Evolution__3rd_Edition

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6 Random
Events in
Population Genetics
T he genotypes at a locus may all have the same fitness.
Then the gene frequencies evolve by random genetic
drift. This chapter starts by explaining why drift happens
and what it means, and looks at examples of random
sampling effects. We see how drift is more powerful in small
than large popuations, and how in small populations it can
counteract the effects of natural selection. We then see how
drift can ultimately fix one allele. The Hardy–Weinberg
ratios are not at an equilibrium once we allow for the
effects of drift. We then add the effects of mutation, which
introduces new variation: the variation observed in a
population will be a balance between the drift to
homozygosity and mutation that creates heterozygosity.