Genetic Drift and Effective Population Size

Abstract

Does it really matter whether a species has low or high levels of genetic variation? Biologists, conservationists, environmentalists, and informed citizens all worry about the impact of environmental change on the ecosphere. Although organisms cannot plan for environmental change, the more variation that exists in a population, the better prepared that population will be to adapt to change when it does occur. Note that the level of genetic variation within a population is dynamic: It reflects an ever-changing balance between processes, both random and nonrandom, which remove variation. Sometimes, the latter can overwhelm the former, leading to low levels of variation that cannot be reconstituted over ecological time scales. Researchers understand that variation arises through mutation and recombination, and they also know that natural selection can remove variation from a population. Moreover, scientists are well aware of the fact that real-life populations are not infinite, as the Hardy-Weinberg model requires us to assume. Together, these factors lead to a relentless loss of variation, a process referred to as genetic drift. Genetic drift is the reason why we worry about African cheetahs and other species that exist in small populations. Drift is more pronounced in such populations, because smaller populations have less variation and, therefore, a lower ability to respond favorably — that is, adapt — to changing conditions. Thus, it's not just the number of cheetahs that worries us—it's also the decreased variation in those cheetahs.