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Lecture V. After The Origin.

Anchiornis huxleyi, a four-winged dinosaur named after Thomas Huxley and close to the ancestry of birds. Feather colors inferred from the size and shape of melanosomes — see Li et al. (2010).

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Lecture 5. After The Origin. (Eiseley, 10-12) I. Evolution Agonistes.

A. The 6th and final edition of The Origin published in 1872. 1. A year later, Louis Agassiz, the last Special Creationist

of note, died.

2. Darwin himself succumbed to his final heart attack (pos-sibly consequent to Chagas disease) in 1882.

3. Wallace (despite scandal and financial problems) re-

mained scientifically active until his death in 1913.

a. During the final decades of the 19th century, a leading exponent and defender of Darwinism – his book by that title published in 1889.

b. In 1907, he published Is Mars Habitable? –a re-sponse to the astronomer, Percival Lowell – in which he argued that Mars was too cold to sup-port life; further that the atmospheric pressure was too low to permit the ex-istence of liquid water.

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B. Evolution became the consensus, but the objections of Jen-kin and Kelvin remained 1. Darwin’s attempts to provide a mechanism (pangenesis)

for rapid, environmentally driven change disputed by the experiments of Galton and Weismann.

2. Evolution was kept alive a. Not by the articulation of a widely acceptable mecha-

nistic theory. b. But by accumulating evidence for descent with modi-

fication from i. comparative anatomy, ii. embryology, iii. the fossil record.

C. Nowhere is this more evident than in the work of Huxley.

1. Evolution didn’t really show up in his research until nearly a decade after The Origin’s publication.

2. And then not with reference to natural selection, but to Haeckel and body plans.

3. The key realizations (for Huxley) were a. Inheritance and modification of body plans the basis

of phyletic evolution. b. Phylogenetic history reflected by development – the

recapitulation doctrine of Ernst Haeckel.

“Phylogeny, or the paleontological development of blood-related forms … is a physiological process which, like all other physiological functions of organisms, proceeds of ab-

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solute necessity by mechanistic means. These means are the atomic and molecular motions which piece organic mat-ter together, and the limitless variegation evidently achieved in the process of phylogenetic development [that] express-es a similar limitless variegation first in the concatenation of organic matter and then in the subtle compounds of which the active plasma of the constituent plastides of all organ-isms is composed.” – quoted by Di Gregorio [d82]

c. Shades of Lamarck – n’est-ce pas? d. Which “variegations” get preserved a non-question. e. Object of the exercise to work out relationships

among living and extinct organisms.

4. Di Gregorio (1982; see also Bartholomew, 1975) con-cludes:

“… I think it follows that Huxley's own research is not the place to look for evidence of any direct impact … of the publication of the Origin of Species, … prior to the late 1860s when systema-tologists … began to adjust. The same line of thought also dis-pels the expectation … that the doctrine of natural selection ought to be featured … in those works of Huxley that deploy the idea of evolution; for that expectation is countered by the observation of the substantial irrelevance of natural selection to the [then] current concerns of systematics. Endorsing evolu-tion and endorsing natural selection are not the same thing. Both his heavily qualified acceptance of natural selection and the survey of his research show that Huxley became an evolu-tionist, but not of the Darwinian kind.” [Emphasis added]

5. So much for “Darwin’s bulldog.”

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D. Huxley nonetheless contributed greatly to the case for de-

scent with modification.

1. Demonstrated anatomical similarities among primates.

2. Demolished Owen’s argument for an extraordinary gap

between hominid and pongid brain structure by showing that both possessed a hippocampus minor.1

3. Reflective of Owen’s refusal to accept descent of Homo

from ape-like ancestors.

4. Quarrel became a public scandal in which the press de-lighted.

1 The hippocampus minor is a ridge in the floor of the posterior horn of the lateral ven-tricle.

Anatomical correspondences among higher primates. From Huxley. T. H. 1883. Evidence as to Man’s Place in Nature.

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“Then HUXLEY and OWEN With rivalry glowing With pen and ink rush to the scratch. ‘Tis Brain vs. Brain Til one of them’s slain By Jove! It will be a good match.

Says OWEN, you can see The brain of a chimpanzee Is always exceedingly small With the hindermost “horn” Of extremity shorn And no “Hippocampus” at all.

