—by means of ON-OFF switches! Insect legs are unbranched and occur on only 3 body-segments. By contrast, arthropods ancestral to insects had two-branched appendages on a large number of body-segments. (Crayfish are not ancestral to insects, but they are somewhat like some insect ancestors. Recall

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the many, multi-branched appendages we saw upon our lab-crayfish.) Each appendage typically had a dual role in locomotion and feeding—however, unbranched, specialized appendages would allow more efficient walking and would therefore be adaptive, at least for locomotion. Now say that the switch eliciting the branching of appendages mutates (this would mean that the switch’s promoter-substance could not bind and that the switch would therefore be turned OFF) in one body-region. Nothing super-weird would happen; instead the organism would have (in the affected region) a set of unbranched appendages that would be more efficient for walking than the branched appendages of the creature’s ancestors. (Note the following “small print,” lawyer-type weasel statement: In this caricature-explanation, I am pretending that the branched appendages mutate from branched to unbranched in one step. In actuality, such would not be the case; one branch could be deleted over many generations by successive non-expression of a switch-key protein called distal-less at the termination of that branch.) Because of the organism’s segmented duplication, the new critter would still have some old-type, unaffected segments that produce the old-time branched appendages capable of gathering food and of locomotion—but (because of improved walking-organs in the mutated segments) the locomotive functions of these unaffected segments would be less important. In these segments, switch-setting mutations that improve feeding efficiency would therefore be adaptive, even if they undercut the segments’ locomotive functions. Thus, within the chain of segments that comprise our hypothetical organism, segments in one region are now somewhat specialized for locomotion, and segments in another region are now somewhat specialized for feeding. Through further mutations, over time, these specializations could be fine-tuned, and the organism would become an increasingly efficient walker and eater. Something like the above scenario really is now accepted as explaining the evolutionary origin of insects. 5. Considering the role of switches in evolutionary changes among salamanders. Insects are perhaps the greatest success-story of the entire animal Kingdom, and evolutionists have profited enormously from their study. However, as a herpetologist, I prefer to contemplate a plausible evolutionary example gleaned from the study of ambystomatid salamanders. Typically these varmints have an initial larval, aquatic stage, incapable of reproduction, and an adult, terrestrial stage that reproduces. Keep that “dual lifestyle” in mind—and recall that the “geographical coordinate” system in a developing embryo has a time dimension. That is to say, various switches are turned ON at some times and OFF at others. Now consider two “timing switches” (or sets of switches): At some pre-set point in developmental time, Switch A elicits the cascade of changes that resorb gills, refine lungs, and restructure limbs for terrestrial locomotion. At a later point, Switch B elicits the cascade of changes that result in the development of adult reproductive organs. Suppose Switch A mutates (= is set permanently to OFF) and Switch B does not (= it will eventually turn ON). In such a case, the result would be the development of reproductive organs in animals that otherwise possess larval (as opposed to adult) characteristics. (Actually, of course, the process is more complex, involving mutation of a switch that controls many other switches. Perhaps the entire process has to do in part with the embryo’s processing of iodine. Anyhow, I hope you get the idea.) The above paragraph describes approximately the scenario that has been hypothesized for the evolution of an actual ambystomatid salamander species that lives in parts of Mexico. As with most other ambystomatids, the adult tiger salamander is terrestrial, and the larva is aquatic. As the Mexican Central Plateau dried out, climatic changes destroyed habitat for adult tiger

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salamanders, but larval habitat remained in abundance. Mutations in time-development switches allowed the evolution of individuals that could reproduce while in a “larval” state. In Central Mexico, with numerous deep ponds surrounded by very dry land, such mutations were highly advantageous, and a new type of salamander emerged. This is called the axalotl, or water-monster. It is basically a reproductively capable tiger salamander larva that eventually evolved a few extra adaptations for its permanently aquatic lifestyle. (Below, an axalotl is shown to the left, and a conventional, adult tiger salamander is shown to the right. Observe that just a few mutations can make a lot of difference!)

