Harlequin ladybirds mating © Arterra / Getty
The existence of sex is a knotty problem. If you could just reproduce by cloning yourself, then all of your genes would be passed on to the next generation. Why, then, would you choose to have sex with another organism, a process that immediately cuts your genetic legacy by half?
One answer is that sex, in which genes are mixed and reshuffled, gets rid of damaging mutations. Another answer is that new combinations of genes allow creatures to keep one step ahead of disease and infection. There is good evidence that creatures tend to choose mates that differ from themselves, especially in those genes involved in fighting disease.
Organisms that generally reproduce asexually tend to accumulate harmful mutations, and become quite samey after a while, allowing infection to take a larger toll than it might have done otherwise.
On the other hand, some organisms appear to have been all but entirely unisexual for thousands or even millions of years. Rotifers – beautiful, microscopic creatures found in ponds – are a good example; some species are only known from females.