Next Huxley replies That OWEN he lies And garbles his Latin quotation; That his facts are not new, His mistakes are not few, Detrimental to his reputation.” [Punch, 1862].

5. Discoveries of non-human

hominids (H. erectus, H. ne-anderthalenthis) later sealed the case.

E. Huxley correctly interpreted fossil Archaeopteryx (discov-ered in 1861) as evidence of a dinosaur-bird connection. Rejected pterodactyl ancestry in favor of Compsagnathus-like ancestor.

Top. Human brain with Hippocampus minor in red. Bottom. Neander-thal skull cap.

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Archaeopteryx lithographica was correctly identified by both Owen and Huxley as a bird. Both rejected ptero-dactyl affinities. Archaeopteryx remains the earliest uni-versally recognized member of the class Aves.

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F. Today, we know that

1. Birds were preceded

by feathered dinosaurs.

2. Feathers a. Were widely distrib-

uted among dinosaur clades.

b. Evolved in stages – 1st for insulation; lat-er for flight

Theropod phylogeny emphasizing two- and four-winged proto-birds.

Dilong paradoxus, a small early Cretaceous tyrannosaurid, was distinguished by its small size (~2m), relatively long forearms and a body covering of proto-feathers.

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G. Evolution of feathers (present in small tyrannosaurids) for

insulation raises the fascinating and controversial topic of dinosaur metabolism – were they homeothermic. 1. One clue the evolution of air sacs that permit higher rates

of avian oxidative metabolism than in mammals – eagle over Everest.

2. Dinosaurs had them.

3. Conceivably a. Air sacs date to Permian / Triassic anoxia and the evo-

lution of hollow-bones dinosaur ancestors.

b. Likewise, dwindling size of their synapsid contempo-raries may have reflected reduced [O2]atm.

Stages in feather evolution. Cross branches (barbules) secured by hooks are necessary for flight.

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Air sacs in a non-avian theropod dinosaur (Majungasaurus) and a living bird compared. Some scientists speculate that air sacs evolved in dinosaur ancestors in the Permian when atmospheric concentrations of oxygen may have dropped to 10%.

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H. Owen and Dinosaurs. 1. Coined the name Dinosauria.

2. Viewed dinosaurs “not as primitive and anatomically

incompetent denizens of an antediluvian world but ra-ther as uniquely sleek, powerful, and well-designed creatures - mean and lean fighting and eating machines for a distinctive and distinguished former world. … [H]e compared the design and efficiency of dinosaurs with modern (read "superior") mammals, not with slith-ery and inferior reptiles of either past or present worlds.” [Gould, 1998]

3. Viewed dinosaur sophistication as an argument against

the dominant view of the day that equated evolutionary progress with transmutation.

4. This – plus his innate combativeness, tendency to self-

aggrandizement and conservative outlook – led to a bit-ter feud with Darwin and his acolytes – Huxley, in par-ticular.

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I. Owen and the Aye-aye. 1. Like Lyell, Owen, never completely at

ease with transmutation, 2. Nonetheless demonstrated that the

mysterious aye-aye was nothing more than a highly modified lemur.

3. Internal organs typically lemuriform.

This despite spectacular specialization for detecting and extracting insects from tree bark. a. large ears b. narrow middle finger c. chisel incisors

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J. Like Huxley, Owen was wedded to the concept of arche-types. 1. His famous vertebrate archetype shown below.

a. Correctly interpreted the vertebrate body plan as fun-

damentally metameric – skeletal and muscular ele-ments sequentially homologous – recall Goethe. Got some of the details wrong, but

b. As in arthropods, much of vertebrate evolution the story of segment fusion / reduction and specialization.

2. To Darwinians, archetypes became common ancestors

destined to evolve via variation and selection.

3. To Owen, they were templates whose transformation into modified descendants was implicit in their design which possibly reflected the Creator’s intent.

Owen’s (1846) vertebtate archetype imagined that the vertebrate skeleton could be viewed as a series of identical segments, some of which were modified to form the skull, appendages, etc.

