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ScienceWeek
EVOLUTION: ON THE EVOLUTION OF SEXUALITY
The following points are made by Rasmus Nielsen (Science 2006 311:960):
1) 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[1] now supports this theory.
2) 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 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].
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.
4) 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. The new work 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.
References (abridged):
1. S. Paland, M. Lynch, Science 311, 990 (2006).
2. J. Maynard-Smith, The Evolution of Sex (Cambridge Univ. Press, Cambridge, UK, 1978).
3. A. S. Kondrashov, Nature 336, 435 (1988)
4. W. D. Hamilton et al,. Proc. Natl. Acad. Sci. U.S.A. 87, 3566 (1990)
5. D. J. Begun, C. F. Aquadro, Nature 356, 519 (1992)
Science http://www.sciencemag.org
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Related Material:
EVOLUTIONARY BIOLOGY: SEXUAL SELECTION AND SPECIATION
The following points are made by K. Kraaijeveld and A. Pomiankowski (Current Biology 2004 14:R419):
1) When Charles Darwin (1809-1882) [1] proposed his theory of sexual selection he was concerned mainly with explaining the widespread occurrence of exaggerated sexual ornaments and courtship displays, as these traits could not easily be explained by natural selection. He also noted that taxonomic groups with more pronounced sexual ornaments tended to have more species. This suggests that sexual selection may elevate the rate at which populations diversify and give rise to new species. A new study [2] of female mate preferences in five populations of an East African cichlid species strongly supports the connection between sexual selection and speciation.
2) With the surge of interest in sexual selection over the past few decades, the question of whether it can lead to speciation has also enjoyed renewed attention. A plethora of theoretical models have investigated the connection, and generally concluded that sexual selection can promote speciation (3). The main evolutionary mechanism proposed invokes the rapid coevolution of female mate preferences and male courtship traits, leading to reproductive isolation between groups of individuals. However, empirical evidence in support of the idea is scarce.
3) An indirect way this idea has been tested involves looking across broad taxonomic groups for a link between the strength of sexual selection and species number. So far, the evidence from these studies has been conflicting. In birds for example, taxa with greater sexual differences in plumage color -- an indicator of sexual selection -- have higher species numbers compared to sister taxa subject to weaker sexual selection [4,5]. However, surveys in other groups (butterflies, mammals, and spiders) have failed to find such an association, and the positive result in birds has not been replicated in a recent reanalysis. It seems premature to conclude from this that speciation is independent of sexual selection. One reason for the lack of a strong linkage is that sexual selection may promote extinction as well as speciation, if it leads to the evolution of traits maladaptive to male and female survival. Another is that sexual selection can even retard speciation under certain conditions. So in the long term, species numbers may only loosely be connected to sexual selection.
4) A more direct way of investigating the connection between sexual selection and speciation is to examine its action in closely related populations. Knight and Turner [2] attempted such a test using populations of the cichlid fish Pseudotropheus zebra from Lake Malawi. The cichlid fishes of the East African lakes, in particular Lake Victoria and Lake Malawi, are renowned for rampant speciation over a very brief period of time -- more than 1000 species have been generated in less than a million years. Some of this diversity is due to ecological specialization, facilitated by the "key innovation" of the cichlid pharyngeal jaw. But many closely related species show practically no differences except in male color, suggesting that sexual selection may be an important additional mechanism of speciation.
References (abridged):
1. Darwin, C.R. (1871). The Descent of Man and Selection in Relation to Sex. (London: John Murray)
2. Knight, M.E. and Turner, G.F. (2004). Laboratory mating trials indicate incipient speciation by sexual selection among populations of the cichlid fish Pseudotropheus zebra from Lake Malawi. Proc. R. Soc. Lond. B 271, 675-680
3. Turelli, M., Barton, N.H., and Coyne, J.A. (2001). Theory and speciation. Trends Ecol. Evol. 16, 330-343
4. Barraclough, T.G., Harvey, P.H., and Nee, S. (1995). Sexual selection and taxonomic diversity in passerine birds. Proc. R. Soc. Lond. B 259, 211-215
5. Owens, I.P.F., Bennett, P.M., and Harvey, P.H. (1999). Species richness among birds: body size, life history, sexual selection or ecology?. Proc. R. Soc. Lond. B 266, 933-939
Current Biology http://www.current-biology.com
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ON EVOLUTION AND SEXUAL REPRODUCTION
The following points are made by Richard E. Lenski (Science 2001 294:533):
1) Why have some organisms evolved the capacity for sexual reproduction, whereas others make do with reproducing asexually? Since the time of August F. Weismann (1834-1914), most biologists have been taught that sex produces variation and thereby promotes evolutionary adaptation. But how does sex achieve this effect, and under what circumstances is it worthwhile?
2) The traditional explanation for sex is that it accelerates adaptation by allowing two or more beneficial mutations that have appeared in different individuals to recombine within the same individual. Without sexual recombination, individual clones that possess different beneficial mutations compete with one another, slowing adaptation by clonal interference. Sex, according to the traditional explanation, allows simultaneous improvements at several genetic loci, whereas multiple adaptations must occur sequentially in clonal organisms.
3) The above explanation, however, has recently come into question. First, sex imposes a 50 percent reduction in reproductive output: If a female can produce viable offspring on her own, why dilute her genetic contribution to subsequent generations by mating with a male? Second, the circumstances under which this kind of model provides sufficient advantage to offset the cost of sex are restrictive, requiring certain forms of selection and environmental fluctuations. Third, alternative models propose that the advantage of sex lies in eliminating deleterious mutations rather than in combining beneficial mutations. Still another hypothesis, involves an interplay between deleterious and beneficial mutations. Finally, empirical tests of these hypotheses have so far failed to produce a clear winner, so the field is ripe for significant experiments.
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