Genetic drift may cause large changes in small populations

In very small populations, genetic drift—the random loss of individuals and the alleles they possess—may produce large changes in allele frequencies from one generation to the next. Harmful alleles, for example, may increase in frequency because of genetic drift, and rare advantageous alleles may be lost. As we will see later, even in large populations, genetic drift can influence the frequencies of alleles that do not influence the survival and reproductive rates of their bearers.

Populations that are normally large may pass through occasional periods when only a small number of individuals survive. During these population bottlenecks, genetic variation can be reduced by genetic drift. How this works is illustrated in Figure 23.8, in which red and yellow beans represent two different alleles. Most of the "surviving" beans in the small sample taken from the bean population are, just by chance, red, so the new population has a much higher frequency of red beans than the previous generation had. In a natural population, the allele frequencies would be said to have "drifted."

Suppose we perform a cross of Aa x Aa individuals of a species of Drosophila to produce an F1 population in which p = q = 0.5 and in which the genotype frequencies are 0.25 AA, 0.50 Aa, and 0.25 aa. If we randomly select 4 individuals (= 8 copies of the gene) from among the offspring to produce the F2 generation, the allele frequencies in this small sample may differ markedly from p = q = 0.5. If, for example, we happen by chance to draw 2 AA homozygotes and 2 heterozygotes (Aa), the allele frequencies in this "surviving population" will be p = 0.75 (6 out of 8) and q = 0.25 (2 out of 8). If we replicate this sampling experiment 1,000 times, one of the two alleles

1) The original population has approximately equal frequencies of red and yellow alleles.

^ A chance environmental event greatly reduces the population size.

3 The surviving population has different allele frequencies from the original population.

Hi .which generates a new population with more red than yellow alleles.

23.8 A Population Bottleneck

Population bottlenecks occur when only a few individuals survive a random event, resulting in a shift in allele frequencies within the population.

Genetic Variation Within Population

Tympanuchus cupido (male)

23.9 A Species with Low Genetic Variation Prairie chickens in Illinois lost most of their genetic variation when the population crashed from millions to fewer than 100 individuals.

Tympanuchus cupido (male)

23.9 A Species with Low Genetic Variation Prairie chickens in Illinois lost most of their genetic variation when the population crashed from millions to fewer than 100 individuals.

The D. subobscura founders probably reached Chile and the United States from Europe aboard the same ship, because the two populations are genetically very similar. For example, the North and South American populations have only 20 chromosomal inversions, 19 of which are the same on the two continents, whereas 80 inversions are known from European populations. North and South American populations also have lower allelic diversity at enzyme-producing genes than European populations do. Only alleles that have a frequency higher than 10 percent in European populations are present in the Americas. Thus, as expected for a small founding population, only a small part of the total genetic variation found in Europe reached the Americas. Geneticists estimate that at least ten, but no more than a hundred, flies founded the North and South American populations.

will be missing entirely from about 8 of the 1,000 "surviving populations."

These numbers show that, as it passes through a bottleneck, a population may lose much of its genetic variation. This is what happened to greater prairie chickens, millions of which lived in the prairies of North America when Europeans first arrived there. As a result of both hunting and habitat destruction, the Illinois population of prairie chickens plummeted from about 100 million birds in 1900 to fewer than 50 individuals in the 1990s (Figure 23.9). A comparison of DNA from birds collected in Illinois during the middle of the twentieth century with DNA from the surviving population in the 1990s showed that Illinois prairie chickens had lost most of their genetic diversity. As a result, both hatching success and chick survival were low. To increase the genetic diversity of Illinois prairie chickens, birds from Minnesota, Kansas, and Nebraska were introduced to Illinois. They interbred with the Illinois birds, restoring much of the genetic diversity of that population, which is now increasing in size.

When a few pioneering individuals colonize a new region, the resulting population is unlikely to have all the alleles found among members of its source population. The resulting change in genetic variation, called a founder effect, is equivalent to that in a large population reduced by a bottleneck. Scientists were given an opportunity to study the genetic composition of a founding population when Drosophila subobscura, a well-studied European species of fruit fly, was discovered near Puerto Montt, Chile, in 1978 and at Port Townsend, Washington, in 1982. In both South and North America, populations of the flies grew rapidly and expanded their ranges. Today in North America, D. subobscura ranges from British Columbia, Canada, to central California. In Chile it has spread across 23° of latitude, nearly as wide a range as the species has in Europe (Figure 23.10).

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