Selection in Natural Populations

By Jeffry B. Mitton | Go to book overview

10
The Sisyphean Cycle

Hence it is the most flourishing, or, as they may be
called, the dominant species -- those which range
widely over the world, are the most diffused in their
own country, and are the most numerous in indi-
viduals -- which often produce well marked vari-
eties, or, as I consider them, incipient species.

C. Darwin, On the Origin of Species by
Means of Natural Selection
( 1859)

Sisyphus repeatedly pushes a boulder up a steep
slope until, on the verge of reaching the peak, it
goes out of control and rolls to the bottom again.
Analogously, an individual in the top end of the fit-
ness distribution has achieved its near maximum of
fitness by an only momentarily effective combina-
tion of genetics and individual history.

G. Williams, Sex and Evolution ( 1975)

At the species level, geographic range is the best
predictor of levels of allozyme variation. Endemic
species have the lowest genetic diversity whereas
regionally distributed and widespread species main-
tain the most diversity.

J. Hamrick and M. J. Godt,
"Allozyme Diversity in Plant Species" ( 1990)

In his book on the evolution of sexual reproduction ( 1975), George Williams coined the term Sisyphean genotype to refer to an optimal genotype selected by biotic and abiotic components of the environment. Williams proposed low heritability of fitness from generation to generation, for the fitness of a genotype is highly dependent on the environment in which it was tested, and environments are rarely constant. Because the optimal genotype varies with environmental conditions, natural selection, like Sisyphus, is never done.

In a sexually reproducing population, a highly heterozygous genotype is a Sisyphean

-157-

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