12-11-06 A look forward. Laying the groundwork I: The birth of - - PDF document

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A look forward….

Laying the groundwork I: The birth of evolutionary theory Darwin was not the first

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1. The earth is young

• James Uscher: 4004 BCE
• 9:00AM Sunday 23 October to be exact

2. Species were immutable

• Species were created by God in exact forms

3. The value of science is to explain God’s creation

• Conflict with the church could be very bad for one’s career
• The rationalists began to set science in direct conflict with the church

4. Imprecise knowledge of species

• Naming and organizing natural diversity was chaotic

Historical constraints to thoughts about nature of life on earth Carolus Linnaeus: I will name them all

• Carl van Linné (1707-1778), Swedish
• proponent of “Natural theology”
• set out to stop the chaos
• binomial nomenclature
• System Natura 1735 (142pp) Later editions

(>2300pp)

• rock star of field biologists
• believed species were immutable

12-11-06 3 Hutton: the world is older than you think

• James Hutton (1726-1797), Scottish
• did not like ad hoc explanations
• did not like “Catastrophism”
• champion of “Uniformitarianism”: the

present is the key to the past

• the Earth must be very old
• Hutton was a huge influence on Charles

Lyell (1797-1875) who wrote highly influential books on geology

Lamarck: Species are not immutable

• Jean Baptist Lamarck (1744-1829), French
• first true museum systematist
• important work on patterns in fossil record
• first to put together two points:
• the world was very old
• species changed over this long time period
• Lamarck most famous for getting the

mechanism of evolution wrong (very sad)

• Lamarck and Erasmus Darwin

corresponded about evolution by acquired characteristics (Hmmm…)

12-11-06 4 Cuvier: Fossils are real

• Georges Cuvier (1769-1832), French
• father of comparative anatomy
• believed species were fixed.
• very influential and unkind to Lamarck
• convinced world that fossils were real
• must have seen same patterns in fossil

record as Lamark, seems he choose to ignore the implications

Malthus: Life is a struggle

• Thomas Malthus (1766-1834), English
• political economist
• published “Essays on the principle of

population” (1790)

• Malthusian principle
• presented many examples of life as a struggle
• “profit of doom”
• Doctrine of Eugenics echoes back to Malthus

12-11-06 5 Young Darwin: I will voyage on the beagle

• Charles Robert Darwin (1809-1882), English
• started life as orthodox member of church
• naturalist on the Beagle
• observed many differences between species among

islands and between islands and mainland

• read Lyell while on voyage
• became a devout Uniformitarian; saw much evidence
• read Malthus two years after return to England; He

sees Malthus differently from all who came before

• drew info from very different sources; saw beyond the
• rthodox

Older Darwin: I say species evolve by natural selection

1. Species produce more offspring than survive to age of reproduction. This leads to a “struggle”. 2. Food and other such resources are limited; more evidence for “struggle”. 3. Population sizes surprisingly stable given the intensity of struggle among

• species. Must be a mechanism for stability.

4. Variation in the form of differences among individuals exists in every species. 5. Variation is heritable.

Ernst Mayr, in his book entitled “The Growth of Biological Thought” distils Darwin’s work to five key observations:

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Older Darwin: I say species evolve by natural selection

1. Competition for finite resources ensures that many individuals within a species are eliminated because of inferior ability to survive and reproduce (low fitness). 2. Natural selection is the result of competition, where the more fit individuals

• utnumber the less fit individuals.

3. The characteristics favored by natural selection are passed on to succeeding generations because such characteristics are heritable.

From these observations, Darwin made three inferences: The consequences: all organisms must have descended, with modification, from common ancestors

Wallace: Hey, I say so too!

• Alfred Russel Wallace (1823-1913), English
• full credit for “co-discovery” of evolution by

natural selection

• had epiphany while delirious from an attack of

malaria (1858)

• had read Malthus as well, and came to same

conclusion as Darwin

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Darwinian evolution fell into disrepute an languished until 1920’s Two problems that Darwin (and Wallace) could not solve: 1. Source of variation 2. Mechanism of inheritance

• Fleming Jenkin demolished Darwin in an article in the North British Review

(1876)

• Jenkins introduced the problem of “blending”

Darwin’s five theories [not in the notes]

Ernst Mayr (1982)

• 1. Evolution of species and over long periods of time
• 2. Common Descent
• 3. Population speciation
• 4. Natural selection
• 5. Gradualism

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Laying the groundwork I: The birth of evolutionary theory Laying the groundwork II: A concise history of the gene Laying the groundwork III: Neo-Darwinism and the evolutionary synthesis

History

Gregor Mendel: Traits endure, they do not blend

• Augsutinian monk interested in plant breeding
• many breeders, none examined problem of heredity via

mathematics

• published in 1865 (six years after publication of The Origin)
• traits do NOT blend (largely ignored)

Mendel’s postulates:

