Heterozygote disadvantage, or underdominance Fitness of heterozyote is less than either homozygote. If the less fit allele gets fixed in a population, natural selection cannot move it toward the more fit, because of the valley in the fitness curve. In finite populations, drift is a consideration Populations may not stay at the predicted equilibrium frequencies. In bottlenecks, drift may counteract selection so that a deleterious allele may increase in frequency. This is especially in underdominance, because the population will move toward on allele or the other.
Drift can overcome the selection pressure that is preventing movement from one peak to the other.
Thus, drift and selection may accomplish together what selection alone cannot - moving a population from one stable peak to another. Scientists are now looking at DNA sequences among species that diverged millions of years ago, to look at the more subtle effects of natural selection.