Allelic frequencies
Allelic frequencies change in populations owing to two factors: natural selection, which is the result of population variation among individual genotypes in their probabilities of survival and/or reproduction, and random genetic drift, which is due to a finite number of individuals participating in the formation of the next generation.
Both natural selection and genetic drift can ultimately lead to the elimination or fixation of a particular allele.
In the presence of mutation and in the absence of selection (that is, under neutral conditions), the rate of neutral evolution of a finite population is equal to the reciprocal of the mutation rate.