Now that you’ve handled one very large box, let’s see if you can deal with another—because we’re now going to consider an example of a switch.

An Example of a Switch. Escherichia coli is a common bacterium (you probably have a few in your body right now) that survives through the enzymatic metabolism of various sugars. E. coli “prefers” (uh, in so far as a bacterium can prefer anything…) the simple sugar, glucose. However, if E. coli lives in an environment without glucose and with lactose, the bacterium will produce beta-galactosidase, an enzyme that breaks lactose down into glucose and galactose. When glucose is present and lactose is absent, E. coli doesn’t “want” to waste the energy required to produce beta-galactosidase. (Can you blame it? I cannot reliably even spell the enzyme!) As one contemporary author puts it, “…the neat trick to understand is how the bacterium makes beta-galactosidase only when lactose is present. … [T]he production of this enzyme is controlled by a switch that resides at the beta-galactosidase gene. The switch is off when lactose is absent, but flips on when lactose is present. There are two key components of the switch, a protein called lac repressor, and the short stretch of DNA sequence near the beta-galactosidase gene to which the lac repressor protein can bind. When the repressor protein binds to this DNA sequence, the gene is off (repressed), and no RNA or protein is made. But when lactose is present, the repressor falls off the DNA, [the promoter-protein can therefore bind,] and RNA transcription and beta-galactosidase enzyme production occur…[Carroll, Endless Forms

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Most Beautiful, 2005].” The action of the LAC Repressor switch is illustrated by the sketch-model below:

Now don’t you folks get lost in this long Special Topic. At this point I have completed my discussion of how contemporary biologists view Darwin’s strict adherence to uniformitarianism. And most evolutionary theorists today believe that doctrinaire Darwinism should loosen up a little and affirm that evolutionary processes do not always run at exactly the same rates. Allow for some fits and starts that punctuate long periods of relative stasis. And even admit that some small, discontinuous steps might lie along the multi-generational progression from one biotype to another. Now I shall go on to consider other contemporary critiques of unrevised Darwinian theory.

2. Criticism of Darwin’s restriction of natural selection to the organismal level.By definition, natural selection can operate only upon discrete entities (well-defined

“things” that have boundaries) that can reproduce (can be “born,” can “give birth,” can provide a link of heritability between ancestors and offspring) and can cease to reproduce, permanently (can “die”). I’ll offer you a new term so that I can write that nasty sentence more briefly: natural

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selection can operate only on Darwinian individuals.33 (Uh, of course that sentence is based upon the definition of a Darwinian individual as a bounded entity that can reproduce and can cease to exist.)

Obviously, an individual organism such as a single liveoak tree or a single mountain lion would fit my definition of a Darwinian individual. And, because no other level of biological organization so obviously fits the definition, Charles Darwin himself argued that natural selection operated almost entirely at the level of individual organisms.34 This is easy to understand: individual varmints can triumph (pass on lots of genes) or can fail (die childless) in Darwin’s survival-of-the-fittest world. Most biologists (and I’m among them!) still affirm that natural selection operates primarily at the level of individual organisms. However, many modern biologists argue that selection at non-organism levels is sufficiently important to be worth mentioning. Now please allow me to write that last sentence in another way, using my new vocabulary-term: some biologists believe that biological entities other than individual organisms can be Darwinian individuals, and therefore biological entities other than individual organisms can evolve through natural selection.

Except for the brilliant Richard Dawkins (whose arguments for selection at the level of genes do not hold water), the most renowned exponents of non-organismal-level selection have also been vigorous proponents of punctuated equilibrium (see above). These theorists have generally argued that the most important non-organismal targets of selection are species. To make such arguments in a worthwhile way, the theorists must accomplish two tasks (1) explaining how species can be Darwinian individuals and (2) demonstrating empirically that species-level selection is sufficiently important to deserve our attention.35

I believe that Task Number 1 has been best addressed by paleontologists, many of whom hold that a species can be assigned (a) a well-defined border, (b) a well-defined time of ending (“death”), and (c) well-defined time(s) of reproduction (being “born” and giving “birth”).

