Evolutionary biology: trait subject to stabilizing selection , what factors determine genetic ...

Genetic diversity increases if the trait becomes a valuable asset to the species. If the trait increases reproductive success or survival, it will be passed on.

The question is basically asking, what's the driving force for new alleles in a population. Genetic drift isn't an explanation, in all fairness. This "forces" only pushes random fluctuations in the numbers of alleles in a population, and not an actual cause. Typically, genetic drift occurs in small populations, where infrequently-occurring alleles face a greater chance of being lost. Once it begins, genetic drift will continue until the involved allele is either lost by a population or is the only allele present at a particular gene locus within a population. Both possibilities decrease the genetic diversity of a population.

Genetic variation is brought about, fundamentally, by random mutation, which is a permanent change in the chemical structure of chromosomes. Genetic recombination also produces changes within alleles.

Hi again, to elaborate, even under stabilizing selection, random mutations may still occur leading to changes in allele frequency equilibria and evolution.