Head Megan L, Lindholm Anna K, Brooks Robert
School of Biological, Earth and Environmental Sciences, The University of New South Wales, Australia.
Evolution. 2008 Jan;62(1):135-44. doi: 10.1111/j.1558-5646.2007.00277.x. Epub 2007 Dec 6.
Demographic parameters including operational sex ratio (OSR) and population density may influence the opportunity for, and strength of sexual selection. Traditionally, male-biased OSRs and high population densities have been thought to increase the opportunity for sexual selection on male sexual traits due to increased male competition for mates. Recent experimental evidence, however, suggests that male-biased OSRs might reduce the opportunity for sexual selection due to increased sexual coercion experienced by females. How OSR, density, and any resultant changes in the opportunity for sexual selection actually affect selection on male sexual traits is unclear. In this study, we independently manipulated OSR and density in the guppy (Poecilia reticulata) without altering the number of males present. We recorded male and female behavior and used DNA microsatellite data to assign paternity to offspring and estimate male reproductive success. We then used linear selection analyses to examine the effects of OSR and density on directional sexual selection on male behavioral and morphological traits. We found that females were pursued more by males in male-biased treatments, despite no change in individual male behavior. There were no differences in sexual behavior experienced by females or performed by males in relation to density. Neither OSR nor density significantly altered the opportunity for sexual selection. Also, Although there was significant multivariate linear selection operating on males, neither OSR nor density altered the pattern of sexual selection on male traits. Our results suggest that differences in either OSR or density (independent of the number of males present) are unlikely to alter directional evolutionary change in male sexual traits.
包括操作性别比（OSR）和种群密度在内的人口统计学参数可能会影响性选择的机会和强度。传统上，由于雄性对配偶的竞争增加，雄性偏向的OSR和高种群密度被认为会增加对雄性性特征进行性选择的机会。然而，最近的实验证据表明，雄性偏向的OSR可能会由于雌性遭受的性胁迫增加而减少性选择的机会。OSR、密度以及性选择机会的任何由此产生的变化实际上如何影响对雄性性特征的选择尚不清楚。在这项研究中，我们在不改变雄性数量的情况下，独立操纵孔雀鱼（Poecilia reticulata）的OSR和密度。我们记录了雄性和雌性的行为，并使用DNA微卫星数据确定后代的父系身份并估计雄性的繁殖成功率。然后，我们使用线性选择分析来研究OSR和密度对雄性行为和形态特征的定向性选择的影响。我们发现，在雄性偏向的处理中，雄性对雌性的追逐更多，尽管个体雄性行为没有变化。雌性经历的性行为或雄性表现出的性行为在密度方面没有差异。OSR和密度都没有显著改变性选择的机会。此外，虽然对雄性有显著的多变量线性选择，但OSR和密度都没有改变对雄性特征的性选择模式。我们的结果表明，OSR或密度的差异（与存在的雄性数量无关）不太可能改变雄性性特征的定向进化变化。
