There is a common belief that the amount that a group or species can evolve is proportional to the time (or at least the number of generations) that the evolution occurs. This is true, but what is often not realized is that in a sexually reproducing population the evolution rate is proportional to the size of the population.

Adaption occurs through mutation. Most mutations are harmful, but a very few are useful. This means that individuals should avoid mutations, and indeed the living processes go to considerable effort to avoid mutations. Mutations in other animals of the same species however are very useful indeed. If they are harmful the animal is likely to die, meaning more resources for you. If they survive, there is a good chance that the mutation is a beneficial one.

The number of useful mutations in a population is in direct proportion to the size of the population. If one individual develops a beneficial gene, it can spread right through the rest of the population exponentially, until it dominates. If two genes appear at the same time, they can both go through to dominate the entire population (provided they do not occupy the same location in the DNA coding).

This is something that non-sexual reproduction will simply not allow. The genes in two animals, each with a different but beneficial gene must compete with each other. Two such genes in a sexual population can simply merge within one individual.

This is why sex exists.