Fixation is a shift in the genetic diversity of a population, specifically when a gene represented multiple alleles reaches a point where only one allele exists fixed in the population. When mutations occur they are either be lost or fixed in the population. Whether a gene is fixed or lost is dependant on selection forces in the population causing changes in allele frequency. Smaller populations have a higher rate of fixation and it occurs at a faster rate than in large populations (Zackay, 2007). Kimura and Ohta were able to develop a mathematical method to determine the probability of fixation and the number of generations a gene will persist (Kimura, 1968).
Several studies have investigated gene fixation. In the 1969 a study induced gene fixation in corn. They induced a mutation in corn that regulated alcohol dehydrogenase production. The mutation did not effect seed production or germination at optimal growing conditions. However when seeds were exposed to harsh conditions individuals homozygous for the mutation did not germinate. This caused fixation of the native allele due to lack of germination during adverse conditions. While the allele was not necessary for growth it was advantageous to survive unfavorable conditions (Schwartz, 1969).
Kimura, M., & Ohta, T. (1969). The average number of generations until fixation of a mutant gene in a finite population. Genetics, 61(3), 763.
Schwartz, D. (1969). An example of gene fixation resulting from selective advantage in suboptimal conditions. American Naturalist, 479-481.
Arie Zackay (2007). Random Genetic Drift & Gene Fixation