Natural selection
The natural selection processes such as sexual, convergent, divergent or stabilizing selection pave the way for allele fixation. One way some of these natural selection processes cause fixation is through one specific genotype or phenotype being favored, which leads to the convergence of the variability until one allele become fixed. Natural selection can work the other way, where two alleles become fixed through two specific genotypes or phenotypes being favored, leading to divergence within the population until the populations become so separate that they are now two species each with their own fixed allele.