Download 12-11-06 1 Laying the groundwork I: The birth of evolutionary theory

12-11-06
A look forward….
Laying the groundwork I: The birth of evolutionary theory
Darwin was not the first
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Historical constraints to thoughts about nature of life on earth
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
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
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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…)
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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
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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
orthodox
Older Darwin: I say species evolve by natural selection
Ernst Mayr, in his book entitled The Growth of Biological Thought
Darwin s work to five key observations:
distils
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.
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Older Darwin: I say species evolve by natural selection
From these observations, Darwin made three inferences:
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
outnumber the less fit individuals.
3. 
The characteristics favored by natural selection are passed on to succeeding
generations because such characteristics are heritable.
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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History
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
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
Gradualists
•  Laboratory research
•  taxonomists; field naturalists;
biometricians
•  Experimental evidence for large
effect of mutation
•  natural selection only relevant to
removing deleterious mutations
•  Speciation by mutation; i.e.,
saltation
•  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
of 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
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usually a gradual process *
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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:
Bitter tasting
Tasty mimics
(Papilio)
“This is extraordinary; I just
didn’t know things like this
existed”
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