fixation

Imagine a tug-of-war between two alleles that ends when one side wins completely: every member of the population now carries only that version. That endpoint is fixation—the state in which an allele has reached a frequency of one (100%), having entirely replaced all alternatives at its locus.

When an allele becomes fixed, all other alleles at that locus are, by definition, lost; the population is monomorphic there, with no variation left to act on. Fixation and loss are the two absorbing outcomes for an allele under drift: once at frequency zero or one, in the absence of new mutation or migration, it stays there.

Both drift and selection can drive an allele to fixation. Drift fixes alleles by chance, more readily in small populations; selection fixes a beneficial allele by steadily favoring its carriers. The pattern of which alleles have fixed between species is a key clue to past evolution.

Fixation is permanent only until something disturbs it. A fresh mutation, an influx of migrants carrying a different allele, or a change in selection can reintroduce variation at a locus that had become uniform.