Also known as the Sewell Wright effect, it involves chance fluctuations of gene frequencies in small isolated breeding populations, possibly leading to rapid evolutionary change (i.e., speciation). What is being claimed is that neutral alleles underlie most of the genetic diversity in natural populations and that they are fixated or eliminated by genetic drift. Another way of looking at this effect is in terms of sampling errors. Errors due to sampling are inversely related to the size of the sample. Thus, the smaller the sample, the larger will be the sampling errors. Translated to living organisms, this means that the smaller the number of individuals in a population, the larger will be changes in allele frequency due to genetic drift. The effects of genetic drift may lead to a considerable alteration in allele frequencies during population bottlenecks (i.e., when unfavourable climatic or other conditions result in a drastic reduction in population size and thereby the risk of extinction). It is important to bear in mind that genetic drift and natural selection are not alternatives for any observed genetic change: drift is a mathematically defined certainty and selection a biologically defined certainty. They play complementary roles in biological evolution and thus they are difficult to separate in practice.
See Allele, Biological evolution, Cultural selection, Founder effect, Genetic (or DNA) recombination, Phyletic gradualism and punctuated equilibrium, Speciation, Theory of natural selection, Ultimate mechanisms (or causes)