Evolutionary genetics

Abstract

Evolution is often thought of in genetic terms, as a change in allele frequencies and in the phenotypes associated with these alleles in populations over time. Several evolutionary processes in addition to selection — mutation, genetic drift, and gene flow — can change allele frequencies and increase the frequency of disease-associated alleles in populations. When they happen to be linked to beneficial alleles, deleterious or disease-associated alleles may also spread by genetic hitchhiking. Frequency dependent selection, heterozygote advantage, and environmental heterogeneity all contribute to the maintenance of genetic polymorphisms, the existence of multiple alleles of a gene. Most genes are pleiotropic; they have multiple phenotypic effects. The spread of alleles of pleiotropic genes depends upon the balance between their beneficial and deleterious effects. Epigenetic mechanisms, heritable changes in gene expression and therefore in phenotype that are not dependent on changes in DNA sequence, include DNA methylation, covalent modifications of histones, and expression of noncoding regulatory RNA molecules. The Hardy-Weinberg model is an idealized model that provides a starting point for thinking about the relationship between allele frequencies and genotype frequencies. The dramatic growth of the human population since the agricultural revolution has resulted in the production of many new, rare, alleles, some of which may be associated with disease. Contrary to what some people believe, the human population is subject to ongoing natural selection.