Recombination breaks down the linkage equilibrium and increases the variance. In Through the Looking Glass, Alice observed that, although she was running very fast, she was not moving anywhere. The Red Queen told her that here, you see, it takes all the running you can do, to keep in the same place. So in order to avoid extinction, a lineage must constantly be evolving and responding to the changing environment. Thus, we expect strong selection for a reproductive mode that allows rapid evolutionary change.
Sexual reproduction permits a rapid response to changing selective forces.
For example, the offspring from a bout of sexual reproduction will be more genetically diverse than the offspring from a bout of asexual reproduction. Therefore, it is likely that at least some of a sexual individual s offspring will have a genotype and phenotype that will be successful in the new, recently changed environment. Recently, attention has been given to parasites. Parasites are likely candidates for a particularly strong and variable selective agent in the environment of a host species. Parasites and their hosts are engaged in a constant struggle, with the host evolving defenses and the parasite evolving mechanisms to counter the host s defenses.