Chapter 17 & 18

Evolution’s Historic Perspective

http://www.pbs.org/wgbh/evolution/religion/revolution/index.html

Popular Beliefs up through the 1800s

Commonly held beliefs were not based on looking at the world, or examining nature.

They were based on stories handed down from generation to generation, in both oral and written forms. The Bible and Catholic dogma was the most influential force on thought.

Therefore, when the world and nature was examined, anything that was observed was interpreted according to what was already believed.

Beliefs based on these forces:The earth was young.Species were made and did not change.

When observations were made that did not fit what was already believed, new theories were contrived to fit the data into the context of what was already believed.

Jean-Baptiste Lamarck (1744-1829)A major contributor to Cell TheoryOne of the first to use “biology” in its modern senseStarted out as an “essentialist” who believed species did not change. But his own work on mollusks convinced him that change had to have occurred.Was the first to propose a cohesive theory of evolution (inheritance of acquired characteristics)

Popular Beliefs up through the 1800s

George Cuvier (1769-1832): paleontologist, father of comparative anatomyHe worked with fossils (he found and named pterodactyl in Bavaria).He thought Lamark’s ideas were ridiculous, and firmly believed in the fixity of a species.He came up with the principle of “correlation of parts” – The notion that comparative anatomy was so accurate that after inspecting a single bone, the class and sometimes genus of an animal could be inferred.This is because the number, direction, and shape of the bones composing each part of a body are always in a necessary relation to all parts such that a great deal can be inferred with only a few bones.That was in 1798.

He believed (because he was really left with no other choice) that the fossil remains he worked with must be that of extinct species.

Because he lived in a society dominated by religious thought – and he was a close personal friend of a cardinal, apparently – he had to fit his observations into the context of the creationist account that the earth was young.

He therefore proposed that “All of these facts, consistent among themselves, and not opposed by any report, seem to me to prove the existence of a world previous to ours, destroyed by some kind of catastrophe”

To reconcile belief with his observations he developed the theory of catastrophism, which is sometimes called “Cuvier’s Compromise”.

Catastrophism was the idea that a young earth’s history was defined by an accumulation of brief yet violent natural catastrophes. Localized natural disasters would whip out all life in one region, and new species from other regions would repopulate the barren area.

From a biography found here: All extinct forms of life found in the fossil record were formed in a dark mysterious antediluvian period (before the flood).

Due to his protestant beliefs, Cuvier carefully described his antediluvian period having multiple floods of the last being the biblical flood.

When defining this period he gave it an unknown time length which corresponded with the concept that earth was in fact very old…

It was assumed that rocks and sand precipitated out of an enormous flood. This theory was called Neptunism.

James Hutton (1726 – 1797) founder of modern geology

His geological observations suggested that the earth was very, very old. Neptunism – and therefore, catastrophism – could not be correct.

They called his theory Plutonism. The earth was hot inside, which was the only way to explain the observed geology.

The notion of “deep time” arose from his work.So did Uniformitarianism: The assumption that the

natural processes operating in the past are the same as those that can be observed operating in the present – “the present is key to the past”

He wasn’t a very good writer, and his ideas didn’t catch on until others popularized his work (mostly, Lyell, see below)

He believed that species could adapt, but the notion of selection giving rise to new species was a “romantic fantasy”

Sir Charles Lyell (1797 – 1875)

His father was a lawyer and botanist. So he followed his father into law. Until his eyes started to get bad; then he took up geology (he spent a lot of his childhood at a famous hunting lodge)

He worked under William Buckland, at Exeter College in Oxford, who did NOT like Hutton’s work on Uniformitarianism, as it countered the existence of the flood. Buckland developed a theory to reconcile the biblical account – Flood Geology -- in 1820. This was found in “the Connexion of Geology with Religion”. In this he reconciled Hutton’s theories with the notion that the word “beginning” in Genesis meant an undefined period of time. Old World Creationism is born.He also wrote the first full account of a fossil dinosaur.

Lyell was rare in that he managed to support himself by writing books (how this helped his eyesight, I have no idea). He wrote Principles of Geology , in which he popularized the theory from 30 years ago (Hutton’s) that the earth was very old, based on geographical data.

Aside

• Freethought is a philosophical viewpoint that holds that beliefs should be formed on the basis of science and logic and not be compromised by emotion, authority, tradition, or any dogma. The cognitive application of freethought is known as freethinking, and practitioners of freethought are known as freethinkers

Charles Darwin (1809 – 1882)Dad was a doctor. Darwin was an apprentice, then went to college for medicine – but didn’t

like how brutal surgery was.He joined a society of naturalists, the Plinian Society, while in college, and studied under a

biologist who advocated Lamarck’s ideas.(Darwin’s grandfather Erasmus, wrote a book that advocated a common descent as well)Darwin’s early work examined marine animals, and his work led to evidence supporting

homology, the ‘radical’ theory that all animals have the same organs, but differ in complexity, thus showing common descent.