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4. Owen’s views on evolution vague and contradictory. Regarding the aye-aye, he wrote: “The terms in which the anatomist would express the sum of his observations on the structural resemblances of the Aye-aye throughout the Lemuridae would be, that the principle of ‘unity of organization’ prevailed through such group. “And such term would have a more intelligible meaning on the hypothesis that these singularly diversified Lemurs were genet-ically related by descent from a common ancestral form “Whilst admitting the general evidence therefore in favour of ‘creation by law,’ I am compelled to acknowledge ignorance of how such secondary causes may have operated in the origin of Chiromys. Darwin seems to be as far from giving a satisfactory explanation of them as Lamarck.” [O62]

K. Meanwhile, Darwin continued working on what amounted

to the completion of his original project.

1. Retreated into use and disuse in Variations of Animals and Plants under Domestication.

2. Established Crustacean nature of barnacles by observation of segmental body plan.

3. Investigations orchids allowed him to demonstrate the

multiple ways in which structures could be modified – in this case flowers and reproductive structures to facilitate services by animal pollinators.

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L. Triumph of the Darwinians

1. Regarding the Darwinian triumph and the battles that preceded it, Jensen (1988) observed that "The battle was not only between the Darwinists and the divines, but between the Darwinists, generally the younger scientists, and the older, more conservative scientists … . At the British Associa-tion meeting in 1894 the Marquis of Salisbury looked back at the 1860 meeting and said that 'in many cases religious apprehen-sion only masked the resentment of the older learning at the ap-pearance and claims of its younger rival'."

2. Huxley and his associates

a. Founded the journal Nature.

b. Founding member of the influential X-Club.

c. Gained control of the Royal Society with the ap-pointment of Hooker in 1873 and THH himself in 1883.

3. Previously noted that Huxley was no selectionist.

a. To his mind, one of Darwinism’s cardinal virtues

was that it allowed for “persistent types.”

b. His editorial in the 1st issue of Nature lauded Goethe – recall serial homology, body plans, etc.

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4. IMO, the Darwinian triumph generally, and Huxley’s rise, in particular, was driven as much by the pursuit of power both within and beyond the realm of sci-ence, as it was by a desire to account for Darwin’s facts.

5. Thus developed the British scientific establishment with its Sirs, Lords and Baronets and its enduring proclivity to intrude itself into social and political af-fairs.

6. In effect, the guiding hand of the old Churchly elite

was replaced by a no less dogmatic secular hand.

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II. Human Origins. A. Darwin took up the problem of human evolution in The

Descent of Man. Emphasized 1. Continuity of human physical and mental development

vis-à-vis lower primates and within the races of man.

2. Sexual selection and its importance to human evolution: hairless body, female beauty; male beard, etc.

3. Relaxation of natural selection as man emerged from

barbarism – adverse societal consequences.

B. On the issue of eugenics, Darwin agreed with his cousin, Francis Galton.

“We civilised men … do our utmost to check the process of elim-ination; we build asylums for the imbecile, the maimed, and the sick; we institute poor-laws; and our medical men exert their ut-most skill to save the life of every one to the last moment. There is reason to believe that vaccination has preserved thousands, who from a weak constitution would formerly have succumbed to small-pox. Thus the weak members of civilised societies propagate their kind. No one who has attended to the breeding of domestic animals will doubt that this must be highly injurious to the race of man.” (Darwin, 1871)

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C. However, he argued that there were “checks” 1. The weak and inferior do not marry as frequently as the

sound.

2. “The men who are rich through primogeniture [inheritance of the entire estate by the first born] are able to select generation after generation the more beautiful and charming women; and these must generally be healthy in body and ac-tive in mind [and bear them healthy children who are likewise healthy and intelligent]..”

D. Moreover,

“The aid which we feel impelled to give to the helpless is mainly an incidental result of the instinct of sympathy, which was originally acquired as part of the social in-stincts, but subsequently rendered, in the manner previ-ously indicated, more tender and more widely diffused. Nor could we check our sympathy, if so urged by hard reason, without deterioration in the noblest part of our nature. The surgeon may harden himself whilst perform-ing an operation, for he knows that he is acting for the good of his patient; but if we were intentionally to neglect the weak and helpless, it could only be for a contingent benefit, with a certain and great present evil. Hence we must bear without complaining the undoubtedly bad ef-fects of the weak surviving and propagating their kind …” [Emphasis added]

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E. Wallace by way of contrast imagined two stages of hu-man evolution.

1. Period of slow morphological evolution including ac-

quisition of bipedalism.

