Zoology · Ch 6 — Evolution
Genetic Drift / Sewall Wright Effect
Genetic Drift / Sewall Wright Effect
Genetic drift, also called the Sewall Wright effect, is a mechanism of evolution in which a population's allele frequencies change from one generation to the next purely by chance — that is, by random sampling error in which individuals happen to survive and reproduce, rather than because of any survival or reproductive advantage. Drift occurs in populations of every size, but its effects are far stronger in small populations, where a chance event affecting even a few individuals can noticeably shift the allele frequencies of the whole group (Fig. 6.9); over time it can cause some alleles, including potentially beneficial ones, to be lost altogether while others become fixed. Genetic drift becomes especially significant under two specific circumstances: the bottleneck effect, where a natural disaster sharply reduces population size so that the survivors carry only a small, non-representative sample of the original population's genetic variation; and the **founder …
What this figure shows. The figure shows a small starting population of beetle icons in genotypes AA, Aa and aa, with a caption reading 'Due to chance events only these beetles give rise to offsprings', and an arrow pointing to a small subset of the icons that are circled or otherwise marked out as the ones that happen to reproduce. A 'Next generation' panel to the right shows the resulting offspring population, with bar-style indicators for 'Frequency of A' and 'Frequency of a' before and after this chance event, showing the allele frequencies shifting even though no selective advantage was involved — purely because only a s …