33 Darwin himself did not use the term, and he would not applaud the way I shall employ it. But I do need a special term, that’s the one I’ve chosen, so you’re stuck with “Darwinian individual.”34 Darwin wrote at length about sexual selection, which may sound as if it operates at some non-organismal level. Actually, sexual selection may lead to the evolution of traits with negative survival value to the individuals that manifest them—but still, selection has its impact at the level of the individual organism. The loss in “survival value” is outweighed by a concomitant gain in “reproductive value.” In my essay on artiodactyls I’ll argue that evolution has selected for large antlers in many deer species, even though those big antlers may be detrimental to the individual males that form and carry them. But these big-antlered males produce more kids.35 By the early 1980’s, Robert May of Princeton University had proved mathematically that under at least some conditions, natural selection at a non-organismal level can occur. But even accepting May’s mathematics, a biologist still faces what I’ll call “the cantaloupe problem.” Physicists are absolutely sure that the orbit of Mars is constrained by the position of the sun and by the fall of a cantaloupe from the fruit-shelf of a Bi-Lo grocery store in Moncks Corner, South Carolina. But is the cantaloupe’s effect worth thinking about? Well, when it comes to non-organismal levels of selection, some biologists believe we’re dealing with a second sun, and others believe we’re dealing with a cantaloupe (or even a lemon). I think we’re somewhere in between!

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Boundedness. The stability that punctuated equilibrium posits for many fossil species comprise a boundary: for any set of geological strata we can, in principle, say which individual organisms are “inside” a species and which individual organisms are “outside” that species. Thus that species may be considered bounded: we can call it “a thing,” an individual. But as I stated above, Darwinian individuals must possess two characteristics beyond mere “thing-ness.”

Species death. For many individual species, the “time of death” criterion is straightforward: extinction is a familiar process, and although we may not know the exact instant of a species’ demise, it is, for many species, definable in principle.

Species “reproduction.” The reproduction of species, as Darwinian individuals, can be more difficult to visualize—and indeed for species that originate through gradual, anagenetic processes (see above), the discrete-birth criterion is simply not fulfilled.36 Proponents of punctuated equilibrium define species-reproduction as the splitting of a species into two daughter-species (one of which can be the surviving parent-species).37 Because the punctuated-equilibrium people often “observe” this 1-species 2-species change as they look from an older stratum to the younger one above it (i.e., as they infer a bifurcation that presumably occurs during the between-time not recorded by contiguous strata), they speak of reproduction as occurring within a geological moment.38 Now, given all that background about “species-individuals,” my next trick will be to argue, by means of a simplistic, hypothetical scenario, that species-level selection is at least possible under some conditions. My made-up example is not as good as May’s work (or Gould’s), but it is less math-heavy (and less wordy).

Ab’s Imaginary-World Argument for the Potential Reality of Species-Level Selection

Let us imagine a simple world whose terrestrial habitats consist entirely of widely dispersed, moderate-sized islands. Each island presents a fairly uniform habitat, but the islands differ substantially from one another. For example, although insects are abundant on all islands, each island is inhabited by its own unique set of insects-species. These insect-sets differ greatly from island to island, but all insect-types have an earthbound larval stage and a flight-capable adult stage, and none are capable of inter-island dispersal. As the mammalogical paleontologists

36 Theorists can argue for species-level selection even if the discrete-birth criterion is not fulfilled: only an assumption of species-reproduction by bifurcation is necessary. In my opinion, however, the argument is messy, and I do not wish to attempt it in this essay.37 It is easier to visualize “species reproduction” if we consider an analogy to asexual organisms—perhaps a bacterium that reproduces by fission, or a hydra that reproduces by budding off a second individual: in either case, reproduction means 1 parent cell 2 daughter-cells.38 Of course this does not mean that proponents of punctuated equilibrium posit times of species-birth, death, and reproduction that would be instantaneous according to a conventional stopwatch or even a conventional day-calendar. As I suggested above, we are talking about a “geological moment.” Thus, say that a species persists for a few million years and that its origin and termination are in principle definable to within a few thousand years. In that case, “birth” and “death” would be in principle definable to within about 0.1% of the species-lifetime—and that’s about how accurate we could define the birth and death of many organism-level individuals.