2. Period of rapid change dominated by enlargement of the brain, which could not be the result of selection.

3. Regarding human intelligence, Wallace emphasized

a. Its uniqueness. b. Lack of intellectual variation among human races. c. His views paralleled those George Mivart who argued

that the critical development in human history was the acquisition of the soul and who wrote a scathing at-tack on The Descent of Man.

4. Mivart, who would later run afoul of the CatholiChurch,

(http://www.newadvent.org/cathen/10407b.htm) further impressed by "the incompetency of 'natural selection' to account for the incipient stages of useful structures."

F. Reception of non-human hominids: Neanderthal man.

1. Announcement (1857) of the type specimen (Nean-

der Valley) antedated publication of The Origin.

2. Previous specimens dated to 1829 (Engis) and 1848

(Gibraltar).

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3. Interpretations varied:

a. Characteristics within the range of variation of

modern man (Huxley).

b. Similar to the “rude tribes of America and Austral-

ia” (Dawson).

c. Engis and Neander skulls belonged to a race re-

sembling Australian aboriginals, the more gracile

Engis skull being that of an intelligent female, the

more robust Neander skull that of a brutish male

(Vogt).

d. Microcephalic idiot.

e. A reversion (atavism) to a condition reminiscent of

man’s ancestral state.

4. Large Neanderthal cranial capacity (1300-1600 cc)

troubling.

a. Suggested that human brains got larger as one de-

scended into the past.

b. Contradicted the Darwinian expectation that human

ancestors would have smaller brains.

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5. Interpretations Homo erectus (1891; cranial capacity

850-1100 cc) similarly mixed. Ranged from patholog-

ical deformity to true intermediate between modern

man and unknown ancestors.

6. Eiseley (1959) interpreted response to fossil homi-

nids in terms of continuing influence of Great Chain

of Being.

a. Now an escalator.

b. Encouraged biolo-

gists to look for

stepwise, unidi-

rectional ape to

man transition.

c. Predilection con-

sistent with 19th

century views on

orthogenesis and

human races.

7. Sorting things out

would have to wait

for 20th

century dis-

coveries by Dart and

Broom.

Australopithecus africranus.

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G. We now know that

1. Primates descended from tree-living Cretaceous insec-tivores.

a. Grasping feet

with opposable big toe; nails.

b. Two groups.

c. Eyes directed forward; binocular vision.

2. Prosimians:

a. Tree shrews, lemurs, etc. b. Nocturnal c. Most in Madagascar.

3. Anthropoids:

a. Tarsiers b. New world monkeys (arboreal; many have prehensile

tails) c. Old world monkeys (arboreal and terrestrial) d. Apes (arms elongate; brachiators and knuckle walk-

ers) e. Hominids.

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4. Hominids. a. Diverged from Great

Apes about 6 m.y.a.

b. Early representatives include Australo-pithecines (Africa).

c. Bipedal

d. Robust and gracile.

5. Evolutionary trends:

a. Reduction in relative arm length;

b. Foreshortening of face and jaw; reduc-tion in tooth size, es-pecially the molars.

c. Increasing size, cra-nial capacity

d. Tools – H. habilis (2 m.y.a.).

e. Increasing consump-tion of meat / fish – may have facilitated evolution of large brains.

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III. Variation and Selection. A. Wallace’s position on the brain notwithstanding, he was

otherwise a strong selectionist – every variation had posi-tive or negative selective consequences – see Gould and Lewontin (Spandrels).

B. Wallace rejected sexual selection. C. Also set about quantifying variability in the field – recall

Darwin’s caustic rejoinder to Sir Wyville Thomson con-cerning lack of variability, which while amusing, doesn’t hold under weak selection:

“If Sir Wyville Thomson were to visit the yard of a breeder, and saw all his cattle or sheep almost absolutely true, that is closely similar, he would exclaim: ‘Sir, I see here no extreme variation; nor can I find any support to the belief that you have followed the principle of selection in the breeding your animals.’ From what I formerly saw of breeders, I had no doubt that the man thus re-buked would have smiled and said not a word. If he had after-wards told the story to other breeders, I greatly fear that they would have used emphatic but irreverent language about natural-ists.” [d80].

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IV. By the Century’s End.

A. Neo-Darwinism 1. Term coined by Romanes.

2. Emphasized primacy of NS

3. Rejected alternative mechanisms of change

a. Especially IAC b. Incompatible with sequestration of the germ.

