Why have sex?

One of the big questions in evolutionary biology is why sex evolved. There are obvious disadvantages to being a gene in an organism which reproduces sexually: there is a 50% chance that you won't end up in any given offspring. So, why do so many organisms reproduce sexually?

One idea, originally proposed by John Maynard Smith, is that selection is a lot more efficient with sexual reproduction. When you reproduce asexually, then selective pressure can only act on whatever complexes of mutations happen to form in given individuals. So, if I have beneficial mutation A and you have B (at different positions), there's no direct way for some future organism to get A and B--though they could start with A and independently mutate B. By contrast with sexual reproduction, an organism with A can mate with one with B and produce an organism with both A and B. So, this confers an advantage in terms of faster selection.

In the Feb 17 Science, Paland and Lynch provide confirming evidence for this theory. (more readable review of this work on which this blog post is partly based here). The water flea (Daphnia pulex) can convert from sexual to asexual reproduction (but not back). Paland and Lynch were able to measure selection rates (by measuring the stability of the amino acid sequences at various loci) in both lineages and the rate of selection was substantially higher in the sexual lineages, providing some confirmation for the faster selection hypothesis.