In population genetics, fixation occurs when every individual within a population has the same allele at a particular locus. The allele, such as a single point mutation or whole gene, will be initially rare (e.g. originating in one individual), but can spread through the population by random genetic drift and/or positive selection. Once the frequency of the allele is at 100%, being possessed by each member, it is said to be "fixed" in the population. Similarly, genetic differences between taxa are said to have been fixed in each species. Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and migration. ... In genetics, an allele (pronounced al-eel or al-e-ul) is any one of a number of viable DNA codings occupying a given locus (position) on a chromosome. ... Short and long arms Chromosome. ... A point mutation, or substitution, is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide. ... For other meanings of this term, see gene (disambiguation). ... Biological inheritance is the process by which an offspring cell or organism acquires or becomes predisposed to characteristics of its parent cell or organism. ... In population genetics, genetic drift is the statistical effect that results from the influence that chance has on the success of alleles (variants of a gene). ... The Galápagos Islands hold 13 species of finches that are closely related and differ most markedly in the shape of their beaks. ... A taxon (plural taxa) is an element of a taxonomy, e. ... In biology, a species is one of the basic units of biodiversity. ...

References

• Gillespie, J.H. (1994) The Causes of Molecular Evolution. Oxford University Press Inc., USA.
• Hartl, D.L. and Clark, A.G. (2006) Principles of Population Genetics (4th edition). Sinauer Associates Inc., USA.
• Kimura, M. (1962) On the Probability of Fixation of Mutant Genes in a Population. Genetics 47: 713–719. PubMed Central