8.7: Genetic drift is a random change in allele frequencies in a population.

Another evolutionary mechanism is genetic drift, a random change in allele frequencies in a population. For example, consider two alleles for a particular trait, let’s say a cleft chin. We’ll assume that it’s a single-gene trait, with two alleles, and that cleft is dominant over smooth chin. (Note: although research on cleft chins suggests a strong genetic influence on the trait, the mode of inheritance hasn’t been determined with certainty.) In this case, individuals with either one or two copies of the dominant allele (CC or Cc) exhibit the cleft chin (FIGURE 8-12). Now suppose that two heterozygous (Cc) people have one child. Which combination of alleles will that child receive? This is impossible to predict because it depends completely on which sperm fertilizes which egg—the luck of the draw. If the couple’s sole child inherits a recessive allele from each parent, will the population’s allele frequencies change? Yes. The new individual’s two recessive alleles (cc) increase the proportion of recessive alleles in the population. It is equally likely that this couple’s only child will receive two dominant alleles (CC). In either case, because a change in allele frequencies has occurred, evolution has happened.

This impact of genetic drift is much greater in small populations than in large populations. In a large population, any one couple having a child with two recessive alleles is likely to be offset by another couple having one child with two dominant alleles. When that happens, the overall allele frequencies in the population do not change, and evolution has not occurred.

One of the most important consequences of genetic drift is that it can lead to fixation for one allele of a gene in a population (see Figure 8-12). Fixation results when an allele’s frequency in a population reaches 100% (and the frequency of all other alleles of that gene becomes 0%). If this happens, there is no more variability in the population for this gene; all individuals will always produce offspring carrying only that allele (until new alleles arise through mutation). For this reason, genetic drift reduces the genetic variation in a population.

The important factor that distinguishes genetic drift from natural selection is that the change in allele frequencies is not related to the alleles’ influence on reproductive success. This doesn’t mean that genetic drift is confined to alleles that have no effect on fitness. Rather, it just refers to the cause of an allele’s change in frequency. Multiple evolutionary mechanisms can occur simultaneously—an allele favored by natural selection, for example, may simultaneously increase or decrease in frequency due to genetic drift.

Two special cases of genetic drift, the founder effect and population bottleneck effect, are important in the evolution of many populations.

Founder Effect A small number of individuals may leave a population and become the founding members of a new, isolated population. The founder population may have different allele frequencies than the original, “source” population, particularly if the founders are a small group. If this new population does have different allele frequencies, evolution has occurred. Because the founding members of the new population will give rise to all subsequent individuals, the new population will be dominated by the genetic features that happened to be present in that group of founding fathers and mothers. This type of genetic drift is called the founder effect.

The Amish population in the United States is thought to have been established by a small number of founders, some of whom happened to carry the allele for polydactyly—the condition of having extra fingers and toes. As a consequence, this trait, while rare, now occurs much more frequently among the Amish than it does worldwide (FIGURE 8-13).

Population Bottleneck Effect Occasionally, a famine, disease, or rapid environmental change causes the deaths of a large proportion of individuals in a population. Because the population is quickly reduced to a small fraction of its original size, this reduction is called a bottleneck. If the catastrophe is equally likely to strike any member of the population, the remaining members are essentially a random, small sample of the original population. For this reason, the remaining population may not possess the same allele frequencies as the original population. Thus, the consequence of such a population bottleneck would be evolution through genetic drift (FIGURE 8-14).

Just such a population bottleneck occurred with cheetahs near the end of the last ice age, about 10,000 years ago. Although the cause is unknown—possibly environmental cataclysm or human hunting pressures—it appears that nearly all cheetahs died. And although the population rebounded, all cheetahs living today can trace their ancestry back to a dozen or so lucky individuals that survived the bottleneck. As a result of this past instance of evolution by genetic drift, there is almost no genetic variation in the current population of cheetahs. (And, in fact, a cheetah is able to accept a skin graft from any other cheetah, much as identical twins can from each other.)