Morrow Edward H, Pitcher Trevor E
Uppsala University, Evolutionary Biology Centre, Norbyvägen 18D, SE-752 36 Uppsala, Sweden.
Proc Biol Sci. 2003 Sep 7;270(1526):1793-9. doi: 10.1098/rspb.2003.2441.
The relationship between sexual selection and extinction risk has rarely been investigated. This is unfortunate because extinction plays a key role in determining the patterns of species richness seen in extant clades, which form the basis of comparative studies into the role that sexual selection may play in promoting speciation. We investigate the extent to which the perceived risk of extinction relates to four different estimates of sexual selection in 1030 species of birds. We find no evidence that the number of threatened species is distributed unevenly according to a social mating system, and neither of our two measures of pre-mating sexual selection (sexual dimorphism and dichromatism) was related to extinction risk, after controlling for phylogenetic inertia. However, threatened species apparently experience more intense post-mating sexual selection, measured as testis size, than non-threatened species. These results persisted after including body size as a covariate in the analysis, and became even stronger after controlling for clutch size (two known correlates of extinction risk). Sexual selection may therefore be a double-edged process-promoting speciation on one hand but promoting extinction on the other. Furthermore, we suggest that it is post-mating sexual selection, in particular, that is responsible for the negative effect of sexual selection on clade size. Why this might be is unclear, but the mean population fitness of species with high intensities of post-mating sexual selection may be especially low if costs associated with multiple mating are high or if the selection load imposed by post-mating selection is higher relative to that of pre-mating sexual selection.
性选择与灭绝风险之间的关系鲜有研究。这很遗憾，因为灭绝在决定现存类群中物种丰富度的模式方面起着关键作用，而现存类群是对性选择在促进物种形成中可能发挥的作用进行比较研究的基础。我们研究了1030种鸟类中感知到的灭绝风险与四种不同性选择估计值之间的关联程度。我们没有发现证据表明受威胁物种的数量根据社会交配系统分布不均，并且在控制了系统发育惯性后，我们的两种交配前性选择指标（两性异形和雌雄异色）均与灭绝风险无关。然而，以睾丸大小衡量，受威胁物种显然比非受威胁物种经历更强烈的交配后性选择。在分析中纳入体型作为协变量后，这些结果依然存在，在控制了窝卵数（两个已知的灭绝风险相关因素）后，结果甚至更强。因此，性选择可能是一个双刃剑过程——一方面促进物种形成，另一方面促进灭绝。此外，我们认为特别是交配后性选择导致了性选择对类群大小的负面影响。原因尚不清楚，但如果与多次交配相关的成本很高，或者如果交配后选择施加的选择负荷相对于交配前性选择更高，那么交配后性选择强度高的物种的平均种群适合度可能会特别低。