4. Among its principal advocates: Wallace and August

Weismann. “Our final conclusion is that, whether we can discover their use or no, there is an overwhelming probability in favour of the statement that every truly specific character is or has been useful, or, if not itself useful, is strictly correlated with such a character.” [Wallace, 1896b]

B. The argument regarding individual vs. single differences

persisted with “mutationists” favoring saltation. 1. Of the latter, Hubrecht had argued that Darwin allowed

for single differences in his earlier writings only to abandon the notion in response to Jenkin.

2. To this, Wallace (1908) took sharp exception.

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C. Increasing interest variation led among other things to the emergence of the so-called “biometrical school”. 1. Focused on the statistics of parent-offspring correla-

tions.

2. Intimately connected to the rise of the eugenics move-ment.

The first regression line constructed by Francis Galton in 1877. The slope is less than one, illustrating reversion to the mean”. From this Galton concluded that evolution must take place by sal-tations. Recall Fleeming Jenkin’s sphere.

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D. Biometrical tradition persists as quantitative genetics. 1. Partitions phenotypic variance into genetic and envi-

ronmental components.

2. Heritability, h2, fraction of phenotypic variance ac-counted for by so-called “additive” effects.

3. Operationally, h2 estimated from parent-offspring re-

gression; i.e., h2 is the slope of the regression: offspring trait val-ue vs. parent trait value.

4. In particular,

�� � �� � ����� ���

where To is the expected off-spring trait value; Tp is the parental mean; and �� is the population mean. a. Assumes ��(��� � �� b. High heritability does not

necessitate a genetic basis.

5. Response to selection. a. Distribution is re-stablished. b. The mean is shifted in the direction of the selection.

Selection for a quanti-tative trait. In this ex-ample, only those indi-viduals for which the trait exceeds some threshold are permitted to reproduce.

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E. Galton’s “discovery” of reversion to the mean led him to be-lieve that selection acting on small variations could not in-duce indefinite deviation from an original type. 1. A large scale displacement would be required to move

the system to a new state of stability.

2. Illustrated his ideas with the response of an asymmetric polygon to external forces.

“Galton’s use of a polygon with slightly asymmetric surfaces to illustrate his hypothesis of organic stability. In the left diagram the polygon rests on face A–B. This is a stable, symmetric configuration from which it will not budge without a forceful nudge. Such a nudge causes the polygon to come to rest on face C–B. This is an asymmetric, unstable face, and a gentle shove will bring the polygon back to rest in its original stable position on face A–B. A harder shove in the opposite direction will, however, bring the polygon to rest on face D–C. This is a stable, symmetric position once again. In short, the polygon has now achieved what Galton refers to as a ‘new system of stability’”. Figure and caption from Gillham (2001).

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3. Today we would explain this in terms of multiple equi-libria with different degrees of stability.

4. The cause of saltation was subsequently taken up by William Bateson of whose work Huxley was approving.

“[I am] inclined to advocate the possibility of consider-able ‘saltus’ on the part of Dame Nature in her variations. I always took the same view, much to Mr. Darwin’s dis-gust, and we used to debate it.” [quoted by Gillham (2001)]

F. Especially among American paleontologists, Darwinism

took a back seat to Neo-Lamarckism. 1. Emphasized both an inherent tendency to progress and

IAC as a means of achieving it.

2. View congenial with apparent phylogenetic linearity.

3. Noteworthy was Edward Cope whose fossil collecting expeditions were supported by the USGS. a. Participated in the so-called “Bone Wars” with Oth-

niel Marsh – professor of geology at Yale and a committed Darwinian.

b. Formulated Cope’s rule – lineages tend to increase in body size over evolutionary time.

4. By the end of the 19th century, many biologists were

confidently predicting the demise of Darwinism.

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G. Peter Bowler (1992) lists six arguments against Darwinism: 1. Discontinuities in the fossil record.

2. Kelvin’s calculated age of the earth.

3. Existence of apparently non-adaptive structures

4. Artificial regularity of evolution.

a. Orthogenetic trends; convergent evolution, e.g., ver-tebrate / cephalopod eye.

b. Orthogenesis initially taken as evidence of a “guiding hand”;

5. NS not a creative force unless variation non-random.

6. Blending inheritance – Jenkin’s argument.

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H. Bowler emphasizes what he sees to be the central problem: “Why did the original Darwinian theory gain wide support despite a number of unresolved problems? Why did the level of opposition increase after the first couple of decades? … All the standard ob-jections to natural selection were formulated in its first decade, yet the theory went on to gain wide popularity.” [Emphasis added]

I. He gives several answers:

1. The fact that Darwin converted the scientific communi-

ty to evolution gave his theory a “leg up” on alterna-tives.