Dad didn’t like any of this; he enrolled Charles at Christ’s College, in the hopes his son would make good money as an Anglican parson. But Darwin liked to ride and shoot, more than study.

He studied botany, collected beetles, and studied (and enjoyed) the writings of William Paley (who argued for the divine design in nature- “the watchmaker” idea).

He graduated 10th in his class of 178, and did not rush to take his orders to be ordained as a parson (similar to a priest). Instead, he wanted to go to the Madeira islands with some classmates, to study its natural history (Alexander von Humboldt wrote a book talking about biogeography that excited young Charles)

To prepare, he took a course in geology (by one of the founders of modern geology who initially proposed the Cambrian and Devonian periods), and later helped assist mapping the geological strata in Whales.

Charles hung out with some friends for a “fortnight”, and upon returning home, found a letter waiting for him.

Charles Darwin (1809 – 1882)It was from Henslow, his university professor. It basically said that

Darwin was “suitable (if unfinished)” for the position of naturalist for the unpaid position of “gentleman’s companion” to Robert Fitzroy, captain of the HMS Beagle.

It was leaving in 4 weeks.

His dad originally objected to the idea, seeing it as a waste of time, but his wife’s brother convinced him to let his son go.

It’s five year mission was to chart the coastline of South America. (originally it was supposed to take 2. whoops.)

http://darwin.thefreelibrary.com/The-Voyage-of-the-Beagle/0-1

The second voyage of the Beagle

Darwin believed in a Creator during his initial voyage.

While in Australia…In the dusk of the evening I took a stroll along a chain of ponds, which in this dry country represented the

course of a river, and had the good fortune to see several of the famous Platypus, or Ornithorhyncus paradoxus... A little time before this I had been lying on a sunny bank, and was reflecting on the strange character of the animals of this country as compared with the rest of the world.

An unbeliever in every thing beyond his own reason might exclaim, "Two distinct Creators must have been at work; their object, however, has been the same, and certainly the end in each case is complete." While thus thinking, I observed the hollow conical pitfall of the lion-ant: first a fly fell down the treacherous slope and immediately disappeared; then came a large but unwary ant; its struggles to escape being very violent, those curious little jets of sand, described by Kirby * as being flirted by the insects tail, were promptly directed against the expected victim.

But the ant enjoyed a better fate than the fly, and escaped the fatal jaws which lay concealed at the base of the conical hollow.

There can be no doubt but that this predacious larva belongs to the same genus with the European kind, though to a different species.

Now what would the sceptic say to this? Would any two workmen ever have hit upon so beautiful, so simple, and yet so artificial a contrivance? It cannot be thought so: one Hand has surely worked throughout the universe.

It took many years before Darwin published his radical theory.

He stated it simply at the beginning of his book:

As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form

Principles of Evolution: Population Genetics

Members of a population vary.Microevolution deals with evolutionary changes within a population (recall

the definition!)Population Genetics concerns itself with allele frequencies within a

population.The gene pool is the total number and amount of alleles for every single gene

of a particular population.Recall simple mendelian geneticsLet the trait “wing size” be represented by “L” and “l”. (Long wings are

dominant)So, individuals can be

LL (homozygous dominant)Ll (heterozygous)ll (homozgous recessive)4

LL Ll ll

If we have 100 flies in a population, we have 200 total alleles (everyone has 2 copies of each factor)

36% of the flies are LL, 48% are Ll, and 16% are ll.

How many L total? 120 out of 200, or 60%How many l? 80 out of 200, or 40%Let’s do a population genetic’s Punnett

square 60% L 40% l

60% L 36% LL 24% Ll

40% l 24% Ll 16% ll

36% 48% 16%

The allele frequency does not change in the next generation. Sexual reproduction alone cannot bring about a change.

Principles of Evolution: Population Genetics

LL Ll ll

The English mathematician Godfrey Harold Hardy and and German physican Wilhelm Heinberg independently came to the same mathematical truth about population genetics.

60% L 40% l

60% L 36% LL 24% Ll

40% l 24% Ll 16% ll

36% 48% 16%

Principles of Evolution: Population Genetics

LL Ll ll

Using simple algebra…

Let p represent “L”Let q represent “l”Since there are only 2 alleles, p+q = 1 (or 100%)

So LL = pp, Ll = pq, and ll = qq.

… p*p = p2 and … q*q = q2

Since the total number of possibilities is simply the sum of all the genotyes, we have:

p2+q2+pq+pq, or to put it in its organized state:

p2+2pq+q2 (think of FOIL from math, (p+q)(p+q))

Since the total sum is 100% (or just 1 in probability math):

p2+2pq+q2 = 1

60% p 40% q

60% p 36% pp 24% pq

40% q 24% pq 16% qq

36% 48% 16%

Principles of Evolution: Population Genetics

Principles of Evolution: Genetics

LL Ll ll

Summary: we know thatp+q = 1p2+2pq+q2= 1 (a binomial

expression)

Therefore, we can derive allele frequencies based on the population statistics! 60% p 40% q

60% p 36% pp 24% pq

40% q 24% pq 16% qq

36% 48% 16%

The Hardy-Weinberg Principle The Hardy-Weinberg Principle states that an equilibrium of allele frequencies in

a gene pool will remain in effect in each succeeding generation of sexually reproducing population as long as five conditions are met.

