Question: why do animals have sex ?

Sam Tazzyman answered on 22 Jun 2011:

This is a great question. The short term answer is “to reproduce”, but what is fascinating is that many organisms such as bacteria, fungi, many species of plants, and even some animals, can reproduce without having sex! In most cases this is called “asexual reproduction”. Since everything evolved from ancient bacteria which are asexual, it must have been the case that sexual reproduction evolved from asexual reproduction. But if you look at the tree of evolutionary relationships between species, the asexual species (ignoring bacteria) are often quite recently evolved, so they must have evolved from sexual species.

So: we know that sexual reproduction can evolve from asexual reproduction, and we know that it can also go the other way, with asexual reproduction evolving from sexual reproduction. Since all the asexual reproduction species are quite “young” (in an evolutionary sense) we think that somehow species that are asexual get a short-term advantage, but go extinct quicker than sexual species.

One possibility for why this is is that in the short term asexual species reproduce quicker than sexual species. Asexual species don’t have to go out and find a mate, and every single one of the species can reproduce on its own, whereas sexual species have to find a mate, and so asexual species have a short-term advantage. However, in the long term, because sexual species have offspring with genes from the mother and the father shuffled up, they can evolve to changing environments better. Asexual species’ offspring are clones of the parent, and so they have much less genetic variety in the species, and so (perhaps) are more likely to go exist.