
The time I remember talking about this most was talking about Hardy-Weinberg equilibrium in pop. bio (and ecology, and general bio, and vert. zoo. and genetics, and probably more times I'm forgetting [you really get tired of it after a while]), which has a number of assumptions. If any of those are violated, evolution occurs. So a population in equilibrium doesn't have changing allele frequencies. If any assumptions are violated, the frequency will change, and evolution occurs. And adaption is a form of evolution as well. An organism has a particular trait that's particularly favorable so it survives better, so its offspring have it, possibly in a more favorable form than before, and so on. As is mutation (which is one of the assumptions, that there's no mutation), which can change the allele frequencies when it occurs, even more if it's favorable.
pumice