What causes genetic drift?

Genetic drift is a phenomenon that refers to the change in the frequency of alleles in a population's gene pool due to random sampling effects, particularly in small populations. When a population is small, chance events can have a more pronounced effect on the genetic composition of that population. This might occur, for example, when a few individuals randomly survive a disaster or when certain individuals happen to reproduce while others do not, purely by chance. As a result, some alleles might become more common while others diminish or disappear entirely, leading to a shift in the genetic makeup of that population.

This concept is critical because it illustrates how variation in allele frequencies can occur independently of natural selection. It highlights the role of chance in the evolutionary process, which can lead to significant changes over generations, especially when populations are reduced in size.

In contrast, large-scale migration of a population generally involves gene flow, which can introduce or remove alleles from the gene pool but does not lead to genetic drift. Similarly, the introduction of new mutations contributes to variation within a population but is not the same as genetic drift. Lastly, random mating within a large population does not significantly affect allele frequencies since the population is large enough that chance events do not drastically alter the genetic structure.