In a clumsy effort to seduce her, I attempted to explain the evolutionary advantages of sexual reproduction to a female friend the other day. One advantage, I said, was introducing an element of genetic competition into the process. She wanted to know why, if two sexes are needed to create genetic competition, why aren鈥檛 there three, four or a million sexes to create even more competition? So why are there only two?
鈥� Some species do have more than two types. Single-celled ciliates have up to 100, and mushrooms tens of thousands. But most organisms 鈥� even single-celled ones 鈥� come in two types.
So why are there two types in most species? In all species, no matter how many types, sex occurs between just two cells and any can mate with any other sex cell that is different from it. So, as your questioner suggests, finding just two types in most species is paradoxical, because having many types would maximise the chances of finding a mate.
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鈥淔inding just two sexes in most species is odd; having more would maximise the chances of finding a mate鈥�
One answer to this problem is that two types is best for co-ordinating the inheritance of 鈥� the part of the cell鈥檚 genetic material that is not contained in the nucleus. However, there is a drawback to this solution. The species with two types fuse cells and potentially run the risk of scrambling this extra material.
The species with more than two mating types do it differently. With three types, the coordination is even more difficult to make error proof, while those with many mating types don鈥檛 fuse cells at all and so are not constrained to having just two types.
Laurence Hurst, Professor of Evolutionary Genetics, University of Bath
Laurence Hurst has written widely on the subject and more information can be found in his following papers 鈥� Ed
鈥淐ytoplasmic fusion and the nature of sexes鈥� (with William Hamilton), Proceedings of the Royal Society B, vol 247, p 189; 鈥淪elfish genetic elements and their role in evolution: the evolution of sex and some of what that entails,鈥� Philosophical Transactions of the Royal Society B, vol 349, p 321; 鈥淲hy are there only two sexes?鈥� Proceedings of the Royal Society B, vol 263, p 415.
鈥� Having taught a difficult lesson on statistical techniques in geography to my secondary school students, I stood before them lost in admiration of the test I鈥檇 written up on the board. Just then my students informed me that there were actually three sexes in this world: men, women and geography teachers. Unfortunately, I am a geography teacher.
Mary Sinclair, By email, no address supplied