• 1. Inheritance is by factors or “particles”
• 2. Particles are present in “pairs” in the breeding adults
• 3. Paired particles segregate independently during the

formation of gametes; each contains only one particle

• 4. Particles can have alternate (dominant/recessive) forms
• first to distinguish phenotype and genotype
• Mendelian genetics re-discovered 30 years later (enhanced

the decline of Darwinian theory)

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Hugo de Vries: Mutation, Mutation, Mutation

• one of three who “re-discovered” Mendelian inheritance
• bred evening primrose for over 20 years
• observed a trait that “jumped” and then bred true: “saltation”
• thought that saltation was inconsistent with gradual Darwinian

evolution

• coined the term “mutation”
• suggested that mutations acted on tiny particles within a cell
• Europe was awash in Lamarkism at the time (1900’s), and de

Vries was unhappy with it

Mendelians:

• 1. Gradualism could act within a population
• 2. Saltation was only mechanism for new species

August Weismann: Your chromosomes determine who you are Theodor Boveri: I think chromosomes carry smaller “things” that determine heredity; and you need a full set of them!

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• T. H. Morgan: No genes, ⎯wait, yes there really are genes
• extremely skeptical (no such thing as genes)
• did not “believe” in genes (or Lamarkism, or Darwinism)
• founded “Fly Room” at Columbia University
• 1910, discovered the white-eyed mutant
• OK, maybe there really are genes
• Evolution by mutation (saltation)

F1 = 100% red-eyed; F2 = 3:1 red:white All males = white eyes All males = y chromosome Linkage groups matched chromosomes

“father of modern genetics” Sturtevant and Morgan: genes are ordered in a linear array on chromosomes

• linkage groups are not perfect
• “breakages” in linkage were not random
• freq of breakage fit a linear model of genes
• reasoned that homologous chromosomes would on

rare occasions exchange bits: recombination

• the observed freq of “broken-linkage” was used to

put a relative distance between genes; linage mapping was born! As always, Morgan was skeptical; until Sturtevant began to predict the frequency of broken-linkage in genes that he had mapped but not directly examined in crosses!

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Conflict between perspectives: mutation verses variation 30 year conflict Mendelians

• Laboratory research
• Experimental evidence for large

effect of mutation

• natural selection only relevant to

removing deleterious mutations

• Speciation by mutation; i.e.,

saltation Gradualists

• taxonomists; field naturalists;

biometricians

• worked with variation observed in

natural populations (as compared with crosses)

• Observed much small scale

variation

• variation was correlated with

geography

• saltation did not fit their data

Laying the groundwork III: Neo-Darwinism and the evolutionary synthesis 1936-1942

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Godfrey Hardy Wilhelm Weinberg

Independent proof (1908) of equilibrium in allele frequencies in a an ideal population

• serve as kernel of population genetics (but wait until 1930’s)

Three “wise men” (theoreticians)

Sewall Wright John Haldane Ronald Fisher

• Developed the mathematical theory that reconciled mutationalism and

gradualism; the “modern synthesis”

• Demonstrated that laws of Mendelian inheritance were consistent with variation

in natural populations

• Showed that such variation could be subject to “positive” Darwinain selection.

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Fisher:

• small & continuous differences were compatible with Mendelian principles
• validated biometricians models that viewed evolution as a shift in the distribution
• f the whole population

Fisher and Haldane:

• developed theory of change in allele frequencies in populations in response to

natural selection Wright:

• developed a comprehensive theory that included the effects of natural selection,

migration, inbreeding, and chance (genetic drift). Other architects of the modern synthesis:

• Theodosius Dobzhansky (Biologist)
• Julian Huxley (Biologist and proponent of Eugenics; brother of Aldous Huxley)
• Ernst Mayr (Biologist)
• G Ledyard Stebbins (Botanist)
• George Gaylord Simpson (Paleontologist)

Theoretical work was integrated into studies of natural populations, leading to a series of books on the subject of evolution.

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Tenets of Neo-Darwinism / evolutionary synthesis

• 1. populations contain genetic variation that arises at random via mutation and

recombination

• 2. populations evolve by changes in allele frequencies
• 3. allele frequencies can change by mutation, migration, drift and natural selection
• 4. most mutations are deleterious [note: nothing here about neutrality]
• 5. most adaptive phenotypic effects are small so changes in phenotype are slow and

gradual

• some such changes (like certain color polymorphisms) can have large

discrete effects

• 6. diversification occurs by speciation
• usually a gradual process *
• usually by geographic isolation ***
• 7. population processes, continued for sufficiently long periods of time, give rise to

changes of greater magnitude such as the divergence of genera, families, etc.

Evolutionary theory has grown tremendously since the synthesis MACROEVOLUTION: the sum of those processes that explain the character-state changes that are characteristic of divergences of species and higher taxonomic ranks (modified from Jeffrey S. Levinton)

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Morgan visits England in 1932: “This is extraordinary; I just didn’t know things like this existed”

Bitter tasting Tasty mimics (Papilio)