You can make both eggs and sperm?
Another neat article, this one's from Nature, a few years back:
Counting the cost of sex
Henry Gee reports on a possible solution of one of the great puzzles of biology: why did sex evolve?
16 March 2000
Sex is an expensive business. There's more to it than flowers, extravagant nights out, school fees and divorce: sex can cost lives and destroy whole species. Sex is so expensive, in fact, that some evolutionary biologists have wondered how so many species can afford such a dangerous luxury -- or even how it could ever have evolved in the first place. Now a team of British researchers offers an explanation.
It's all a matter of perspective. When evolutionary biologists talk about sex, they are talking specifically about sexual reproduction, and rating it against the alternative strategy of asexual reproduction -- simply making clones of oneself.
On the face of it, asexual reproduction looks like a sounder strategy for propagating a species. After all, every member of an asexual species can clone itself as much as it likes, whereas in a sexual species only females have babies. Males are just baubles that lounge around and play golf. Females have to do twice as much work as a member of an asexual species just to keep numbers up. By rights, asexual species should wipe out sexual ones, simply by weight of numbers.
Yet sexual species are everywhere. They are not driven to extinction by reproductively more efficient (if less interesting) asexual clones. This is why evolutionary biologists are hung up about sex.
The usual explanation is that the inherent variation of sexual species more than makes up for the cost of indigent males. When you send in the clones, one looks a lot like another. Sex, on the other hand, shuffles parents' genes, and keeps offspring varied so that sexual species maintain flexibility in the face of changing circumstances.
But there's a catch: variation is an investment that matures over many generations, whereas the cost of males is immediate. Asexual species would 'win' long before sexual species' variation had a chance to make itself felt.
C. Patrick Doncaster and colleagues from the University of Southampton, UK, have found a way out of the dilemma, by looking at how sexual and asexual species would compete in an environment of limited resources: much like real life, in fact.
They have used a computer simulation to model a situation where sexual and asexual species compete for the same resource. The asexual species breeds more than the sexual species, and corners more of the resources. Conventional wisdom would say that the asexual species would then breed more and more, driving the sexual species to equilibrium. But that, the group report in Nature1, does not happen.
The reproductive advantage of asexual species, it turns out, is not as great as biologists had imagined. The reason lies with the monotony of clones. Every cloned individual looks and behaves very much like every other, so they tend to compete for resources far more intensively with each other than with members of sexual species. Second, clones tend to have much more specific preferences than individuals of sexual species. Conversely, because sexually reproducing individuals are varied, they compete less intensively with each other, and can exploit a much wider range of resources. These factors take the heat off sexual species.
So asexual species do not inevitably drive sexual species to extinction. Instead, the two reach an equilibrium, a kind of uneasy balance in which the asexual species' greater capacity for reproduction is countered by its own monotony. This may allow sexual species to invest in their secret weapon -- their own variation -- against the day when circumstances change and they can seize supremacy.
Doncaster, C. P., Pound, G. E. & Cox, S. J. The ecological cost of sex. Nature 404, 281 (2000).