January 21, 2012, 6:55 am
By David Barash
Sex is a problem for evolutionary biologists, a very big problem. (Let’s be clear: we don’t personally have any more difficulty with it than does anyone else; it’s strictly a professional problem!) And here it is: By all rights, sex shouldn’t exist. Ask most non-biologists what sex is “for,” and they’d probably answer “reproduction,” but they’d be wrong.
In fact, sex is quite simply a terrible way to reproduce.
The reality is that living things can easily make babies without sex, and many do just that. Lots of animals breed asexually, via parthenogenesis (development of an unfertilized egg, without any involvement by males); the list includes many insects, crustaceans, rotifers, flatworms, snails, even some vertebrates including certain species of shark, lizard and the occasional bird. And it’s quite common in plants.
The evolutionary conundrum is that compared to asexual reproduction, breeding sexually poses a daunting number of disadvantages, so many, in fact, that a number of highly regarded evolutionary theorists have concluded rather glumly that sex may actually be a biological liability, something that we—and other species as well—are regrettably stuck with. Here is a brief catalog of its downsides, reasons why sex is such an evolutionary dilemma. (Fear not, however. I’ll conclude, next time, with its likely saving grace.)
First, there are a number of ecological downsides. For starters, every individual who reproduces sexually needs to find a partner, which is often easier said than done. The Indonesian rhinoceros, for example, has become so rare, in part, because male and female literally have difficulty encountering each other, it seems. And we all know people who have their own troubles finding an acceptable mate. How awkward, and how much easier it would be if breeding were simply up to each would-be parent, acting on her own!
According to Aristophanes in Plato’s Symposium, ancestral human beings didn’t have this problem, because they were a bit different from today: Each had two heads, four arms and legs, and two sets of genitals, one male and one female. Every individual was thus a complete, reproductively competent package. But these characters, called Androgynes, grew uppity and self-important, so Zeus decided to diminish them by hurling thunderbolts, as was his wont. Each Androgyne was neatly split in two, and as a result, we have been doomed ever since to search the world for our missing other half. And indeed, people—like Indonesian rhinos—often fail in their quest.