Isolations and genetic drift

Date: 2015-10-07; view: 440.

The frequency of alleles can change from generation to generation as a result of chance alone in a small gene pool. This phenomenon is known as genetic drift.

Random mating involves individuals pairing by chance, not according to their genotypes or phenotypes. Nonrandom mating involves individuals inbreeding and assortative mating. Inbreeding is mating between relatives to a greater extent than by chance; inbreeding can occur if dispersal is so low that mates are likely to be related and does not change allele frequencies, but it does decrease the proportion of heterozygotes and increase the proportions of both homozygotes at all gene loci.

Assortative mating occurs when individuals tend to mate with those that have the same phenotype. Assortative mating divides a population into two phenotypic classes with reduced gene exchange.

Genetic drift is changes in allele frequencies of a gene pool due to chance or random events. This can occur in large or small populations. Genetic drift causes gene pools of two isolated populations to become dissimilar as some alleles are lost and other are fixed.

Genetic drift occurs when founders (or colonizers) establish a new population, or after a genetic bottleneck and resultant interbreeding. The founder effect is a case of genetic drift in which rare alleles, or combinations of alleles, occur in higher frequency in a population isolated from the general population. Founding individuals contain a fraction of the total genetic diversity of original gene pool. The alleles carried by founders is determined by chance alone. Consider the Pilgrim colonists in New England. By no means did they represent all the genetic variation of the human species or even genetic variations among Europeans.

When a population is started by one or a few individuals who randomly separate from a larger population, chance may dictate that allele frequencies in the new population may be very different from those of the original population. Many species on islands (such as the famous Darwin's finches on the Galápagos) display founder effects. The Galápagos Islands are volcanic islands off the coast of South America. They had fewer types of organisms than the South American mainland. The island species varied from the mainland species, and from island-to-island. Each island had a variation of tortoise that correlated with different vegetation and environmental conditions on that island. Galápagos finch species varied by nesting site, beak size, and eating habits (figure 32.1).

Figure 32.1 Divergence of the Galapagos finches from ancestral colonizers from the South American mainland.