Sex produces parasite-resistant offspring, making sexual organisms better able to adapt to changing parasitic environment than asexual organisms are.
Sexual reproduction produces variable offspring (offspring that are not genetically identical to their parents), whereas asexual reproduction doesn't. Asexual females produce daughters, granddaughters, and so on that are genetically identical to Mom.
Asexual reproduction is also called clonal reproduction, and all the resulting descendants of a single asexual female are referred to collectively as a clone. All the members of a clone are identical except for possible rare random mutations that may have occurred in DNA during the reproductive process. Sexual females, although they produce only half as many daughters, produce daughters that aren't identical to Mom or to one another.
Evolutionary biologists since Darwin have pondered the possible advantages of producing variable offspring. The first argument — that because natural selection requires variation on which to act, having variable offspring makes it more likely that your descendants will evolve faster than the other critters — seems like a no-brainer until you consider the following:
i The parent sexual organism clearly has a very fit combination of genes already. It's survived, found food, dodged predators, and found a mate; now it's reproducing.
i Sexual reproduction is dicey — you're more likely to break up a perfectly good set of genes with a proven track record in the current environment than hit on an even more fit combination.
So why take the gamble?
The key words here are current environment. In the current environment, this sexual organism does just fine. But what happens when the environment changes? This year's very fit gene combinations won't necessarily be very fit next year. The evolutionary interaction between parasites and their hosts can lead to such year-to-year changes in the hosts' environment.
The parasites experience selection pressure to infect their hosts better, whereas the hosts experience selection pressures to better resist infection by their parasites. In this sort of system, the genes of last year's really fit host won't be really fit next year, because the parasites will have evolved to infect it better.
With parasites evolving to better infect their hosts, an asexual host will be in trouble, because all its offspring will be exact copies of itself, but they will have to fend off better-adapted parasites. A sexual host, on the other hand, produces variable offspring with gene combinations the parasite population has not yet had a chance to adapt to. For this reason, the offspring of sexually reproducing organisms are less susceptible to attack by the parasites. So although the parent organism may not fare too well with new or better-adapted parasites, its offspring will.
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