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of our newly imagined world, we are not interested in the insects—except as potential food for two mammal-types that we shall shortly invent. And, from the point of view of these mammals, all the world’s insects are good to eat.

Now, in order to illustrate selection at the species-level, we need to begin our world’s imaginary geo-history with two imaginary mammalian species, whose imaginary existence and “species-progeny” we shall track through geological time. The first imaginary species is an aerial insectivore, a bat, that is proficient at capturing adults of at least some insect species in our imaginary world. The second imaginary species is a terrestrial insectivore, a shrew-mole, that is proficient at capturing larvae of at least some insect species in our imaginary world. Individuals in both of these species live about the same length of time and produce about the same number of babies. Initially, both species enjoy similar levels of genetic diversity. Both experience similar rates of mutation. Both species have at least some unique bone-structures that fossilize very well. And, at the beginning of our world’s relevant history, both species are restricted to one island!

Remember (and we’re almost through with the set-up of this little scenario) that we are paleontologists, who will be tracing the imaginary geo-history of mammals in this imaginary world through a long series of many imaginary fossil-bearing strata.

And, oh, I need to tell you two more things about our made-up world and its made-up mammals: (1) Once in a great while, and from variable directions, the winds of our imaginary archipelago blow hard, far, and wide. (2) Our imaginary shrew-mole—unlike some real shrew-moles—is a very bad swimmer that hates water and drowns easily. (Can you now guess the rest of the story?)

To begin our analysis of our made-up world let’s ask and answer a few questions:Q1: Which species might we expect eventually to disperse to the various islands?A1: The bats may not be capable of flying from island to island, but the shrew-moles definitely cannot disperse—and we might expect the bat-biotype eventually to be blown by the rare winds to every piece of land within the archipelago.

Q2: Does the answer to Q1 make appeal to differences between the capabilities of individual organisms or to the difference between the capabilities of the two species?A2: Some bats are probably better fliers than others; some of the shrew-moles may be able to tolerate water slightly better than others. Nevertheless, A1 above (“bats are the biotype that will disperse”) depends upon characteristics that are coherent to species, not individual organisms!

Q3: Given the theory of allopatric speciation (“different-place” speciation; see the body of this essay’s text, far above), which original species might we expect eventually to split into more daughter-species?A3: We should expect the bat-species eventually to split into more daughter-species. Founder-stocks of bats, eventually blown to the various places in the archipelago, will be subjected to the differing selective pressures of the different islands. Over time, this should lead

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to the origin of several daughter-species of bats. (Here are two notes: [1] We are assuming that bats are not shuffled too often across the matrix of islands; such over-frequent mixing could make the archipelagic bat population effectively panmictic—and thereby preclude allopatric speciation. [2] Proponents of punctuated equilibrium would expect these bifurcations to occur in “brief geological moments.” And indeed the dispersal-events could be almost instantaneous—though the follow-up construction of obviously different biotypes would take longer.)

Q4: If bat-species proliferation exceeds shrew-mole-species proliferation, then is this “origin of bat species” due to organismal-level selection or to species-level selection?A4: Some biologists might say “organismal level,” and some might say “species level.” But I say, “The origin of the additional bat species is due to both organismal-level selection and species-level selection!” If you consider local adaptations that lead to different bat-species on different islands, then clearly organismal-level selection is the driving force. We can, however, ask Q4 in a different way:

