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Evolution_why_sex - PERSPECTIVES EVOLUTION Why Sex Rasmus...

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17 FEBRUARY 2006 VOL 311 SCIENCE www.sciencemag.org 960 PERSPECTIVES W hy sex? This has been one of the most fundamental questions in evolution- ary biology. In many species, males do not provide parental care to the offspring. Clearly, the rate of reproduction could be increased if all individuals were born as females and reproduced asexually without the need to mate with a male (parthenogenetic reproduc- tion). Parthenogenetically reproducing females arising in a sexual population should have a twofold fitness advantage because they, on aver- age, leave twice as many gene copies in the next generation. Nonetheless, sexual reproduction is ubiquitous in higher organisms. Why do all these species bother to have males, if males are associ- ated with a reduction in fitness? The main solu- tion that population geneticists have proposed to this conundrum is that sexual reproduction allows genetic recombination, and that genetic recombination is advantageous because it allows natural Darwinian selection to work more effi- ciently. New empirical evidence supporting this theory now comes from a study by Paland and Lynch on page 990 in this issue ( 1 ). One reason why selection works more effi- ciently in the presence of recombination—that is, the exchange of genetic material between chromosomes—is that selected mutations tend to interfere with each other in the absence of recombination ( 2 , 3 ). Imagine, for example, a beneficial mutation (A) arising in one individual and another beneficial mutation (B) arising in another gene in an individual that does not carry mutation A. In the absence of recombination, mutation B would be eliminated when mutation A reaches a frequency of 100% in the popula- tion, and vice versa. No individual carrying both beneficial mutations could be created, and only one of the mutations could eventually reach a frequency of one in the population. Recom- bination speeds up the rate of adaptive evolu- tion because it allows several beneficial muta- tions to be combined in the same individual. Likewise, when multiple deleterious mutations are present in the population, recombination has the potential for creating new offspring chromosomes with fewer deleterious mutations than either of the parental chromosomes. The famous population geneticist John Maynard- Smith compared this situation to having two cars: one with a broken engine and one with a broken transmission. Neither of them can run, but if you can replace the broken part in one car with a part from the other car you can produce a new functional car. Recombination allows bro- ken parts to be shuffled among chromosomes, allowing new combinations to arise for selec- tion to act on. Under suitable assumptions regarding the way deleterious mutations affect organismal fitness, the advantage of recombi- nation in eliminating deleterious mutations can outweigh the twofold cost of sex ( 3 ).
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