Well, yes, strictly speaking – that is, if you want to reproduce. Mammals and most other vertebrates, at any rate, require a male at some point in the process to produce some gametes – sperm, specifically – which must make their way to an egg and fuse with it to make a zygote. This is usually achieved by the delightfully biological processes of copulation, ejaculation and insemination, though some aquatic vertebrates make use of the decidedly less titillating (although less risky, in some ways) method of external fertilization. That particular practice is even more widespread when we consider sex in invertebrates, though internal fertilization is still popular among insects and other large groups.
We, as humans, have a more recreational approach to sex than most species, which is why we’ve devised a number of means by which we can engage in it whilst avoiding some of the more unpleasant potential side effects (horrible diseases, babies, etc. etc.). So, for most of us, the question “Why sex?” seems misguided, at best.
But, scientifically speaking, it’s a stumper. There are a number of objections to engaging in sex, but the major issue is that sex generally requires, as we have established already, males (am I right, ladies?). The problem with males is that those lazy, selfish bastards don’t produce offspring of their own, which cuts the potential rate of reproduction of a typical sexual population in half. That’s bad news if you’re competing with asexual populations that don’t face this particular handicap – they’ll outproduce you in no time at all, and then it’s curtains, Charlie. That should lead us to expect that most species will be asexual, but in fact, most multicellular eukaryotes (in particular, animals, fungi and higher plants) are sexual at least part of the time, and doing just fine. That’s pretty strong evidence to suggest that sex is actually enormously advantageous for these kinds of organisms. Determining what about it is advantageous has been the pursuit of some of the best minds in evolutionary biology over the last four decades or so.
We’ve done pretty well at coming up with theoretical explanations that account for sex. Keep in mind that “theory” means something pretty specific in evolutionary biology, and that is mathematical modeling of the problems and potential solutions; so this is sophisticated, sometimes mind-bending stuff, not vague hand-waving. So, here’s a short list of theoretical advantages to engaging in sex:
Fighting off parasites (and, from the parasites’ perspective, attacking hosts): sex facilitates rapid coevolution; an evolutionary “arms race.” This is the Red Queen hypothesis.
Adaptation: sex produces a wide variety of offspring genotypes, increasing the odds of adapting to a changing environment.
Protection from mutations: without sex, nasty mutations stack up in your genome, ultimately hurting your fitness, quite severely under some theories.
Complementation: sex can prevent deleterious mutations from negatively affecting the phenotype much of the time (i.e., we have recessive deleterious alleles).
There are dozens more. What has proved more difficult than generating theories has been gathering empirical evidence to support or refute them. Happily, that side of the science of sex has been gathering steam over the last few years, and there is increasing evidence to support some of these theories. For example, we now have definitive evidence that asexual species often do accumulate mutations at a more rapid pace than sexual species, although the consequences for fitness are harder to establish. And now, this month, an elegant study released in the journal Science provides clear evidence in favor of the Red Queen hypothesis: sex is good because it gives you an advantage in fighting off parasites.
The inimitable Carl Zimmer posted about this study on his blog earlier this month, and I certainly won’t try to imitate him; I strongly recommend reading his post, it’s a short and clear description of the study. But if you don’t want to click through: sexual and asexual populations of nematode worms were reared in environments with and without parasites, and the asexual worms clearly did worse in the presence of parasites, ultimately being wiped out.
What I have to say about this is that it’s tremendously exciting to be able to demonstrate the predictions that we make coming true in actual organisms, and it’s mildly astonishing that this study has not been done before. Many of the theories for the advantages of sex have only become practically testable in the last ten years or so as we have become able to very cheaply get lots and lots of DNA sequence data, but this is a pretty old-school experiment. Evolutionary science definitely has a people-power issue – only so many people are working on these problems. That can make it both a wonderful area in which to do research, due to relatively low risk of being “scooped” by competitors, but also a frustrating one when your realize that there are gaps in your knowledge of a system that no one can fill, because no one has worked on it. So this is good news. To be clear, this doesn’t prove that parasites are THE reason for sex; chances are that it is one of a number of contributing factors, some of which are undoubtedly listed above. It sure is nice to have solid evidence that it is part of the picture, though.