Q4*: At some point in imaginary geological time, we look across our imaginary world to observe many species of bats and few species of shrew-moles. Of course this constellation of species is the result of evolution. But is the bat-species / shrew-mole-species ratio more reasonably attributed to sorting based on individual-organism variation within species—or is it more reasonably attributed to sorting based on between-species variation (in dispersal abilities)?A4*: The broad pattern of mammalian phylogeny in our made-up world is reasonably attributable, at least in part, to selection at the species-level. “Bat-ness” (the entire package of what it means to be a bat, including many gene-driven characteristics and the even the non-linear effects that emerge from combinations of those characteristics) implies a biotype more proficient at wind-borne over-water dispersal than “shrew-mole-ness” does. Thus the eventual composition of species across the islands of our invented archipelago, constructed by evolution, is convincingly attributable to selection at the species-level. (Note: As the number of bat-species proliferates, we might technically need at some point to speak of “clade-level selection” rather than “species-level selection. In that case we would still be affirming the possibility of natural-selection pressures at biological levels beyond that of individual organisms.)

The above scenario is my best argument that, at least for some types of analysis, evolutionary models based on species-level selection are useful and appropriate. Critics could of course argue that that the scenario sketched above is simplistic to the point of parody. In my defense, I would say, “I could make it more realistic, but then it would be harder to understand—and a whole lot longer.” If the critics were patient with me and asked that I indicate how I would construct a longer and more realistic scenario, then this is how I would begin: “Let us consider two “species” of virus that are pathogenic in human beings. Smallpox mutates slowly, and people easily developed a vaccine against it. HIV, on the other hand, mutates very rapidly, and this virus has thus far survived the human quest for a vaccine. If we consider the different mutation rates to be species characteristics….”

Earlier in this Special Topic I indicated that proponents of species-level selection had two tasks. The first would be to demonstrate that, under at least some conditions, species-level

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selection would be theoretically possible. I’ve done my best to show that such is the case, and I think I have succeeded. The second task would involve “demonstrating empirically that species-level selection is sufficiently important to deserve our attention.”39 This is not a trivial task; nor is it one toward which my education or research interests direct me. The literature on species-level selection is vast, but it is mostly theoretical, and clear empirical examples are hard to find within the current, published corpus. My limited perusal suggests that much of the actual fieldwork deals with marine snails40—though Elizabeth Vrba has also made interesting observations on African hoofed mammals. For students particularly interested in the topic, I might suggest examining Gould’s tome on evolutionary theory (you could scavenge around among pages in the higher 600’s, but I’ll warn you, the book is not light reading). In any case I shall for now leave the cantaloupe-issue undecided, though I shall admit my prejudice that eventually species-level selection shall be adjudged moderately important in evolutionary theory.

Historical notes concerning selection at non-organismal levels. Although criticized by some modern biologists, a strictly organismal approach to natural selection allowed Darwin to present a well-structured theory of elegant simplicity. (By contrast, Wallace’s skitterings between organismal and non-organismal levels appear somewhat undisciplined, perhaps even malarial.) The brilliance of Darwin is further illustrated by his arguments that intricate, coordinated, multi-species ecosystems are developed as a result of competition among individual organisms. Probably Darwin owes this insight in part to the work of economist Adam Smith. In Wealth of Nations (1776) Smith argues that the unfettered competition of individual economic actors eventually produces the most efficiently structured, complex economic system. (Clever historians of ideas might wish to note that—perhaps because decent people are unwilling to tolerate Malthusian levels of suffering in their economic systems—Smith’s ideas may be more purely realized in nature than in human enterprise. And students who know me personally will observe how the immediately previous sentence grows in part from my political ideology: I think that the practice of pure Smithsonian economics would be downright sinful.)

Now let’s take stock once more of where we are, in this long Special Topic. We first dealt with the conversation between strict uniformitarianism and punctuated equilibrium. Then we considered the question of whether natural selection could operate on non-organismal levels. Now, finally, we must address the issue of form’s importance in the channeling of evolutionary processes.

3. Overwhelming primacy of adaptation over form. Darwin believed that almost all characteristics of any species reflected the pressures of natural selection and could therefore be best explained by an analysis of the species’ adaptation-history. Of course he knew that adaptations were limited by phylogenetic history as well as by the laws of physics and ecological economics, but he felt certain that these factors were of relatively minor importance in explaining evolution (box). In other words, the Great Man’s basic idea was that natural selection could do pretty much anything. Some modern biologists (including one of my former teachers)

39 Remember what I called “the cantaloupe problem.”40 Apparently the fossil record often documents an increase in the number of species that brood their young relative to the number of species with larvae that float and feed in the water column.

