What will happen to FST when populations are separated with no migration or selection?

FST, or the fixation index, is a measure of population differentiation due to genetic structure. When populations are separated with no migration or selection, they become increasingly isolated in terms of genetic variation. Over time, the genetic differences between these populations accumulate because any allele present in one population is not being shared with the other.

As a result of this genetic drift and the lack of gene flow, the populations can diverge significantly from each other. When FST approaches 1, it indicates that the populations have become fully differentiated, meaning that almost all genetic variation is found between the populations, and there is very little to no genetic similarity between them. Hence, under these conditions, FST will increase towards 1 as populations continue to diverge without any introduction of new alleles from migration or changes due to selection pressures. This reflects the increasing genetic distance and complete separation between the populations.