Experimental evolution of sex (revised)
"I show that a similar cost of sex exists when asexual mutants arise... but not when the species is a self-fertile hermaphrodite.... Although individual fitness (expected reproductive success) is assumed to be equal for sexual and asexual females, the heritability of fitness is... twice as high in asexual females" -- Richard Michod, Darwinian Dynamics
I should be working on my book, but a paper that just came out in Nature got me thinking about sex. A population with half males and half females will grow only half as fast as one consisting only of females that self-fertilize or clone themselves. So, many people have asked why sex evolved.
That's an interesting question, but I'm not sure about the rationale. As noted by Michod, a population of self-fertilizing hermaphrodites doesn't have any intrinsic growth advantage over a population of hermaphrodites that mostly cross-fertilizes. So is the problem sex, or males?
Evolutionary changes in gene frequency over generations depend on whether individuals with a given gene survive and reproduce more than other members of their population, not on the consequences for overall population growth. (Individuals can move between populations.) So we really have two related questions:
1) why do genes for producing male offspring persist? and
2) why do genes for cross-fertilization persist in species that can self-fertilize?
From an individual perspective, it's not apparent that producing male offspring is always a bad idea. Do couples with two sons have fewer descendants than those with two daughters? It can depend on the sex ratio in the population. If a human couple produces one offspring of whichever sex is in the minority, their offspring may have an easier time finding a mate.
But what about cross-fertilization? If a female cloned herself, her offspring would have all of her genes, rather than just half of them. So the frequency of genes for self-fertilization would tend to increase, unless individuals resulting from cross-fertilization were more likely to survive and reproduce. An offspring with half as many of one's genes, but a 2.1-fold better chance of survival (maybe because a sexual partner contributes different disease-resistance genes) gives a greater increase in fitness. So, one key to understanding the evolution of sex (cross-fertilization) is to measure the survival of individuals with one parent versus two, under conditions that plausibly occurred at critical points in a species ancestry.
This week's paper, "Mutation load and rapid adaptation favour outcrossing over self-fertilization", set out to "recapitulate the evolutionary process under the specific conditions predicted to favour either selfing or outcrossing." Levi Morran, Michelle Parmenter, and Patrick Phillips used the nematode, C. elegans, which consists of males and hermaphrodites. (This mix, and the lack of pure females, suggests there can be individual benefits to maleness, whatever the consequences for the population as a whole.) They used genetic manipulation to make populations that only self-fertilized or never self-fertilized, exposed them to high mutation rates or to a bacterial pathogen, and let them evolve.
Their hypothesis was that cross-fertilization would limit the accumulation of bad mutations: if two individuals with two different bad mutations mate, some of their offspring will get both mutations and die, but some will get neither, whereas self-fertilizing populations may not have any mutation free individuals. Sure enough, mutations accumulated in the self-fertilizing population, resulting in decreased fitness. Cross-fertilization was also beneficial to the populations exposed to the pathogen: the population made to cross-fertilize evolved resistance, perhaps because they could combine good mutations from different parents.
As discussed above, however, just because some trait benefits the population as a whole doesn't guarantee that it will evolve. So a more interesting results used populations where the amount of cross-fertilization was allowed to evolve. An increase in mutation rate caused two different strains to evolve more cross-fertilization. Pathogen exposure seemed to have a similar effect, although there was a lot of variability.
Previously, it was thought that even the low rate of natural cross-fertilization in this species was enough to provide most of the benefits, but they saw improvements with additional cross-fertilization. This was achieved by increasing the percentage of males, which they suggested would provide additional benefits via sexual selection. If males fight over females and the healthiest males win, or if females choose the healthiest males, maybe we aren't so useless to our populations after all.