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affirm, by contrast, the evolutionary importance of constraints inherent in the form of a biotype and in the internally specified pathways of its development. These folks argue that factors other than natural selection are important in shaping the evolutionary trajectory of living organisms. Therefore, they criticize Darwin for (1) his belief in the near-omnipotence of natural selection and (2) his near-total neglect of form-considerations.41 I do not intend to present a lengthy forms-and-pathways critique of Darwinian selectionism. Nevertheless, I do wish to indicate a potential line of argument—after the box that is presented below.

Natural Selection versus Phylogenetic History: for Darwin, No Big Deal

Consider (as we shall, multiple times, during this semester) the noble turtle. Every biologist accepts that turtles evolved from very un-turtle-like ancestors. And contemporary turtle species have different sets of evolutionary adaptations, presumably developed in response to the pressures of natural selection. However, turtle-ness necessarily involves having a shell. Over geological time, the turtle-shell developed through a profound set of mutations. And after turtles had shells, they experienced further adaptations based upon their “shelled-ness.” It is difficult to imagine any future series of mutational events that would allow the turtle biotype to “back out of its commitment to a shell” (as it were)—especially since every back-out step would have to produce an ecologically “fit” biotype. (Statistically, the probability of any such back-out series is effectively zero.) Thus, turtles are turtles; their descendents are constrained to be turtles, and throughout any imaginable future, the evolution of turtles is overwhelmingly likely to consist in refinements of recognizable turtle-ness. The hands of natural selection have been tied by the form of the turtle biotype that evolution created. Darwin recognized this sort of phylogenetic-historical constraint; he thought the whole point was reasonably obvious, and he did not believe that sustained meditations upon it would yield substantial dividends in understanding the nature of the living world.

Natural Selection versus The Laws of Physics and the Economics of Ecology: for Darwin, No Big Deal

In class and lab we have considered the principle of surface-to-volume ratios, and throughout this course I shall repeatedly emphasize their importance. To introduce this box, let

41 Even though I support the idea that natural selection is the primary force behind evolution, I have substantial sympathy with the proponents of form and development. (See my argument, below, on legs and lizards.) This is especially true when some biologists treat selectionism almost as a religion. No matter what sort of organism-attribute you may point out, these “selectionists” will offer an adaptive explanation. Indeed, if an adaptive explanation cannot be demonstrated through ecological research or theory, these folks will make one up: “I mean, this weirdness must have some adaptive function….” That does not sound like scientific reasoning to me.

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me simply re-state a biologically important truth of geometry: if an object maintains the same shape, then as the object becomes larger, its volume will increase more rapidly than its surface area. Please keep that fact in mind as you read what follows. First, here is an example of how the laws of physics limit the power of natural selection. Insects must nourish all the volume of their bodies by exchanging gas (swapping off accumulated CO2 and acquiring more O2) through their external surfaces. At some size level, the volume of an insect’s body could not be supported by through-the-surface gas exchange. Therefore, truly enormous insects are a physical impossibility. Second, here is an example of how ecological economics limits the power of natural selection. Evolution cannot produce a shrew with an adult mass of less than about half a gram. This is because shrews maintain high body-temperatures by burning lots of calories. Beyond some minimal size (c. 0.5g), a hypothetical shrew’s body-heat would drain so rapidly through the tiny animal’s surface (enormous, compared with the animal’s miniscule volume) that the varmint could not afford the effort of finding enough food to support its thermoregulation. That is, the “economics” of fuel and thermoregulation preclude the evolution of “buckshot shrews,” no matter how well they might perform most shrew-ey tasks. Of course Charles Darwin also knew about surface-to-volume ratios, and he would not deny my generalizations about size-limitations on insects and shrews. He simply did not believe that such physics/economics arguments were particularly useful in explaining most biological characteristics.