1: No Mutations2: No Gene Flow3: Random Mating4: No Genetic Drift5: No Selection

Any violation of these conditions will result in a shifting of allele frequencies – and a change in the population over time – evolution.

Example: Industrial melanism

1: No Mutations

We now know that mutations will happen, and be random. Mutations that affect an allele’s expressed phenotype will result in a change in the population.

Most traits have several phenotypes – they are polymorphic.A polymorphism is simple a difference within a population’s

genotype.About 30% of fruit fly genes are polymorphic (Lemontin &

Hubby)Combinations of alleles (not just 1 difference) may confer

selective advantage. This will arise during recombination.Daphnia, pg. 304.

2: No Gene Flow

Definition: Movement of alleles between populations. Result of the migration of breeding individuals.

Increases genetic variation of a population by introducing new alleles produced by mutation in another population.

Makes gene pools similarReduces allele frequency differences between 2

populationsCan prevent speciationTends to decrease genetic diversity amongst different

populations of the same species.

3: Nonrandom matingInbreeding: Breeding between relatives to an extent higher than random

chanceDecreases heterozygote frequency, increases homozygote frequency.Danger of increasing recessive abnormalities

Assortive Mating: Mating between the same phenotypeCauses subdivisions within a populationHomozygotes increase, heterozygotes decrease.Can be seen in the human species!

Sexual Selection: Males compete for the right to reproduce; females choose males with particular phenotypes.(These phenotypes generally relate somehow – directly or indirectly – to the male’s ability to provide resources for the female and her young)

4: Genetic Drift

Change in allele frequency due to chance.Large populations are less affected than small.Loss of all breeding individuals with a particular allele by chance will shift

the allele frequency. Usually happens only to small populations.

Experimental Evidence: Drosophila107 bottles, each with 8 Ll. After the bottles are populated, 8 random males + 8 random females are

removed and put into a new bottle.After 19 generations,

25% of the bottles were 100% aa25% were 100% AA50% of the bottles were Aa

Therefore, give the starting conditions (a “founder” population of 8 random flies), there is a 50% chance that the new population will completely lose this polymorphism and become uniform in this trait.

More Evidence: Cypress trees in CA

4: Genetic DriftBottleneck Effect: near extinction from natural

disaster/overhunting/deforestation/habitat lossExample: Cheetah

Founder’s Effect: a small population breaks off from a larger. Rare alleles – or rare combinations of alleles – occur at a higher frequency

in a population isolated

(key difference: the remaining populations are still alive)

5: Selection• When there is an unequal chance that offspring will survive and

reproduce.• Conditions:

– Inheritable variation within a population (phenotypic variations are genetic)

– Differential adaptiveness – some of these variations affect the ability of an organism to adapt to its environment (to be “comfortable”)

– Differential reproduction – those more adapted will have more offspring. “Their fertile offspring will make up more of the next generation’s gene pool”.• This ultimately determines “fittness”.

• Relative Fitness: Comparing the ability of one phenotype to reproduce better or worse than another.

http://www.biologyinmotion.com/evol/

http://www.truthtree.com/evolve.shtml

Types of Selection

• Most traits acted upon by natural selection are polygenic – traits governed by more than one pair of alleles, located at different genetic loci.

• In general, most traits (phenotypes) fit within a RANGE, or distribution, that usually falls onto a bell curve.

Types of Selection

Directional Selection occurs when an extreme phenotype is more fit.

Examples: HorsesInsects (DDT)Malaria (chloroquinone)

Types of Selection

Stabilizing Selection occurs when an intermediate phenotype is favored.

Examples: Swiss Starling Clutch Size

determined byyolk productionlength of mating

Human Birth Weight

Types of Selection

Disruptive Selection occurs when either extreme phenotype is favored.

Examples: British Land Snails

Maintenance of Variation

• Selection works to decrease variation• Mutation increases variation• Recombination increases variation• Gene flow increases variation• Naturatl selection REDUCES the range of

phenotypes – does not eliminate them• Disuprtive selection PROMOTES

polymorphism

Maintenance of Variation

• Diploidy and the heterozygote:– Only alleles that are “exposed” are subject to natural

selection– Therefore, the heterozygote can be seen as a

“protector” of recessive alleles that would otherwise be selected out of a population.

– Example: Sickle Cell Allele• Balanced Polymorphism: The ratio of

phenotypes is equivalent from one generation to the next.

Lines of Evidence

Besides direct experimentation:Comparative anatomy (homologous vs. analogous

characters),Biogeography: The study of the distribution of different

species of organisms around the planet and the factors that influenced that distribution

fossil record, molecular biology (e.g., amino acid and nucleotide

sequencing, ubiquity of genetic code and fundamental biochemical pathways)

Embryology and development.