Problems with Mutation and the Evolution of the Sexes

Hi Lyston, There have been a couple of people who have touched on what you now seem to be indicating is your main question: the evolution of sexual reproduction. On the other hand, a lot of the cranky answers probably derive from your irritating and pointlessly obnoxious (not to mention arrogant) approach, with which you began right out of the starting gate in the OP. That being said, it might be interesting for other participants and potential lurkers to take a moment to review the various hypotheses that have been put forward to answer the question. In other words, this isn’t really a reply to you, since I generally don’t waste time talking to twits. If you want to participate honestly in the discussion, then I’m happy to talk with you, but drop the attitude.The origin and maintenance of sexual reproduction is an area of active research. That means that the question has not been satisfactorily answered as yet (i.e., no concensus has developed). One of the problems is that sexual reproduction - or something resembling it - apparently evolved way down at the base of the organismal tree. For instance there are several haloarchea (eg., Halorubrum) which use recombination - i.e., sexual reproduction - in the full sense of the word. There are also several single-celled eukaryotes that also use actual recombination (such as Plasmodium) during reproduction. Finally, in one sense bacterial conjugation - where genetic material is transferred between one type of bacteria and another via an exchange of plasmids - can be considered a different form of “sexual reproduction” that evolved in a completely different domain of life. In other words, not only is sexual reproduction not limited to modern multi-celled organisms, but it apparently started evolving very shortly after life itself appeared.So how did all the hanky-panky get started? There are several reasonable hypotheses (Catholic Scientist provided a wiki quote covering a number of them way back in message 5 - to which I notice you didn’t bother to reply). In general, the hypotheses can be divided into genetic and ecological explanations. CS’s wiki quote covered most of the genetic ones - if you have any questions on those, I’ll try and answer them although I’m far from an expert in genetics.I personally prefer the ecological approach. Not, I hasten to add, because the answer is intrinsically better than the more purely genetic approaches, but rather because from training and experience I have a tendency (my friends sometimes say “a purblind determination” ) to view most questions in biology in that light. The most compelling of these explanations derive from an application of what is known as the Red Queen Hypothesis (with a tip o’ the hat to Matt Ridley, who first applied the term from its broader meaning to the specific case of sexual reproduction). For those of you who are unfamiliar with the term, it derives from Louis Carroll’s wonderful book Through the Looking Glass, where Alice met the Red Queen who said, “In here, it takes all the running you can do to stay in the same place.” In essence, it is a description of the co-evolutionary arms race between a predator and its prey (or a parasite and its host). In this race, the key selection pressure on each population is its counterpart in the relationship. Simplistically, as a f’rinstance, as a predator becomes more successful in catching and killing it’s prey, the prey population may suffer a decline to a point where rare individuals with a more effective strategy or physical ability to escape come to predominate (i.e., the population is said to have adapted to the new capability of its predator), thus leading to a decline in the predator population until a more successful strategy or physical ability comes to predominate, and so the cycle begins anew. Obviously there are limitations on how far this co-evolution can continue, primarily because the predator-prey relationship isn’t the only selection pressure on the two populations. Eventually something resembling an equilibrium is likely to develop, although there will continue to be fluctuations around this equilibrium.One of the really interesting aspects of the Red Queen, and how I see it applying to the evolution of sex, is the parasite-host relationship (this isn’t the hypothesis’ only possible application to the question, but I think it is one of the easiest to illustrate, and has the advantage of having some good studies to back it up). Take a hypothetical population of clonal organisms with genotype A. If a parasite manages to infect this genotype, it will by definition be able to infect every single individual carrying the genotype. Because they’re clonal, the only thing the organisms in this case can do is to wait for the really rare beneficial mutation to appear in its lineage. Since the particular mutation may or may not appear, the population may be doomed to extinction. Let’s say the mutation DID appear, creating genotype B for instance, now we have type A in serious decline due to its parasite load in comparison to B (which for the moment is running around parasite-free). As A declines, its parasite also declines, creating selection pressure for it to adapt to the now dominant type B. If/when it does, B starts declining, A is already infected and in decline, so the clones have to come up with a genotype C, putting pressure on the parasite to adapt to C, and the Red Queen rollercoaster is well and truly underway.Now, what happens if our host population is capable of throwing up variation much faster than the parasite can? Maybe even in a single generation? Well, the parasite is now in serious trouble. This is what recombination through sexual reproduction does. It is a much faster way of creating variation than waiting around for a lucky mutation. Beginning with a simple exchange of genetic material (i.e., as in bacterial combination), as organisms and their parasites got more complex - and the inter-relations between them got more complex - the Red Queen refined the simple exchange of genetic material into something more closely resembling actual sexual reproduction. From there, it’s only a matter of continuing to refine this adaptation to where in some species we now have two genders.For those interested, here are several articles that support this issue:Dybdahl M.F, Lively C.M. 1998, “Host-parasite coevolution: evidence for rare advantage and time-lagged selection in a natural population” Evolution.;52:1057-1066;Ladle R.J. 1992 “Parasites and sex: catching the Red Queen” Trends Ecol. Evol.7:405-408;Lively C.M, Craddock C, Vrijenhoek R.C. 1990 “Red Queen hypothesis supported by parasitism in sexual and clonal fish.” Nature. 344:864-867;Moritz C, McCallum H, Donnellan S, Roberts J.D. 1991 “Parasite loads in aparthenogenetic and sexual lizards (Heteronotia binoei): support for the Red Queen hypothesis.” Proc. R. Soc. B. 244:145-149;Radtkey R.R, Becker B, Miller R.D, Riblet R, Case T.J. 1996 “Variation and evolution of class I MHC in sexual and parthenogenetic geckos” Proc. R. Soc. B. 263:1023-1032;Tobler M, Schlupp I, 2005 “Parasites in sexual and asexual mollies (Poecilia, Poeciliidae, Teleostei): a case for the Red Queen?” Biol Lett. 1(2): 166-168.