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ScienceWeek
ETHOLOGY: RELATIVE SIZE AND MATING BEHAVIOR
The following points are made by M. Andersson and J. Wallander (Nature 2004 431:139):
1) Five decades ago, the German evolutionary biologist Bernhard Rensch (1900-1990)(1) presented an intriguing rule for the differing sizes of male and female animals. He found that in several groups (clades) that each contain related species, male size relative to female size increases with the body size of the species. Rensch's rule has since been verified in animals as diverse as arthropods, reptiles, birds and mammals, including primates(2). The causes behind the rule, however, have remained unclear(2-5). Why are males much larger than females in many animals with large body size? And why, in the same clade, are males similar or even smaller than females in species with small body size(1,2)?
2) Szekely et al(6) carried out a comparative analysis of shorebirds that demonstrated such trends, and came up with some thought-provoking conclusions. They demonstrate that the trend in sexual size difference (SSD) can be explained by two aspects of sexual selection (which arises from competition over mates), and the interaction between them. One aspect is the strength of sexual selection involved; the other is the agility of the male's display. The outcome of the analysis is similar whether it is based on body mass or wing length as a measure of size.
3) Males of many animals compete in fights for mates, and such contests favor large body size. Through genetic correlations between the sexes(2,4,5), such competition may also lead to some increase in female body size, though less so than in males. In consequence, the mean size of each sex will increase, and so will the relative size difference between them. Increased male-biased SSD therefore tends to become associated with large body size. On the other hand, there are several smaller species in which sex roles are reversed and females compete strongly for males. Such species tend to have female-biased SSDs(5).
4) Some forms of male competition can favour smaller males. For example, in species where males compete by acrobatic aerial displays, there may be strong sexual selection for small male body size. For geometrically similar animals, agility increases with reduced body size, and this might lead to higher mating success of smaller males in certain birds. Through genetic correlations it may also lead to some reduction in female body size. This in turn can help to explain why female-biased SSD increases with reduced body size in some birds and other animals with agile male display(5).
References (abridged):
1. Rensch, B. Evolution Above the Species Level (Columbia Univ. Press, New York, 1959)
2. Fairbairn, D. J. Annu. Rev. Ecol. Syst. 28, 659-687 (1997)
3. Clutton-Brock, T. H. et al. Nature 269, 797-800 (1977)
4. Maynard Smith, J. The Evolution of Sex (Cambridge Univ. Press, 1978).
5. Andersson, M. Sexual Selection (Princeton Univ. Press, 1994).
6. Szekely, T., Freckleton, R. P. & Reynolds, J. D. Proc. Natl Acad. Sci. USA 101, 12224-12227 (2004)
Nature http://www.nature.com/nature
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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.
Science http://www.sciencemag.org
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