Why we do it

 作者:杞副     |      日期:2019-03-08 08:11:02
By Jon Copley WHY have sex? Pooling your genes with someone else’s seems like a waste of your genetic potential, so the question has puzzled evolutionary biologists for decades. But now researchers think they have the answer. Sex may allow organisms to break a “speed limit” that restricts the rate of evolution of asexual species. From a selfish perspective, transmitting all your genes to your offspring through asexual reproduction seems a better prospect than just passing on half of them. Also, populations without separate sexes can grow twice as fast as those with males and females (see “Sisters are doing it for themselves”, New Scientist, 17 August 1996, p 32). So sex must have something else going for it. Now, in studies of Escherichia coli bacteria, which reproduce asexually, Arjan de Visser of Wageningen Agricultural University in the Netherlands and his colleagues have shed some light on what this is. Populations of bacteria usually include a few individuals with higher than normal mutation rates. These “mutator” bacteria can provide beneficial mutations that allow the population to respond rapidly to selection. But de Visser suspected that increasing the supply of mutations does not always increase the rate of evolution. To test this, the researchers increased the mutation rate of E. coli by introducing genes that code for faulty DNA repair enzymes. They then allowed several different-sized populations of these bacteria to evolve for 1000 generations in conditions where glucose—an important food molecule—was in short supply. The team sampled the populations on several occasions during the experiment and gauged how well they were adapting to the conditions by making them compete against their ancestors. Increasing the mutation rate did increase the supply of beneficial mutants and speed up the rate of adaptation. But the researchers found that large populations did not adapt as rapidly as small ones (Science, vol 283, p 404). De Visser suggests that when a population is awash with beneficial mutants, as in a large population with a high mutation rate, the mutants end up competing against each other until only one remains. This slows the process of adaptation, setting an evolutionary “speed limit”. But competition between beneficial mutants is a problem only for asexually reproducing species. “In a sexual population, competing beneficial mutants can be brought together,” says de Visser. He thinks this allows organisms that reproduce sexually to break the evolutionary speed limit that constrains asexual species. “It is a valuable contribution,” says Rick Michod,