Genetic drift has a large effect in small populations.
Genetic drift is the random change in allele frequencies from generation to generation. By “random,” we mean that frequencies can either go up or down simply by chance. An extreme case is a population bottleneck, which occurs when an originally large population falls to just a few individuals.
Consider a rare allele, A, with a frequency of 1/1000. Habitat destruction then reduces the population to just one pair of individuals, one of which is carrying A. The frequency of A in this new population is 1/4 because each individual has two alleles, giving a total of four alleles. In other words, the bottleneck resulted in a dramatic change in allele frequencies. It also caused a loss of genetic variation as much of the variation present in the original population was not present in the surviving pair. That is, the surviving pair carries only a few of the alleles that were present in their original population. A population of Galápagos tortoises that has very low levels of genetic diversity probably went through just such a bottleneck about 100,000 years ago when a volcanic eruption eliminated most of the tortoises’ habitat.
Genetic drift also occurs when a few individuals start a new population, in what is called a founder event. Such events occur, for example, when a small number of individuals arrive on an island and colonize it. Once again, relative to the parent population, allele frequencies are changed and genetic variation is lost.
Earlier, we considered the fate of beneficial and harmful mutations under the influence of natural selection. What about neutral mutations? Natural selection, by definition, does not govern the fate of neutral mutations. Consider a neutral mutation, m, which occurs in a noncoding region of DNA and therefore has no effect on fitness. At first, it is in just a single heterozygous individual. What happens if that individual fails to reproduce (for reasons unrelated to m)? In this case, m is lost from the population, but not by natural selection (which does not select against m). Alternatively, the m-
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Like natural selection, genetic drift leads to allele frequency changes and therefore to evolution. Unlike natural selection, however, it does not lead to adaptations since the alleles whose frequencies are changing as a result of drift do not affect an individual’s ability to survive or reproduce.
The impact of genetic drift depends on population size (Fig. 21.13). If m arises in a very small population, its frequency will change rapidly, as shown in Figs. 21.13a and 21.13b. Imagine m arising in a population of just six individuals (or three pairs). Its initial frequency is 1 in 12, or about 8% (there are a total of 12 alleles because each individual is diploid). If, by chance, one pair fails to breed and the other two (including the one who is an Mm heterozygote) each produce three offspring, and all three of the Mm individual’s offspring happen to inherit the m allele, then the frequency of m will increase to 3 in 12 (25%) in a single generation. In effect, genetic drift is equivalent to a sampling error. In a small sample, extreme departures from the expected outcome are common.
On the other hand, if the population is large, as in Figs. 21.13c and 21.13d, then changes in allele frequency from generation to generation are much smaller, typically less than 1%. A large population is analogous to a large sample size, in which we tend not to see large departures from expectation. Toss a coin 1000 times, and you will end up with approximately 500 heads. Toss a coin 5 times, and you might well end up with zero heads. In other words, in a small sample of coin tosses, we are much more likely to see departures from our 50 : 50 expectation than in a large sample. The same is true of genetic drift. It is likely to be much more significant in small populations than in large ones.
Quick Check 5 Why, of all the evolutionary mechanisms, is selection the only one that can result in adaptation?
Quick Check 5 Answer
Adaptation is the fit between an organism and its environment. Of all the evolutionary mechanisms, only selection causes allele frequencies to change based on how they contribute to the success of an individual in terms of survival and reproduction. This means that allele frequencies in the next generation are ultimately governed by the environment. Because phenotype is in part determined by genotype, organisms become adapted to their environment under the influence of selection over time.