So, let me recapitulate. Even Darwin recognized the existence of some constraints against the near-omnipotence of natural selection—he just didn’t consider those constraints worthy of a substantial role in his theory of evolution. A number of modern evolution-theorists disagree. Some of these folks initiate their counter-arguments by meditating on why certain types of imaginable organisms do not exist.42

Any creative student could imagine a whole lot of interesting varmints that don’t actually exist. Given a background in chemistry, physics, and biomechanics, the student could probably invent a host of critters that would actually work pretty well in the real world. Evolution-theorists sometimes express these thoughts in a fancier ways, such as (1) “The adaptive topography of potentially workable biotypes is sparsely populated with actual, realized organisms.” (2) “Evolutionary lineages run in separate channels, and, typically, the empty spaces between these channels are vast.” Now, let me suggest an explanation that may be worthy of consideration. General biotypes are plastic to natural selection and can be altered in many ways. However, these “many ways” are not unlimited—and although selective pressures may come from all sides, evolutionary change within a biotype-lineage is constrained to flow most easily along relatively few existentially available trajectories.

To provide examples of this point I shall discuss evolutionary alterations in squamate (=

42 I don’t believe that anybody considers such meditations to comprise a formal argument against selectionism. They are merely an introduction; they start people to thinking.

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snakes, “lizards,”43 etc.) limb-patterns. Consider the absence of forelimbs in pythons.44 The deployment of certain genetic signals in the developing python embryo redefines the regionalization of the basic (ancestral) squamate skeleton. This has two effects. First, middle-body-type vertebrae are constructed from the snake’s head45 to the cloaca. Second, a pectoral (forelimb) girdle is not formed. This sort of re-regionalization is not unusual; indeed, leglessness has evolved dozens of times within the Order Squamata. Presumably this reflects natural-selective pressures towards leglessness under some environmental conditions—but loss of limbs may also be an “easy” evolutionary trajectory for some squamate lineages to follow. Research on legless lizards suggests that regional re-deployment of homeotic-gene46 expression results from relatively simple sets of mutations (silly box, below).

So I have argued that, for some lizards, “zero legs” is easy. On the other hand, although I can conceive of numerous environmental niches in which 6-legged lizards would enjoy adaptive advantages over their 4-legged relatives, such a biotype has never evolved: easy evolutionary pathways toward reptilian hexapody simply do not exist (same silly box). This, then, has been my argument that sometimes developmental form holds sway over simple natural selection.

Silly Box about Switches and Lizard Legs

I don’t want to trivialize the complex and wonderful findings of researchers working on homeotic genes—so please consider what follows to be a simplistic metaphor rather than a bio-genetic explanation. Anyhow, for your edification or amusement, I am willing to risk a certain level of professional ridicule, so I’ll ask you to consider the following metaphorical set of genetic instructions:

Metaphorical Genetic Instructions: Developing squamate embryo, you must now build a set of legs right here. If the underlined “w” easily mutates into a “t,” then pathways exist by which leglessness is evolved with reasonable ease. By contrast, the genetic instructions required to construct an additional pair of legs may require vastly more complex mutations to the original genetic instructions.

43 Perhaps you are wondering why I used quotation marks around the word lizards. That is because, at least when we’re talking about taxonomic categories, snakes should be considered as highly specialized varieties of lizards.44 Technically, pythons have (very small) hindlimbs. These are externally visible as tiny spurs near the cloaca (the eliminatory vent). They play a role in python courtship, and I shall discuss these spurs if we find time this semester to talk about varieties of living snakes.45 Actually the construction of thoracic vertebrae extends anteriorly only to the atlas vertebra. But that is the vertebra against which the skull articulates.46 Homeotic genes are an ancient and highly conserved set of genetic instructions involved in the mapping of embryos and in the activation of “switches” within specific embryonic regions at specific times. An elegant and readable discussion of these “toolkit genes” may be found in Carroll’s Endless Forms Most Beautiful (2005).

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