Population genetics is the branch of biology that studies the distribution and change in allele frequencies under the influence of the following four processes: natural selection, genetic drift, mutation and gene flow. The population genetics is concerned with the study at the level of an entire population of organisms. To the point of view of population genetics, according to the evolutionary view, the process of evolution consists of a series of gene substitutions.
- Main article: Population
A population is a group of individuals pertaining to the same species living in a defined geographical area and effectively or potentially interbreed.
Gene pool is the full set of alleles which can be found in the genetic material of each of living individuals of given population or, in other words, is the totally of the genes of a given sexual population. 
The four processes
- Main article: Natural selection
Natural selection is also known as the survival of the fittest. It is an observable effect of nature and is considered a verifiable mechanism responsible for biological evolution. Natural selection does not create new traits in organisms: it only favors the spreading of advantageous pre-existing traits, and disfavors the spreading of disadvantageous pre-existing traits. In other words, selection is the inbreeding of favored genes, which reduces the diversity of genetic information in a population, and (in the absence of some other source for genetic diversity to outpace selection) produces a purebreed or genetic homozygote for the trait in question. The result is that organisms become highly tailored to their environment over time, and harmful mutations are kept from spreading throughout the population.
- Main article: Genetic drift
Genetic drift is the establishment of certain alleles due to random sampling of the gene pool. Genetic drift refers to the net decrease in genetic variability and heterozygosity over time. In stable populations, genetic drift causes genetic variation to decrease significantly more quickly than mutation can add new variation. Genetic Drift is a stocastic or random genetic process.
- Main article: Mutation
- Main article: Gene flow
Gene flow is the transfer of alleles or genes from one population to another. In other definition, is the propagation of genes from one breeding population to another by migration, possibly leading to changes in allele frequency. The overall effect of gene flow is that it hinders the genetic divergence between populations and increases the genetic variation within populations.
Even before the science of genetics was born the inheritance of quantitative characters has been the subject of intense research. Subsequently it was the work of the British biologist Ronald A. Fisher starting in 1918 that provided a theoretical basis for the inheritance of quantitative characters. Along with Fisher and the American biologist Sewall Wright, the British geneticist J.B.S. Haldane was one of the three founders of the field of study of population genetics.
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