2. Emergence of disciplinary specialization promoted a proliferation of rival opinions each reinforced by its ability to account for discipline-specific concerns.

3. Emergence of biology as an experimental, i.e., laborato-

ry-based, science, particularly with regard to the study of heredity.

4. Theistic concerns; dissatisfaction with unalloyed mate-

rialism; resurgence of more conservative theories that admitted to an innate tendency to progress, if not to Di-vine Purpose, and were reflected by embryological de-velopment.

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V. Case of the Midwife Toad. A. Among the proponents of

IAC was Paul Kammerer.

B. Performed various experi-ments to support the theory – one series involved the “midwife toad” – actually a frog.2

1. Most frogs

a. Mate in water. b. Males have “nuptial

pads” on forelimbs and fingers to aid in mount-ing.

2. Midwife toads mate on

land – no pads.

3. Kammerer forced MTs to mate in water. After several generations, observed black bumps similar to nuptial pads of other frogs / toads. Presto! – Lamarck had been right all along.

2 MT males carry the eggs around with them on their backs – hence the name.

Kammerer’s experiment. From var-gas (2009).

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C. But the bumps on one specimen turned out to be India ink injected under the skin – revealed while Kammerer “tour-ing.”

D. Kammerer shot himself.

1. Final nail in the neo-Lamarckian coffin.

E. More recently,

1. Arthur Koestler (1971) suggested Kammerer had been set up by Nazi sympathizers. a. Nazi ideology eschewed Lamarckism. b. Kammerer was Jewish.

2. Vargas (2009) suggested that Kammerer had stumbled on transgenerational epigenetic inheritance.

F. By 1926 (year of Kammerer’s suicide), Mendelism already

widely accepted within the scientific community. 1. MT scandal confirmed in the popular mind what geneti-

cists had already concluded.

2. Only in Soviet Russia, did neo-Lamarckianism survive (as Lysenkoism).

G. Mendelism became the basis of the “modern synthesis.”

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VI. The Modern Synthesis.

A. Rediscovery of particulate in-heritance (PI).

B. Initially interpreted as evidence

for saltations. 1. Due to the work of Hugo de

Vries who studied color morphs in Oenothera (prim-roses). a. Self-fertilizing. b. Have a unique form of

meiosis, the result of which is the existence of multiple co-occurring races.

c. When de Vries grew the seeds of natural hybrids, he observed the occasional appearance of new, apparently stable forms that he called “mutations.”

2. Result was De Vries’ “mutation theory” – speciation by

abrupt jumps. a. Appealed to opponents of NS b. Rejected by Wallace and other neo-Darwinists.

3. De Vries’ “mutations” turned out to be chromosomal pol-

ymorphisms, the stability of which was maintained by balanced lethals.

Oenothera lamarckiana.

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Left. Lethal factors located in the gametes of Oenothera muricata. The rigens complex is inviable due to a lethal factor within the pollen. The curvans complex is incapable of producing functional ovules. Right. Zygotic lethal factors in Oenothera lamarckiana. The homozgotes velans x velansand gaudens x gaudens are not viable.

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Maintanence of hterozygosity by a pair of balanced lethals. Homozygosity at either locus (red) results in infertility or inviability.

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C. Rediscovery of Mendel’s paper shortly followed by the real-ization that there were exceptions to his Law of Independent Assortment.

D. Reality of genes established. 1. Located on chromosomes – confirmed suspicions of

19th century workers that latter the loci of heredity.

2. Chromosomes could be mapped by computing re-combination frequencies of alleles on the same chro-

mosome. a. Recombination resulted

from crossing over. b. Today we recognize chromosomal, genetic (linkage)

and physical maps.

Three types of chromosomal mapping.

Recombination results from crossing over during meiosis.

