Evolutionary Genetics
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© 1997
David H.A. Fitch
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Lecture notes

Evolutionary genetics:  A brief perspective

I.  The incorporation of genetic theory into evolutionary theory

Although Darwin was very successful at convincing his contemporaries about the fact that evolution had occurred, he was much less successful at convincing his colleagues that his mechanism of Natural Selection was the major mechanism of evolutionary change.  This was mainly because there was no satisfactory explanation for inheritance, or for how variation originated.  Darwin himself was plagued by his inability to understand inheritance, and was dissatisfied by his own theory of inheritance, pangenesis and blending inheritance, because blending suggested that variation should be halved each generation and would rapidly be lost.  (Darwin called it his "well abused theory of Pangenesis", and postulated rapid origins of new variation.  Maybe if Darwin had improved his math skills...?)

In 1883, A. Weismann (1834-1914) proposed that the germ plasm was separate from (and thus immune to influences from) the soma.  Indeed, this principle is reflected in the "Central Dogma" of molecular biology.  The "central dogma" of the NeoDarwinian view is that evolution can only occur by genetic change.

There are some notable exceptions, such as "gene amplification", cortical inheritance of disturbed patterns of cilia inherited in ciliated protozoa, and inherited "states of cell activation" in Daphnia.

In 1865 (only 6 years after Darwin's Origin), in an Augustinian monastery in Brno, a Czech town, Gregor Mendel discovered some of the "rules" of diploid inheritance.  Mendel's work was not widely known until it was rediscovered in the 1900s.

Mendel demonstrated particulate inheritance, dispensing with the problems of blending inheritance.  This mode of inheritance was initially used to argue against natural selection being a strong force:  Since variants or mutants observed by early geneticists had discrete effects, and species differed discretely, species could have arisen by discrete, perhaps systemic changes (perhaps with a direction determined by orthogenesis or other mechanisms).

With the "Evolutionary (or Modern) Synthesis", Darwin's theory became reconciled with the facts of genetics,  especially with the facts that (1) acquired characters are not inherited and (2) continuous variation has the same Mendelian basis as discrete variation.  The theoretical foundations for evolutionary genetics were laid down in 1908 independently be Hardy, Weinberg and Tschetverikov, and subsequently developed by R. A. Fisher (1890-1962) and J. B. S. Haldane (1892-1964) in England and Sewall Wright (1889-1988) in the US.  Additional work (e.g., by J. Huxley, T. Dobzhansky) brought these and other fields (like paleontology) together.

II.  Major tenets of the Evolutionary Synthesis

A.  Populations have genetic variation that continuously arises by undirected processes (mutation and recombination)

B.  Populations evolve by changes in gene frequencies through:
1.  Genetic drift
2.  Gene flow
3.  Natural selection

C.  Most adaptive variants have slight phenotypic effects, so that phenotypic changes are gradual

D.  Diversification arises by speciation (cladogenesis), usually occurring via gradual evolution of reproductive isolation

E.  These processes, continued for a sufficiently long period of time, produce changes sufficient to delineate higher taxonomic levels

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