Because you get to read (and write) sentences like this "A long-standing enigma in evolutionary biology is the widespread occurence of sex, in spite of its twofold cost" (Galvani 2003). I assume, of course, that the "costs" are the energy expenditures required to maintain two separate sexes and find mates in order to produce offspring, plus not being able to pass on all of your genes (where as asexual organisms don't have to worry about those things). Now, before I take all the fun out of sex, sexual organisms do have genetic recombination, so they give their offspring more chances of surviving in more diverse environments . Some organisms are hermaphroditic, so they can self-fertilize. Also, studies seem to indicate that parthenogenic lineages have reduced chances of extinction (perhaps due to the adaptibility thing mentioned earlier). Plus, the genetic recombination that occurs during meiosis in sexually-reproducing organisms helps create offspring with not quite all the mutations of mommy and daddy combined (just some). Whereas, if you produce asexually, your kids are getting all of your mutations, good or bad. Anyway, I think that's more than two, so I'll stop now. If you want to read the article (which is really about virus mutation and virulence in humans, also interesting), go here.
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