Most mutations are bad. After all, randomly changing some piece of an organism's DNA is far more likely to mess up something that was working just fine than it is to improve upon something that wasn't. Every once in a while, however, some random change is actually beneficial and makes an organism better at doing whatever it is that particular organism does. One possible benefit of sexual reproduction is the ability to combine these rare but beneficial mutations more rapidly.
Imagine a couple of beneficial mutations; call them mutation A and mutation B. Both are rare, but they do occasionally occur. Either of the mutations makes the organisms more fit than the organisms without the mutations, but having both is better still.
Sex makes it easier for the two mutations to end up in the same organism. If an individual with mutation B mates with an individual with mutation A, some of the offspring should end up with both beneficial mutations. If this population were an asexual one, each individual would be reproducing in a clonal fashion, and the only way a lineage with mutation A would end up with mutation B (or vice-versa) would be if that mutation occurred in that lineage.
Studying snails and worms
Curt Lively and coworkers tested the theory that sex produces parasite-resistant offspring by using a freshwater snail that lives in New Zealand and is parasitized by a trematode worm. Through their investigation, the researchers provide strong support for the theory that sexual reproduction is advantageous in the presence of parasites.
Lively chose this particular snail because the species has both sexual and asexual forms. The species can reproduce sexually, with a female mating with a male to produce sexual female and male snails. Sometimes, however, offspring have an extra set of chromosomes and are triploidinstead of diploid, which means that they have three sets of chromosomes instead of just two sets. The triploid snails appear to be similar to the diploid snails in every respect except one: They don't need to mate to reproduce. That means that all the triploid snails are females that reproduce asexually, creating identical triploid daughters.
In any given lake, both sexual and asexual snails coexist. The researchers used genetic techniques to determine that many different clones often coexisted in the same location. Each of these clones was the result of a separate instance of sexual reproduction between two diploid snails that resulted in a triploid offspring. Because the different clones have different genes, it's reasonable to assume that one or more of these clones would be better at doing all the things that snails do. The better, more fit clones may outcompete all the others in the short run, but they clearly have not eliminated all the other clones nor the sexual snails.
Lively and company were ready to test some of the specific predictions of the theory that the presence of the parasite was responsible for maintaining sexual reproduction in this system.
Prediction 1: Sexual individuals should be more common in locations that have more parasites
Measuring the density of parasites and the frequency of sexual reproduction at many lakes throughout New Zealand, Lively and company found that as the density of parasites increased the frequency of sexual reproduction increased as well. They also measured the frequency of sexually reproducing snails in both shallow and deep areas within individual lakes and found the same thing: As the density of parasites increased, the proportion of snails that reproduced sexually increased.
Prediction 2: If parasites are adapting to infect their hosts better, parasites should be better at infecting the host snails from their own lake rather than the snails from other lakes
The researchers performed two sets of experiments to test this prediction. They collected parasites and snails from two lakes on opposite sides of the southern New Zealand Alps and brought them back to the laboratory. (The distance between the lakes made it unlikely that the snails or their parasites had ever encountered each other in nature.) Then they measured the degree to which the parasites from each lake were able to infect snails from the two lakes. They found that for each lake, the parasites were better able to infect the coexisting snails than the snails from the other side of the mountains.
Next, they chose three lakes that were much closer to each other — close enough that ducks could easily fly between them, transporting the parasitic worms from one lake to the next. They again collected worms and snails from the lakes and brought them back to the laboratory to measure the ability of the parasites to infect snails from their own lake as well as the other two. In all three cases, they found that the parasites were better able to infect the snails from their own lake, and the effect was strong enough that it was not overwhelmed by the genetic mixing that could be occurring among the three lakes.
Prediction 3: The genotypes that were most common in the past should be most susceptible to the current parasites
In testing this prediction, the researchers focused on the clonal, asexual snails. For their test subjects, they selected clones that possessed the clonal genotypes most common in previous years. Using these clones, they performed two sets of experiments.
They tested the susceptibility of clones from the recent past to the parasitic worms presently inhabiting the lakes from which the clones were collected. What they found was that the clones that had been common in the past were more readily attacked than the clones that had been rare.
If you're a worm, you're well served by having characteristics (naturally selected, of course) that make you better able to overcome the defenses of the most common clone genotypes. A parasite better able to attack these clones would leave more descendents (because there are more of these "common" snails than the others) and therefore would increase faster than those of parasites attacking snails with rare types.
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