Anderson Jill T, Eckhart Vincent M, Geber Monica A
Department of Genetics, University of Georgia, Athens, Georgia, 30602.
Department of Biology, Grinnell College, Grinnell, Iowa, 50112.
Evolution. 2015 Sep;69(9):2249-61. doi: 10.1111/evo.12745. Epub 2015 Aug 27.
Sister taxa with distinct phenotypes often occupy contrasting environments in parapatric ranges, yet we generally do not know whether trait divergence reflects spatially varying selection. We conducted a reciprocal transplant experiment to test whether selection favors "native phenotypes" in two subspecies of Clarkia xantiana (Onagraceae), an annual plant in California. For four quantitative traits that differ between subspecies, we estimated phenotypic selection in subspecies' exclusive ranges and their contact zone in two consecutive years. We predicted that in the arid, pollinator-scarce eastern region, selection favors phenotypes of the native subspecies parviflora: small leaves, slow leaf growth, early flowering, and diminutive flowers. In the wetter, pollinator-rich, western range of subspecies xantiana, we expected selection for opposite phenotypes. We investigated pollinator contributions to selection by comparing naturally pollinated and pollen-supplemented individuals. For reproductive traits and for subspecies xantiana, selection generally matched expectations. The contact zone sometimes showed distinctive selection, and in ssp. parviflora selection sometimes favored nonnative phenotypes. Pollinators influenced selection on flowering time but not on flower size. Little temporal variation in selection occurred, possibly because of plastic trait responses across years. Though there were exceptions and some causes of selection remain obscure, phenotypic differentiation between subspecies appears to reflect spatially variable selection.
具有不同表型的姊妹分类群通常在相邻分布区域占据截然不同的环境，但我们通常并不清楚性状差异是否反映了空间变化的选择作用。我们进行了一项 reciprocal transplant 实验，以测试在加州的一种一年生植物——黄花克拉花（Onagraceae）的两个亚种中，选择作用是否有利于“本地表型”。对于亚种间存在差异的四个数量性状，我们连续两年在各亚种的专属分布区域及其接触带估计了表型选择。我们预测，在干旱、传粉者稀少的东部地区，选择作用有利于本地亚种小花克拉花的表型：小叶、叶片生长缓慢、早花和小花。在亚种黄花克拉花分布的更湿润、传粉者丰富的西部地区，我们预计选择作用会偏向相反的表型。我们通过比较自然授粉个体和补充花粉的个体，研究了传粉者对选择作用的贡献。对于繁殖性状以及亚种黄花克拉花，选择作用总体上符合预期。接触带有时表现出独特的选择作用，而在小花克拉花亚种中，选择作用有时有利于非本地表型。传粉者影响了对开花时间的选择，但对花的大小没有影响。选择作用几乎没有时间上的变化，这可能是由于多年间性状的可塑性反应。尽管存在例外情况，且一些选择作用的原因仍不明确，但亚种间的表型分化似乎反映了空间变化的选择作用。
