Why do we have sex? Not humans in particular: why does anything have sex?
We’re so used to sex (as a concept, at any rate) that it doesn’t seem like the kind of thing that requires an explanation. But from an evolutionary point of view it’s actually a bit of a puzzle.
I was going to write about a great paper in this week’s Science on the topic. But first I’ll try to explain why it’s a puzzle, because I think it’s interesting.
Evolutionary theory predicts that creatures ought to behave so as to spread copies of their genes. There are two main ways to do this: having offspring, and helping relatives (which are likely to share the same genes) have offspring. This is a simplistic way of putting it, but the prediction actually accounts for an awful lot of what animals (and other organisms) do. It explains why they try so hard to survive, why they tend to be nice to close relatives, why they compete with one another for mates.
However, the simple version of this theory doesn’t explain a key point: why mate at all? If the point is to contribute genes to the next generation, why throw half of yours away by having sex? Why not reproduce asexually instead, and have offspring that are genetically identical to you?
For humans, of course, it’s a moot point – you can’t reproduce with yourself, try as you might. But we evolved from asexual organisms, and most other animals (and many fungi, plants and eukaryotes) have evolved two sexes as well. Since so many things do it, there must be something good about sex. But no one knows what.
Actually, that’s not true. People have come up with a few reasons sex might have evolved. One of them involves coevolution and parasites, my two favourite things.
The basic idea is that parasites and their hosts are constantly co-evolving: parasites evolve to be better at infecting hosts, and hosts evolve better immune systems. Immune systems are underpinned by many genes. The less these genes change through the generations, the easier it is for parasites to evolve ways of evading the immune system. Organisms that have sex pass on more diverse genes, and therefore better resistance to parasites, to their offspring.
That’s the gist of it, anyways. It’s called the Red Queen Hypothesis, after the Red Queen’s Race in Through the Looking Glass. There are lots of mathematical models that support the theory, but not much experimental evidence. Which brings us to the Science paper.
The authors of this paper tested the idea that sex is important for host-parasite coevolution with a simple experiment. They used three strains of of C. elegans, (a nematode worm): one that reproduced only sexually, one that reproduced only asexually, and one that could do both. They created some worm populations, each containing only one of these types, and then they introduced an infectious bacterium into the populations.
The results were pretty satisfying. The bacteria drove the asexual populations extinct in just a few generations, while the sexual populations survived infection. In the population that could reproduce either sexually or asexually, the rate of sexual reproduction increased from 20% to 70%.
The authors did a couple more tests to see whether coevolution was really behind these changes. They compared bacteria at the end of the experiment to frozen ancestral stocks of the same bacteria, and found that the experimental bacteria was more infectious: it had evolved to become better at infecting C. elegans. In turn, the coevolved C. elegans could resist infection much better than ancestral worms from the same strain.
So: parasites and their hosts coevolve in the lab, and sexual hosts evolve much more quickly than asexual ones. It doesn’t prove the Red Queen theory of sex, but it strongly supports it.