Genetic drift is a random change in how common different versions of a gene are in a population from one generation to the next. Specific gene versions, also called alleles, can become more common, become less common or disappear entirely. Crucially, this happens entirely by chance, regardless of whether or not a gene version helps the organisms survive and reproduce.
For species survival, genetic drift can pose a major threat by steadily reducing gene diversity, especially in small populations. Losing this genetic diversity leaves a population with fewer inherited tools to help individuals survive new diseases or changing environmental conditions. Protecting a vulnerable species therefore involves more than just maintaining its numbers; it also involves preserving its genetic diversity.
Imagine a small population of wild plants with two versions of a gene (i.e., two alleles) that have no effect on survival or reproduction. By chance, plants carrying one version might produce more of the seeds that successfully grow that year. As a result, that version becomes more common in the next generation, even though it provided the plants with no survival advantage.
This is distinct from natural selection, which occurs when inherited differences affect survival and reproductive success.
Genetic drift occurs in populations of all sizes, but usually has much stronger effects in small ones. As an analogy, a small handful of colored beads taken from a large bag of beads can easily have very different proportions of colors from the beads in the bag as a whole. A much larger handful will usually represent those proportions more closely. Reproduction involves a similar sampling effect when gene copies pass between generations.
Over successive generations, by chance some versions of genes may disappear from a population. Others may become "fixed," meaning that they are the only versions remaining. Separate populations can therefore become genetically different even in identical environments.
Population Bottleneck
A population bottleneck occurs when there is a dramatic drop in a population's size. The survivors may carry only part of its former genetic diversity, so that even if their descendants later become numerous, the lost gene versions are not automatically restored.
For example, intensive hunting reduced the population of Northern elephant seals to an estimated 20 in the late nineteenth century. Although they eventually rebounded to more than 220,000 individuals, the species experienced a substantial genetic loss and will continue to carry the genetic scars of its former bottleneck long into the future.
Likewise, prairie chickens, a ground-nesting bird, which once numbered in the millions across North American grasslands, were reduced in Illinois to fewer than 50 birds by the 1990s as a result of massive habitat loss for agricultural use. The consequent genetic drift together with inbreeding caused egg hatching rates to plummet. Conservationists ultimately had to transport hundreds of prairie chickens from neighboring states to restore the population's genetic diversity.
A founder effect occurs when a few individuals establish a new population. Imagine several birds reaching an island and breeding there. These "founders" carry only a sample of the original population's gene versions. An allele that was rare in the original population might be common among the island's birds simply because one or more founders happened to carry it.
Similarly, when populations become isolated from one another, genetic drift causes each group to lose different gene versions over time. The number of active breeders in a population also matters: even a large population can be vulnerable to genetic drift if only a small fraction of individuals actually pass on their genes to the next generation.
Conservationists can limit these risks by maintaining sufficiently large breeding populations and enabling appropriate movement between separated populations. However, moving organisms of the same species requires careful assessment because populations may be adapted to different local conditions or carry genetic differences that could adversely affect survival and reproduction.