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E. Realization that particulate inheritance compatible with in-dividual variations the critical advance. 1. In 1917, Fisher published “The Correlation Between

Relatives on the Supposition of Mendelian Inheritance”

2. Argued that quantitative traits under what now called “polygenic inheritance.”

“For stature the coefficient of correlation between brothers is about .54, which we may interpret by saying that 54 per cent of their variance is accounted for by ancestry alone, and that about 46 per cent must have some other explanation. “It is not sufficient to ascribe this last residue to the effects of the environment. Numerous investigations by Galton and Pearson have shown that all measurable environment[al fac-tors] have much less effect on such measurements as stat-ure. Further, the facts collected by Galton respecting identical twins show that in this case … the variance is far less. The simplest hypothesis … is that such features as stature are de-termined by a large number of Mendelian factors, and that the large variance among children of the same parents is due to the segregation of those factors in respect to which the par-ents are heterozygous.”

3. There followed ~30 pages of impenetrable calculations.

4. Fisher’s paper opened the floodgates.

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F. Along with Sewall Wright and J. B. S. Haldane, Fisher de-veloped a genetical theory of natural selection. 1. Included Fundamental Theorem of Natural Selection:

�� �

� ��

2. Sewall Wright deduced an analogous result that became

the basis of the theory of adaptive topographies:

∆� ���1 ��2��

��

�

Evolution on a 2-D adaptive landscape. Dashed lines are contours of equal fit-ness. Plus signs indicate fitness maxima; minus signs, minima. Left. Increased mutation or reduced selection leads to increased variability. Center. Converse. Right. An environmental shift can place a population on a new peak, which se-lection then causes the population to climb.

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3. Fisher emphasized “mass selection” in large popula-tions; Wright, evolution in local populations subject to selection and drift - Shifting Balance Theory.

G. The resulting “modern synthesis” (term coined by Julian

Huxley in 1943) was a theory of gene frequencies as deter-mined by 1. Selection; 2. Mutation; 3. Migration; 4. Chance

F. Subsequently incorporated were 1. Systematics (Mayr) 2. Field Observations / selection in nature (Dobzhansky) 3. Paleontology (Simpson).

G. The Synthesis rejected

1. Orthogenesis. a. Inherent tendency of lineages to evolve in a linear

fashion – e,g,, horses, titanotheres. b. Even as Darwin rejected Lamarck’s Power of Life.

2. Racial senescence.

a. Mal-adaptation as lineages aged. b. Examples: Gryphaea and the”Irish Elk.”

3. The “Lamarckian heresy”.

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The Modern Synthesis.

The Modern Synthesis (top) compared with Lamarck (bottom left) and Darwin (bottom right). Note the simplification with time.

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VIII. After the War.

A. Neutral theory 1. Motivated by experimental

observation of ubiquitous het-erozygosity.

2. Conflicted with idea that het-erozygosity maintained by balancing selection.

3. Latter would => excessive

genetic load.3

4. Solves problem by assuming that most protein varia-tions selectively neutral.

B. Phenotypic plasticity /

reaction norms. 1. Genotype – phenotype

relation mediated by environment.

2. Not to be confused with IAC.

3 Consider the case of heterozygote advantage where the fitnesses of the three geno-types are (1-s), 1 and (1-t), where s and t ≤ 1. A quick calculation reveals that the mean fitness, �� � 1 � �� � ��⁄ . Historically, the concept was closely associated with argu-ments for eugenics.

Variation at the esterase-5 lo-cus in Drosophila pseudoob-scura as detected by electro-phoresis. From Lewontin and Hubby (1966).

Norms of reaction can induce phe-notypic reversal by environment.

44

C. Development. 1. Not all evolutionary im-

portant genes have small effect.

2. Genes homology ≠ trait homology.

3. Developmental constraints /

heterochrony.

4. Allometric transformations.

D’Arcy Thompson’s transformations. Here shown is a pattern of defor-mations that map the human skull into that of a chimp. Such transformations are producible by heterochrony.

The three axes of hetero-chrony.

45

D. Saltations 1. Pseudogene resurrection.

2. Developmental origins of novelty.

3. Punctuated equilibrium.

4. Mass extinction.

E. Renewed interest in evolu-

tionary progress. 1. Body Size ⇔ Complexi-

ty ⇒ secular increase in both.

2. Selective explanation of the Power of Life

3. Promoted by John Tyler Bonner (Why Size Mat-ters).

4. Alternative is a random walk bounded at one end (min-imum size / complexity consistent with life) and un-bounded at the other.

Alternative explanations for the evolution of increasing complexity over geologic time.