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.
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
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