Data from: Phenotypic selection favors missing trait combinations in coexisting annual plants

Trade-offs among traits are important for maintaining biodiversity, but the role of natural selection in their construction is not often known. It is possible that trade-offs reflect fundamental constraints, negative correlational selection, or directional selection operating on costly, redundant traits. In a Sonoran Desert community of winter annual plants, we have identified a trade-off between relative growth rate and water-use efficiency among species, such that species with high relative growth rate have low water-use efficiency and vice versa. We measured selection on water-use efficiency, relative growth rate, and underlying traits within populations of four species at two study sites with different average climates. Phenotypic trait correlations within species did not match the among-species trade-off. In fact, for two species with high water-use efficiency, individuals with high relative growth rate also had high water-use efficiency. All populations experienced positive directional selection for water-use efficiency and relative growth rate. Selection tended to be stronger on water-use efficiency at the warmer and drier site, and selection on relative growth rate tended to be stronger at the cooler and wetter site. Our results indicate that directional natural selection favors a phenotype not observed among species in the community, suggesting that the among-species trade-off could be due to pervasive genetic constraints, perhaps acting in concert with processes of community assembly.性状间的权衡对于维持生物多样性至关重要，但自然选择在其形成过程中所扮演的角色却鲜有明确阐释。性状权衡现象可能源于根本性约束、负相关选择，抑或是作用于高成本冗余性状的定向选择。在索诺兰沙漠（Sonoran Desert）的冬季一年生植物群落中，我们已在多个物种种间鉴定出相对生长速率（relative growth rate）与水分利用效率（water-use efficiency）之间的权衡关系：相对生长速率较高的物种，其水分利用效率往往较低，反之亦然。我们在两个气候平均条件存在差异的研究站点，针对四个物种种群内的水分利用效率、相对生长速率及其潜在相关性状开展了选择压力测定。物种种内的表型性状相关性并不符合种间的权衡模式。事实上，对于两个水分利用效率较高的物种而言，相对生长速率较高的个体，其水分利用效率也同样偏高。所有研究种群均受到针对水分利用效率与相对生长速率的正向定向选择。在温暖干燥的站点，针对水分利用效率的选择压力通常更强；而在凉爽湿润的站点，针对相对生长速率的选择压力则更为显著。我们的研究结果表明，定向自然选择所青睐的表型在该群落的现有物种中并未被观测到，这提示种间性状权衡可能源于普遍存在的遗传约束（genetic constraints），或许还与群落组装（community assembly）过程协同发挥作用。