The question of how sexual organisms evolved from asexual ones was recently raised. This i

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The question of how sexual organisms evolved from asexual ones was recently raised. This is one the most puzzling questions in evolutionary biology today. Sex is thought to be maintained because of recombination. Imagine a hypothetical asexual organism with two genes, A and B. Further imagine that, at time zero, the organism has allele A1 at locus A and allele B1 at locus B. Now, if a mutation to from allele A1 to A2 was beneficial and a mutation from B1 to B2 was also beneficial, the hypothetical organism would have to have two independent mutation events in it's lineage to reach the A2/B2 genotype. One mutation would occur (say A1 to A2) then this resulting lineage would have to wait for the B2 mutation to occur. But, if the organism were sexual, the A1 to A2 mutation could occur in one organism and the B1 to B2 mutation could occur in another and these alleles could be brought togethor by recombination. In other words, populations can evolve faster if they are sexual. This is especially true if the mutation rate is high and/or the environment is unstable. It also saves lineages from Mullers ratchet (asexual lineages can't "lose" deleterious mutations, they just accumulate and fitness constantly decreases). Note that this is a group selectionist argument. The populations fitness is increased at the expense of the individuals. Individuals, instead of making offspring that are 100% identical to themselves, produce offspring with only half their genes. This is often called the 50% cost of sex (of course this number actually gets lower as the inbreeding coefficient gets higher). In stable environments, lower recombination rates are selected for (because recombination may just break up co-adapted sets of alleles). Even more striking, some organisms revert to asexual reproduction (or parthenogenesis) when the environment is stable and undergo sexual reproduction in a changing environment. Two examples are yeast and rotifers. The rotifers are a particularly interesting example in that, when males are produced, they are degenerate -- unable to feed themselves. They simply expend all their stored energy to make sperm. Other organisms have reverted to parthenogenesis altogethor. Two examples are the dandelion _Taraxacum officinale_ and lizards in the genus _Cnimidophorus_. The lizards are neat because in order for the female to repro- duce, another female has to "mount" her to simulate sex and get the right hormones flowing -- if that doesn't scream modification with descent, I don't know what does! All the above examples, and there are more partheno's out there, have recently evolved from sexual species(*). The only group of ancient, parthenogenetic organisms known is the rotifer genus _Bdelloidea_ (not the same as the rotifers mention- ed above). Most asexual lineages seem to go extinct quickly on a geologic scale. Before anyone mentions bacteria, be aware that they do indeed have sex -- much wilder sex than (from a phylogenetic pers- pective) any animal, plant, fungus or protist could imagine. They swap DNA (the essential process behind sex) with almost anything. So, over the long term, sex seem to be an advantage (to groups if not individuals). Of course, proving something is adaptive now is not the same as showing how it arose. This is, at the current time, a complete mystery. *** Begin rabid, iconoclastic speculation *** (take the following with one salt lick) I happen to think that in the vast majority of sexual species, sex is not adaptive; I believe it is retained as a historical constraint. I think the mechanisms of sexual repro- duction in most species have been fine tuned so much that it is impossible to reverse them and organisms are just stuck with it. "Why do think anything so gawd-awful silly?" Well, I'll give you two reasons. One, a few species do revert to parthenogenesis (and one author claims that these species then outcompete their sexual rivals; I've got to check the evidence before I believe this, though.) Two, I don't think genetic variation is always available for organisms to make transitions -- even to simple adaptive solutions. I think most sexual species are just stuck with a way of life that benefited their ancestors. (Gee, and just about a year ago I was almost a caricature of a pan-selectionist.) The ultimate adaptive strategy (IMHO) would be being able (like the rotifers) to be asexual in a stable environment why mix your successful genes with someone elses?) and sexual in an unstable one (maybe your mate has an allele or two that would compliment some of yours). Of course, sex might return to being adaptive. In fact, it may right now be returning to be adaptive. With the rate that humans are destroying the biosphere, there are probably numerous niches being opened up for some nice, sexual generalist to radiate into. Who knows. *** end mouth frothing speculation *** (Now have a few beers to go with that salt lick -- try a Harpoon Ale if you can find it in your neck of the woods. Else try Dock Street; it's great tasting ale with a massive hoppy flavor -- but I digress. In fact, I soon hope to digress to the local pub -- but that is another digression. And digressions, like if I were to out of the blue call Lionel Tun, "Looney Toon", should be frowned upon as they distract the readers -- but I digress. I could attempt to explain this stupid digression in terms of how lousy my day has been, but alas, that would cause me to digress further.) Here's a book that reviews recent thought on the evolution of sex. I'm posting this instead of the reams of references I have because you can find them all in this book. Michod and Levins, 1988, The evolution of sex, an examination of current ideas, Sinauer, Sunderland, Mass Chris Colby ## email: ## "The college idealists who fill the ranks of the environmental movement seem willing to do absolutely anything to save the biosphere, except take science courses and learn something about it." - P.J. O'Rourke "Parliament of Whores"


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