Stick insects in the genus Timema have evolved asexual reproduction on five different occasions in their evolutionary history, according to a new study in this month’s Evolution [$-a]. Why? Well, it turns out that from an evolutionary perspective, sex isn’t always a good thing.
A Timema walking stick.
Photo by WallMic.
The problem comes down to the mathematics of evolutionary fitness. Natural selection favors individuals who make more copies of their genes in the next generation – that’s the most basic definition of the “fittest” who survive. In most sexually reproducing organisms, each parent contributes half of the genes necessary to build each offspring. So for every two babies a parent makes with someone else, her genome is replicated once – half for each baby. Consider the possibilities if this parent can instead make a baby all by herself: for each baby, her entire genome is reproduced. That means that, all else being equal, an asexual critter has twice the fitness of a sexual one.
So it makes sense that asexual reproduction might pop up pretty frequently in the evolution of any group, let alone Timema – a mutant who gains the ability to reproduce asexually should be able to overrun a population of sexual competitors with ease. The question turns out to be not, why are some critters asexual? but why are any critters sexual?
One hypothesis is that sex helps in arms races against parasites, by shuffling genes to generate new combinations of defensive traits. This is called the Red Queen hypothesis because the parasite-host arms race recalls the Red Queen’s advice to Alice in Through the Looking Glass, that in looking-glass land, “It takes all the running you can do, to keep in the same place.” Population genetic studies have shown evidence of Red Queen dynamics in some species [$-a], but it’s not clear how widespread they are. Currently, more biologists favor the alternative hypothesis that sex is important in counteracting the Hill-Robertson effect, which prevents useful genes from spreading through a population if they are associated with damaging genes [$-a].
Under either hypothesis, sex is in some sense more useful in the long term than in the short term. That is, an asexual mutant can overrun a population faster than its offspring are killed by parasites or disadvantaged by the Hill-Robertson effect. This conflict should lead to a specific pattern: evolutionary lineages switch to asexuality rapidly if an asexual mutant arises, then die off when parasites or other hazards of natural selection catch up with them. This is what we see in Timema – several species have given up on sex, but all of them have recent sexual ancestors. Not only does giving up sex make life less exciting – it’s probably an evolutionary dead end.
M. Dybdahl, A. Storfer (2003). Parasite local adaptation: Red Queen versus Suicide King Trends in Ecology & Evolution, 18 (10), 523-30 DOI: 10.1016/S0169-5347(03)00223-4
P.D. Keightley, S.P. Otto (2006). Interference among deleterious mutations favours sex and recombination in finite populations Nature, 443 (7107), 89-92 DOI: 10.1038/nature05049
T. Schwander, B.J. Crespi (2009). Multiple direct transitions from sexual reproduction to apomictic parthenogenesis in Timema stick insects. Evolution, 63 (1), 84-103 DOI: 10.1111/j.1558-5646.2008.00524.x