Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA, 93106, USA.
Department of Mathematics and Computer Science, Westmont College, Santa Barbara, CA, 93108.
Am J Bot. 2022 Nov;109(11):1757-1779. doi: 10.1002/ajb2.16016. Epub 2022 Aug 12.
The study of phenotypic divergence of, and selection on, functional traits in closely related taxa provides the opportunity to detect the role of natural selection in driving diversification. If the strength or direction of selection in field populations differs between taxa in a pattern that is consistent with the phenotypic difference between them, then natural selection reinforces the divergence. Few studies have sought evidence for such concordance for physiological traits.
Herbarium specimen records were used to detect phenological differences between sister taxa independent of the effects on flowering time of long-term variation in the climate across collection sites. In the field, physiological divergence in photosynthetic rate, transpiration rate, and instantaneous water-use efficiency were recorded during vegetative growth and flowering in 13 field populations of two taxon pairs of Clarkia, each comprising a self-pollinating and a outcrossing taxon.
Historically, each selfing taxon flowered earlier than its outcrossing sister taxon, independent of the effects of local long-term climatic conditions. Sister taxa differed in all focal traits, but the degree and (in one case) the direction of divergence depended on life stage. In general, self-pollinating taxa had higher gas exchange rates, consistent with their earlier maturation. In 6 of 18 comparisons, patterns of selection were concordant with the phenotypic divergence (or lack thereof) between sister taxa.
Patterns of selection on physiological traits measured in heterogeneous conditions do not reliably reflect divergence between sister taxa, underscoring the need for replicated studies of the direction of selection within and among taxa.
研究密切相关分类群中功能性状的表型分歧和选择，为检测自然选择在推动多样化中的作用提供了机会。如果野外种群中选择的强度或方向在与它们之间表型差异一致的模式下在分类群之间存在差异，那么自然选择就会加强分歧。很少有研究为生理特征的这种一致性寻找证据。
使用植物标本记录来检测姊妹分类群之间的物候差异，而不受收集地点长期气候变化对开花时间影响的影响。在野外，在 13 个 Clarkia 两个分类群对的 13 个野外种群中，记录了营养生长和开花期间光合作用速率、蒸腾速率和瞬时水分利用效率的生理差异，每个分类群对都包括自交和异交分类群。
从历史上看，每个自交分类群的开花时间都早于其异交姊妹分类群，而与当地长期气候条件的影响无关。姊妹分类群在所有焦点特征上都存在差异，但分歧的程度和（在一种情况下）方向取决于生活阶段。一般来说，自交分类群的气体交换率较高，与其较早的成熟度一致。在 18 次比较中的 6 次中，选择模式与姊妹分类群之间的表型分歧（或不存在分歧）一致。
在异质条件下测量的生理特征的选择模式不能可靠地反映姊妹分类群之间的分歧，这突显了在分类群内和分类群间重复研究选择方向的必要性。
