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 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 1
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) 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 unhappy with it Mendelians: 1. Gradualism could act within a population 2. Saltation was only mechanism for new species 2
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! 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” 3
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! Conflict between perspectives: mutation verses variation 30 year conflict Gradualists Mendelians • taxonomists; filed naturalists; • Laboratory research biometricians • Experimental evidence for large • worked with variation observed in effect of mutation natural populations (as compared • natural selection only relevant to with crosses) removing deleterious mutations • Observed much small scale • Speciation by mutation; i.e., variation saltation • variation was correlated with geography • saltation did not fit their data 4
Laying the groundwork III: Neo-Darwinism and the evolutionary synthesis 1936-1942 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) 5
Three “wise men” 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. 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). 6
Other architects of the modern synthesis: • Theodosius Dobzhansky (Biologist) • Julian Huxley (Biologist) • 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. 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 7
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) Morgan visits England in 1932: Bitter tasting Tasty mimics ( Papilio ) “ This is extraordinary; I just didn’t know things like this existed ” 8
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