Why Sex?
Science 17 February 2006:Vol. 311. no. 5763, pp. 960 - 961DOI: 10.1126/science.1124663
Why sex? This has been one of the most fundamental questions in evolutionary 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 reproduction). Parthenogenetically reproducing females arising in a sexual population should have a twofold fitness advantage because they, on average, 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 associated with a reduction in fitness? The main solution 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 efficiently. 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 efficiently 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 population, 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. Recombination speeds up the rate of adaptive evolution because it allows several beneficial mutations 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 broken parts to be shuffled among chromosomes, allowing new combinations to arise for selection to act on. Under suitable assumptions regarding the way deleterious mutations affect organismal fitness, the advantage of recombination in eliminating deleterious mutations can outweigh the twofold cost of sex (3).
However, the selection theories are not free of contradictions and problems. Some of them rely on so-called group-selection arguments, where adaptive properties are properties of a whole population and not of individuals. If sexually reproducing individuals and their offspring do not have an immediate selective advantage in otherwise asexual populations, it is hard to see how populations can ever evolve from asexual to sexual reproduction. Additionally, the best explanations regarding deleterious mutations rely on strong assumptions regarding the distribution of selective effects (3), and there may be other factors favoring sex, such as increased resistance to pathogens (4). An observed genomic correlation between the rate of recombination and variability within species (5) suggests that there is an interaction between selection and recombination, but a direct difference between sexual and asexual populations has been hard to establish.
However, the new study by Paland and Lynch (1) provides direct empirical support for an excess accumulation of mutations in asexually reproducing populations compared to sexual populations. They examined different populations of the small crustacean Daphnia pulex, a type of water flea. Daphnia are excellent organisms to study in this regard because parthenogenetic Daphnia populations have arisen multiple times from sexual populations. Comparing asexual and sexual populations of Daphnia is, therefore, the perfect tool for examining the population genetic consequences of sexual reproduction (see the figure).
Paland and Lynch (1) compared the number of mutations with possible functional effects (nonsynonymous mutations) to the number of mutations with no functional effects (synonymous mutations) in 14 asexual and 14 sexual Daphnia populations. They observed a clear excess of nonsynonymous mutations in the asexual populations. They also estimated that close to 90% of the nonsynonymous mutations were subject to selection. These results suggest that the asexually reproducing species carry a higher load of deleterious mutations and that selection is not as efficient in the asexual as in the sexual populations. It does not directly demonstrate that selection against deleterious mutations is what maintains sexual reproduction, but the results do confirm the most important component of the selection theory: Asexual reproduction leads to an accumulation of deleterious mutations. It seems that males are allowed to exist after all, because they help females get rid of deleterious mutations.
The study is also interesting from another point of view. The estimate of the proportion of new mutations in Daphnia that are under selection is fairly high (>90%). Over the past 30 years, the paradigmatic theory in molecular evolution has been the Neutral Theory (6), which assumes that the vast majority of genetic polymorphisms have little or no selection acting upon them. However, the study by Paland and Lynch (1), and other recent studies (7, 8), suggest instead that many or most polymorphisms may be under selection. Slowly, our weltanschauung in evolutionary biology is changing from a static view of a largely optimized genome to a dynamic view of organisms constantly challenged by selection and struggling with the large genetic load imposed by deleterious and new advantageous mutations segregating in the population.
Why sex? This has been one of the most fundamental questions in evolutionary 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 reproduction). Parthenogenetically reproducing females arising in a sexual population should have a twofold fitness advantage because they, on average, 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 associated with a reduction in fitness? The main solution 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 efficiently. 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 efficiently 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 population, 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. Recombination speeds up the rate of adaptive evolution because it allows several beneficial mutations 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 broken parts to be shuffled among chromosomes, allowing new combinations to arise for selection to act on. Under suitable assumptions regarding the way deleterious mutations affect organismal fitness, the advantage of recombination in eliminating deleterious mutations can outweigh the twofold cost of sex (3).
However, the selection theories are not free of contradictions and problems. Some of them rely on so-called group-selection arguments, where adaptive properties are properties of a whole population and not of individuals. If sexually reproducing individuals and their offspring do not have an immediate selective advantage in otherwise asexual populations, it is hard to see how populations can ever evolve from asexual to sexual reproduction. Additionally, the best explanations regarding deleterious mutations rely on strong assumptions regarding the distribution of selective effects (3), and there may be other factors favoring sex, such as increased resistance to pathogens (4). An observed genomic correlation between the rate of recombination and variability within species (5) suggests that there is an interaction between selection and recombination, but a direct difference between sexual and asexual populations has been hard to establish.
However, the new study by Paland and Lynch (1) provides direct empirical support for an excess accumulation of mutations in asexually reproducing populations compared to sexual populations. They examined different populations of the small crustacean Daphnia pulex, a type of water flea. Daphnia are excellent organisms to study in this regard because parthenogenetic Daphnia populations have arisen multiple times from sexual populations. Comparing asexual and sexual populations of Daphnia is, therefore, the perfect tool for examining the population genetic consequences of sexual reproduction (see the figure).
Paland and Lynch (1) compared the number of mutations with possible functional effects (nonsynonymous mutations) to the number of mutations with no functional effects (synonymous mutations) in 14 asexual and 14 sexual Daphnia populations. They observed a clear excess of nonsynonymous mutations in the asexual populations. They also estimated that close to 90% of the nonsynonymous mutations were subject to selection. These results suggest that the asexually reproducing species carry a higher load of deleterious mutations and that selection is not as efficient in the asexual as in the sexual populations. It does not directly demonstrate that selection against deleterious mutations is what maintains sexual reproduction, but the results do confirm the most important component of the selection theory: Asexual reproduction leads to an accumulation of deleterious mutations. It seems that males are allowed to exist after all, because they help females get rid of deleterious mutations.
The study is also interesting from another point of view. The estimate of the proportion of new mutations in Daphnia that are under selection is fairly high (>90%). Over the past 30 years, the paradigmatic theory in molecular evolution has been the Neutral Theory (6), which assumes that the vast majority of genetic polymorphisms have little or no selection acting upon them. However, the study by Paland and Lynch (1), and other recent studies (7, 8), suggest instead that many or most polymorphisms may be under selection. Slowly, our weltanschauung in evolutionary biology is changing from a static view of a largely optimized genome to a dynamic view of organisms constantly challenged by selection and struggling with the large genetic load imposed by deleterious and new advantageous mutations segregating in the population.
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