Microevolutionary forces

Allele frequencies can change from generation to generation. Microevolution = allele frequencies within a population from one generation to the next.

Mutation: change in the genetic information during gametogenesis (heritable). In a population genetics context, mutations convert one allelic form of a gene into another. The effects of mutations are the that mutations generate new alleles or copies of alleles (recurrent mutation). This alters allele frequency. Magnitude of change in a single generation is usually negligible.

How is mutations rate measured? Typically calculated as the number of new mutant alleles per given number of gametes per locus. Best measured molecular data. Mutation rate of eukaryotic genes typically ranges from 10-4 to 10-7 per generation.

The role of mutation in microevlution: Mutations are the ultimate source of genetic variation. Introduce new alleles into populations. Evolution is not possible without variation. Whether a mutation is useful or not is purely chance, new variation is undirected. Most mutations are either neutral of mildly deleterious. Chance of a favorable mutation is very low. The fate of the new allele depends on the other microevolutionary forces.

Genetic drift: random changes in the frequency of alleles in a population from one generation to the next. Driven by chance alone. Population genetics case of random sampling error associated with transmission of genetic variation from one generation to the next.

What are the effects of genetic drift? Random fluctuations in allele frequencies, allele frequencies in different populations follow different paths, ultimately all but one allele will be lost from a population. Drift is a neutral process giving non-directional change. As all individuals are homozygous for the same allele, hetrozygosity and allelic diversity in a population fall.

What are the causes of genetic drift? Persistently small population size, bottleneck, founder effect- founders of a new population possess a small and non-representative fraction of genetic variation present the ancestral population.

Selection: Defined as the differential reproduction of genotypes, reflecting that some genotypes have more or less offspring than others because of their genotype. Usually measured in terms of fitness and or the selection coefficient.

What is fitness (W)? Proportionate contribution by an individual, or by a genotype, of offspring in the populations. W for the most fit genotypes = 1. W for the other genotypes < 1 and is proportional to their reproductive output relative to the most fit genotype. Fitness is relative (rather than absolute) – selection for one genotype is automatically selection against another genotype and so on.

The role of selection in microevolution: acts to increase/maximize the frequency of advantageous traits in a population. Directional selection reduced genetic diversity within populations. Overdominance maintains diversity. Environmental heterogeneity in time, selection can maintain diversity within populations; in space, selection can drive changes between populations. Selection may cause populations to either converge or diverge. This drives adaptive change.

Gene flow: the exchange of genetic material among populations. Occurs via the movement of individuals from one population to another and subsequently the incorporation of the migrant genes into the recipient population.

What are the effects of gene flow? Closed populations tend to diverge due to a combination of genetic drift, localized selection and mutation. Gene flow promotes genetic homogeneity among populations. It can also introduce new genetic variants into the recipient population.

The role of gene flow in microevolution acts to increase variation in populations and decrease divergence between populations. Low levels of gene flow still allow adaptation to local conditions. High levels of gene flow swamp local selection.







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