Re:Back in a time before you can imagine (Score 2, Informative)

In terms of evolution, sexual reproduction as compared to asexual reproduction is 'worse', it is less effecient and more time-consuming. So their must be a pay-off: Parasitism is one of the main selection pressures on many animals, so most animals have a very good defence against them, and so parasites in turn evolve rapdidly to overcome their hosts defences. Offspring are likely to be in an enviroment where they are parasitised by organisms well adapted to parasitising their parents. So producing offspring with with a higher genetic variability and therefore less like their parents is beneficial. Sexual reproduction does this. An example is aquatic snails, asexual females are common in an enviroment with relatively few trematode parasites, both males and females are common in enviroments with lots of them.