Variability, built up by selection, is decreased by sex.

In the top panel, an initial population in Hardy-Weinberg proportions with a 40% frequency of allele A is subject to selection with fitnesses given by the solid curve. Because the fitness surface exhibits positive curvature, the result of selection is a population with a great degree of variability in height (middle panel). Asexual reproduction in such a population preserves this variation (bottom left), but sexual reproduction with random mating brings the population back into Hardy-Weinberg proportions and reduces variation (bottom right). This example illustrates the fact that sex does not always increase variation.