Ashman T L, Diefenderfer C
Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
Am Nat. 2001 Mar;157(3):334-47. doi: 10.1086/319192.
We explored the idea that sex ratio represents a unique context for selection on attractive traits by manipulating sex ratio and pollinator abundance in experimental populations of a gender-dimorphic wild strawberry Fragaria virginiana. We found that increasing the frequency of functional males (the pollen-bearing morph) increased the frequency of pollen-collecting syrphid flies in the pollinator assemblage, decreased pollinator visitation to less preferred morph (females), and decreased the degree of pollen limitation of females. Moreover, sex ratio influenced the strength of selection on petal size through female fitness but did not alter the strength of selection through male fitness components, suggesting that sex ratio can alter the gender bias of selection on an attractive trait. This study of context-dependent selection has important implications for the evolution of sexual dimorphism in attractive traits. First, it suggests that only certain conditions generate male-biased selection and, thus, could lead to selection-driven male-biased petal size dimorphism. Second, it suggests that flexible pollinator foraging may be an important mechanism by which sex ratio influences selection on attractive traits. Finally, it implies that variation in sex ratio could limit the evolution of sexual dimorphism and/or could maintain genetic variation in attractive traits.
我们通过在两性异形的野生草莓弗州草莓（Fragaria virginiana）的实验种群中操纵性别比例和传粉者数量，探讨了性别比例代表了对有吸引力性状进行选择的独特背景这一观点。我们发现，增加功能性雄株（产生花粉的形态）的频率会增加传粉者组合中采集花粉的食蚜蝇的频率，减少传粉者对较不受欢迎形态（雌株）的访问，并降低雌株的花粉限制程度。此外，性别比例通过雌性适合度影响对花瓣大小的选择强度，但并未改变通过雄性适合度成分的选择强度，这表明性别比例可以改变对有吸引力性状的选择的性别偏向。这项关于背景依赖选择的研究对有吸引力性状的两性异形进化具有重要意义。首先，它表明只有某些条件会产生雄性偏向的选择，因此可能导致选择驱动的雄性偏向的花瓣大小二态性。其次，它表明灵活的传粉者觅食可能是性别比例影响对有吸引力性状选择的重要机制。最后，它意味着性别比例的变化可能会限制两性异形的进化和/或维持有吸引力性状的遗传变异。
