Population genetics – Wikipedia, the free encyclopedia

Population genetics is the study of the distributions and changes of allele frequency in a population, as the population is subject to the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population structure. Studies in this branch of biology examine such phenomena as adaptation and speciation.

Population genetics was a vital ingredient in the emergence of the modern evolutionary synthesis. Its primary founders were Sewall Wright, J. B. S. Haldane and R. A. Fisher, who also laid the foundations for the related discipline of quantitative genetics.

Traditionally a highly mathematical discipline, modern population genetics encompasses theoretical, lab and field work. Computational approaches, often using coalescent theory, have played a central role since the 1980s.

Biston betularia f. carbonaria is the black-bodied form of the peppered moth.

Population genetics is the study of the frequency and interaction of alleles and genes in populations.[1] A sexual population is a set of organisms in which any pair of members can breed together. This implies that all members belong to the same species and live near each other.[2]

For example, all of the moths of the same species living in an isolated forest are a population. A gene in this population may have several alternate forms, which account for variations between the phenotypes of the organisms. An example might be a gene for coloration in moths that has two alleles: black and white. A gene pool is the complete set of alleles for a gene in a single population; the allele frequency for an allele is the fraction of the genes in the pool that is composed of that allele (for example, what fraction of moth coloration genes are the black allele). Evolution occurs when there are changes in the frequencies of alleles within a population; for example, the allele for black color in a population of moths becoming more common.

Natural selection is the fact that some traits make it more likely for an organism to survive and reproduce. Population genetics describes natural selection by defining fitness as a propensity or probability of survival and reproduction in a particular environment. The fitness is normally given by the symbol w=1-s where s is the selection coefficient. Natural selection acts on phenotypes, or the observable characteristics of organisms, but the genetically heritable basis of any phenotype which gives a reproductive advantage will become more common in a population (see allele frequency). In this way, natural selection converts differences in fitness into changes in allele frequency in a population over successive generations.

Before the advent of population genetics, many biologists doubted that small differences in fitness were sufficient to make a large difference to evolution.[3] Population geneticists addressed this concern in part by comparing selection to genetic drift. Selection can overcome genetic drift when s is greater than 1 divided by the effective population size. When this criterion is met, the probability that a new advantageous mutant becomes fixed is approximately equal to 2s.[4][5] The time until fixation of such an allele depends little on genetic drift, and is approximately proportional to log(sN)/s.[6]

Natural selection will only cause evolution if there is enough genetic variation in a population. Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance. But with blending inheritance, genetic variance would be rapidly lost, making evolution by natural selection implausible. The HardyWeinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. According to this principle, the frequencies of alleles (variations in a gene) will remain constant in the absence of selection, mutation, migration and genetic drift.[7] The HardyWeinberg "equilibrium" refers to this stability of allele frequencies over time.

A second component of the HardyWeinberg principle concerns the effects of a single generation of random mating. In this case, the genotype frequencies can be predicted from the allele frequencies. For example, in the simplest case of a single locus with two alleles: the dominant allele is denoted A and the recessive a and their frequencies are denoted by p and q; freq(A)=p; freq(a)=q; p+q=1. If the genotype frequencies are in HardyWeinberg proportions resulting from random mating, then we will have freq(AA)=p2 for the AA homozygotes in the population, freq(aa)=q2 for the aa homozygotes, and freq(Aa)=2pq for the heterozygotes.

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Population genetics - Wikipedia, the free encyclopedia