Okay, here’s my contribution to the week. We spoke a bit about levels of selection, so I thought I’d throw up a paper that shows where an individual’s interest is counter to a group’s interest. There’s also a bit about sex in there, which might be useful to start thinking about now. The paper is How to go extinct by mating too much: population consequences of male mate choice and efficiency n a sexual-asexual species complex by Katja U. Heubel, Daniel J. Rankin and Hanna Kokko.  2009.

First a bit of background, there’s this guppy-like fish genus called Poecilia. In several of the species in that genus, when they interbreed their offspring are these things called gynogens. They’re always female, and their offspring are clones. But while each egg they produce contains all the genetic material their offspring need, they still need to have the egg come into contact with sperm from one of the parental species to jump start the egg’s development. So, the gynogen species need to live around parental species to get the sperm off the males. Males theoretically get no benefit from fertilizing one of these eggs, since their genetic material isn’t passed on in the offspring (though one study found that females of the male’s species are more likely to mate with him if they see him having sex with one of gynogens. This puzzles everyone). This makes gynogens effectively sperm parasitizers.

What’s the problem here? Well the asexual fish should be able to produce twice the number of offspring as the sexual fish, since asexuals only need one individual of their own species to produce another, and sexual species need two individuals (this is officially called the ‘two fold cost of sex’). Thus, the gynogens should increase their population size much faster than the sexual species living in the same area, and should end up out-competing the sexual species for males by sheer numbers.  This causes the sexual species to go extinct, and then the asexual species soon follows because it no longer has any males to get sperm off of.

Two possible factors might throw this off: If males produce enough sperm, they should be able to fertilize all the females, sexual and asexual, and both lineages should persist. Or, males  might preferentially mate with their own species. These traits aren’t necessarily good for an individual to evolve though – sperm is costly to produce, and if the male is too discriminating, it might accidently pass up a chance to mate with a female of its own species, or waste too much time trying to tell whether a potential mate is a gynogen or not (these fish look very similar). The paper looked at these two traits, and constructed a model to see what levels of these traits would be best for the population as a whole (in other words, what amounts of male discrimination and sperm production combined lead to the long-term survival of the species?). They then compared the expected rates to rates in the wild.

I’ll spare the math because well, it’s mathy, and I’ve written a novel already. But, they found that their predicted numbers, and numbers observed in most gynogen populations didn’t match up. They pointed out that local populations have been observed to go extinct, and also that these fish are good colonizers, so as long as the extinction rate is low, individuals might ‘escape’ by colonizing a new area, and the whole game would play out over again. There’s quite a few other factors to consider with these fish (one being that the asexual species don’t completely overlap in niche with the sexual species, which would lessen a lot of competition), but I thought this example was interesting because it talked about group selection, and because gynogens are friggin weird and awesome.

For further reading about asexual fish (and uh, other vertebrates) I suggest Clonality by John Avise. It’s in Woodward. 🙂