I do think that the maintenance of sex is a context in which group selection, selection between species, has actually been important. That's not for its origin or its early evolution, but just for maintaining it. My reason for believing that is that if you look at the taxonomic distribution of parthenogens, I mean, what kind of animals or plants are parthenogenetic, by and large, what you find is that the odd species or variety, or sometimes genus, is parthenogenetic. You tend not, in general, to find whole large groups of organisms which are parthenogenetic. There's a famous and rather embarrassing exception to this rule, the Bdelloid Rotifers, but by and large, you find parthenogens spotted about through the taxonomic distribution. Most organisms are sexual and then just a few parthenogens. Now, that is what you expect if what is happening is that parthenogenic mutants are arising, they're becoming established in a population, and the population is lasting for perhaps a few hundreds of thousands or millions of years, but it doesn't radiate out to produce a whole new big taxonomic group, but goes extinct, because of the disadvantage of being parthenogenetic. Could I say as a sort of interesting digression, I sometimes, almost as a parlour game, ask myself: Can I think of any other characteristics which are distributed taxonomically, like parthenogenesis, because if you could, then there would be good reason to think maybe they were being maintained by group selection, if you like, species selection. Now then you might ask, well, how can species selection work in relation to sex? And if it could work in relation to sex, why not everything else? The answer to that is, I think, first of all that, the advantages of sex - and I'll talk about those in a moment in more detail - the advantages of sex are, the most obvious one is that confers a greater capacity of a rapid evolution [to] the population, so sex is actually giving a characteristic to a species, it's not to the individual, the individual doesn't actually benefit from the sex, it's the population as a whole that does. The other thing is that normally, when one's thinking about the maintenance of something by group selection, the problem is that any variation within the population, you have some characteristic which is disadvantageous within the population but advantageous at the group level, selection acting on variation within the population will always win against selection between populations, just for simple mathematical reasons. But in sex, we very, very rarely come across a population in which both sexual and asexual organisms coexist.
[Q] Even if you did, the asexual part would have, in a sense, isolated themselves away from the rest of the game. They're no longer part of the free exchange of genes, and therefore... isn't it right that the rules of the game have sort of been changed?
Well, I see what you're saying, and it's quite a profound problem actually, because it's asking, all right, when you talk about selection going on within a population, what's the population you're talking about? I think the best way of thinking of the population you're talking about is that it's that group of individuals who are competing essentially for the same resources.
[Q] And even if they've separated themselves off genetically, they're still ecological competitors for exactly the same resource.
That's right. I mean, there were Poecilispsid fishes that people work on in North America, where they're a mixture of sexual and parthenogenetic individuals, and they are living in the same rivers competing with the same food and the same space and so on, so I think one can think of them all as a single population, even though some are exchanging genes and others aren't. But it is very striking that it's clear that the origin of a new parthenogenetic variety is not something that happens very often, it probably happens exceedingly rarely. So you tend not to find mixed populations, you tend to find sexual populations or occasionally you find a population that's been taken over by parthenogenesis, but you only rather rarely find them competing with one another, you do sometimes, but it's... and it's those cases that we ought to really concentrate on, of course, in trying to understand what's happening. I do think that a group selection explanation, at least for maintaining sex, at least as part of the explanation if not the whole explanation, is a plausible approach.
