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Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics
Summary

Charles Darwin's The Origin of Species revolutionized biology. He constructed a theory of evolution based on the principles that variation existed within populations, that variation was heritable, and that the phenotype of the individuals in the population changed through generations because of natural selection. These basic tenets of evolutionput forward in 1859, before any knowledge of Mendelian geneticshave required only minor modification since that time. The study of changes within a population, or population genetics, relates the heritable changes in populations or organisms to the underlying individual processes of inheritance and development. Population genetics is the study of inherited variation and its modification in time and space.

Identifiable inherited variation within a population can be studied by observing morphological differences between individuals, examining the differences in specific amino acid sequences of proteins, or even examining, most recently, the differences in nucleotide sequences within the DNA. These kinds of observations have led to the conclusion that there is considerable polymorphism at many loci within a population. A measure of this variation is the amount of heterozygosity in a population. Population studies have shown that, in general, the genetic differences between individuals within human races are much greater than the average differences between races.

The ultimate source of all variation is mutation. However, within a population, the quantitative frequency of specific genotypes can be changed by recombination, immigration of genes, continued mutational events, and chance.

One property of Mendelian segregation is that random mating results in an equilibrium distribution of genotypes after one generation. However, inbreeding is one process that converts genetic variation within a population into differences between populations by making each separate population homozygous for a randomly chosen allele. On the other hand, for most populations, a balance is reached for any given environment among inbreeding, mutation from one allele to another, and immigration.

"Directed" changes of allelic frequencies within a population occur through the natural selection of a favored genotype. In many cases, such changes lead to homozygosity at a particular locus. On the other hand, the heterozygote may be more suited to a given environment than either of the homozygotes, leading to a balanced polymorphism.

Environmental selection of specific genotypes is rarely this simple, however. More often than not, phenotypes are determined by several interacting genes, and alleles at these different loci will be selected for at different rates. Furthermore, closely linked loci, unrelated to the phenotype in question, may have specific alleles carried along during the selection process. In general, genetic variation is the result of the interaction of evolutionary forces. For instance, a recessive, deleterious mutant may never be totally eliminated from a population, because mutation will continue to resupply it to the population. Immigration also can reintroduce the undesirable allele into the population. And, indeed, a deleterious allele may, under environmental conditions of which we are unaware (including the remaining genetic makeup of the individual), be selected for.

Unless alternative alleles are in intermediate frequencies, selection (especially against recessives) is very slow, requiring many generations. In many populations, especially those of small size, new mutations can become established even though they are not favored by natural selection, simply by a process of random genetic drift.

© 1999 by W. H. Freeman and Company. All rights reserved.