Why is there sex?

Post of the Month: May 1999

by Felipe

Subject:    Re: Fair is Fair
Newsgroups: talk.origins
Date:       May 26, 1999
Message-ID: 01bea7d7$7fe872e0$b80a6899@villanova.villanova.edu

ah-HAH! Now that's a good question. And one that I feel a little more suited to answer.

Before we go further, it seems as if you have more than one question buried in your post -- and perhaps this is what lead to previous confusion. First, there is an evolution of multicellularity. Then there is the evolution of apoptosis in multicellular organisms. And then you ask about sex. The first two questions are ones that I will defer to folks who know better than I do (although I think the short answer to both is "complexity"). The final one is something I know enough about to be dangerous.

And it gets even trickier. I think I posted a reasonable list of recent reviews, with the exception of one from last year in TREE, the reference to which I seem to have lost. The problem gets even trickier because sexual organisms, e.g. sexual females, give up half their longterm reproductive potential by having males. That is, a sexual female has females (who will in turn bear offspring) and males (who won't). In contrast an asexual female will have all female offspring, who will all bear offspring. If both kinds have similar sized broods, then asexual females can increase in the population twice as fast as sexual females, all else being equal. This is sometimes refered to as the two-fold cost of sex.

Yet the bulk of multicellular organisms are sexual, animals at least, which makes people think that sexuality should have some advantage.

The advantage seems to be in fairly rapidly changing environments, especially in the face of disease or parasites. Being able to rapidly recombine genes with another individual often lets an individual's offspring stay one step ahead of whatever diseases are up to. In contrast, asexual organisms have to wait for beneficial mutations. This wait is especially onerous if adaptation entails more than one mutation. While one asexual strain may receive the first beneficial mutation, and the second the second, they're still both screwed because they have to wait for the second and first mutation, respectively, to occur in their strain. Could they recombine then some of the progeny of both (given sex) would be resistant and survive. Thus under variable environments sex might overcome the two-fold cost. This idea seems to be borne out by observation and experimental evidence. Without citing sources (it's late; I'm tired), species for which there are both sexual and asexual strains tend to be sexual where there is a higher risk of infestation by parasites. Further, asexual organisms tend not to have similar brood sizes as asexual organisms, especially after a few generations. Sorry for the vagueness.

Gillespie's recent book, A concise guide to population genetics, has a nice chapter about the problem of the evolution of sex. I'll try to find the TREE (Trends in Ecology and Evolution) article ... here it is, cut and pasted:

TI: Recent advances in understanding of the evolution and maintenance of sex
AU: Hurst, LD; Peck, JR
SO: Trends in Ecology & Evolution [TRENDS ECOL. EVOL.], vol. 11, no. 2, pp. 46-52, 1996

Now if you REALLY want to dive into it, roll up your sleeves and put on your propellor-beanie, then read Charlesworth. Moderately heavy stuff, though.

Hope others can help with explaining how programmed cell death might actually improve an cell's chance of leaving it's genes, given that it is a somatic cell. Turns to another interesting question, the definition of an individual.

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