In species where the females get nothing useful from their mates, they seem to choose on aesthetic criteria alone. - Matt Ridley

The things that kill animals or prevent them from reproducing are only rarely physical factors. Far more often other creatures are involved—parasites, predators, and competitors.

Parasites provide exactly the incentive to change genes every generation that sex seems to demand. The success of the genes that defended you so well in the last generation may be the best of reasons to abandon the same gene combinations in the next. By the time the next generation comes around, the parasites will have surely evolved an answer to the defense that worked best in the last generation. It is a bit like sport. In chess or in football, the tactic that proves most effective is soon the one that people learn easily to block. Every innovation in attack is soon countered by another in defense.

Determinism looks backwards to the causes of the present state, not forward to the consequences.