Department of Biology, Life Sciences Centre, Dalhousie University, 1459 Oxford Street, Halifax, Nova Scotia, B3H 4R2, Canada.
New Phytol. 2012 Mar;193(4):1039-1048. doi: 10.1111/j.1469-8137.2011.04013.x. Epub 2012 Jan 6.
Measures of selection on floral traits in flowering plants are often motivated by the assumption that pollinators cause selection. Flowering plants experience selection from other sources, including herbivores, which may enhance or oppose selection by pollinators. Surprisingly, few studies have examined selection from multiple sources on the same traits. We quantified pollinator-mediated selection on six floral traits of Lobelia cardinalis by comparing selection in naturally and supplementally (hand-) pollinated plants. Directional, quadratic and correlational selection gradients as well as total directional and quadratic selection differentials were examined. We used path analysis to examine how three herbivores--slugs, weevils and caterpillars--affected the relationship between floral traits and fitness. We detected stronger total selection on four traits and correlational selection (γ(ij)) on three trait combinations in the natural pollination treatment, indicating that pollinators caused selection on these traits. Weak but statistically significant selection was caused by weevil larvae on stem diameter and anther-nectary distance, and by slugs on median-flower date. In this study, pollinators imposed stronger selection than herbivores on floral traits in L. cardinalis. In general, the degree of pollen limitation and rate of herbivory are expected to influence the relative strength of selection caused by pollinators or herbivores.
植物传粉者通常被认为是引起花部特征选择的主要因素，因此，对开花植物花部特征的选择度量多是基于这一假设。然而，植物还会受到其他来源的选择，包括食草动物，这些选择可能会增强或对抗传粉者的选择。令人惊讶的是，很少有研究同时检验来自多个来源的对同一性状的选择。我们通过比较自然授粉和人工授粉（手动）植株的 6 个风铃草属花部特征，量化了传粉者介导的选择。我们检验了定向选择、二次选择和相关选择梯度以及总定向选择和二次选择差。我们使用路径分析来检验三种食草动物——蛞蝓、象鼻虫和毛毛虫——如何影响花部特征与适合度之间的关系。我们发现，在自然授粉处理中，四个特征的总选择更强，三个特征组合的相关选择（γ（ij））更强，这表明传粉者对这些特征施加了选择。象鼻虫幼虫对茎直径和花药蜜腺距离以及蛞蝓对中间花日期也产生了微弱但具有统计学意义的选择。在这项研究中，传粉者对风铃草属花部特征的选择强于食草动物。一般来说，花粉限制的程度和食草动物的发生率预计会影响传粉者或食草动物引起的选择的相对强度。
