Population genetics examines the genetic composition of groups of individuals and how this composition changes with time.
A Mendelian population is a group of interbreeding, sexually reproducing individuals, whose set of genes constitutes the population’s gene pool. A population’s genetic composition can be described by its genotypic and allelic frequencies.
The Hardy–Weinberg law describes the effects of reproduction and Mendel’s laws on the allelic and genotypic frequencies of a population. When a population is large, randomly mating, and free from the effects of mutation, migration, and natural selection, the allelic frequencies do not change and the genotypic frequencies stabilize after one generation in the Hardy–Weinberg equilibrium proportions p2, 2pq, and q2, where p and q equal the frequencies of the alleles.
Nonrandom mating affects the frequencies of genotypes but not those of alleles. Inbreeding increases the frequency of homozygotes while decreasing the frequency of heterozygotes.
Recurrent mutation eventually leads to an equilibrium, with the allelic frequencies being determined by the relative rates of forward and reverse mutation.
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Migration, the movement of genes between populations, increases the amount of genetic variation within populations and decreases the difference in allelic frequencies between populations.
Genetic drift is change in allelic frequencies due to chance. Genetic drift arises when a population consists of a small number of individuals, is established by a small number of founders, or undergoes a major reduction in size. Genetic drift changes allelic frequencies, reduces genetic variation within populations, and causes genetic divergence among populations.
Natural selection is the differential reproduction of genotypes; it is measured by the relative reproductive successes (fitnesses) of genotypes.
Evolution is genetic change that takes place within a group of organisms. It is a two-
A species can be defined as a group of organisms that are capable of interbreeding with one another and are reproductively isolated from the members of other species.
Species are prevented from exchanging genes by prezygotic or postzygotic reproductive isolating mechanisms.
Allopatric speciation arises when a geographic barrier prevents gene flow between two populations. Sympatric speciation arises when reproductive isolation exists in the absence of any geographic barrier.
A phylogeny can be represented by a phylogenetic tree, consisting of nodes that represent organisms and branches that represent their evolutionary connections. Two different approaches to constructing phylogenetic trees are the distance approach and the parsimony approach.
Different parts of the genome show different amounts of genetic variation. In general, those parts that have the least effect on function evolve at the highest rates.
The molecular clock hypothesis proposes a constant rate of nucleotide substitution, providing a means of dating evolutionary events by looking at nucleotide differences between organisms.
Genome evolution takes place through the duplication of genes to form gene families, whole-