Data from: Spatial variation in herbivory, climate and isolation predict plant height and fruit phenotype in Plectritis congesta island populations

Climate and herbivory can each drive natural selection on plant traits, but may interact to give rise to different patterns in trait distributions when surveyed across island populations. These different patterns may arise because the occurrence of ungulate herbivores often varies across archipelagos, potentially leading to strong and abrupt spatial heterogeneity in the direction or intensity of natural selection. In contrast, climate tends to vary gradually and thus is more likely to lead to gradual clines in trait values. Population isolation may also affect trait values, given that random genetic drift may fix alleles or traits in the absence of gene flow, or because gene flow between populations with similar or opposing selection pressures may augment or swamp the effects of selection. Here, we estimate the independent and interactive effects of deer, climate and isolation on fruit phenotype and plant height in 285 Plectritis congesta populations at 77 island and 44 mainland sites in western North America. Plectritis congesta is a palatable winter annual with two fruit phenotypes based on a simple Mendelian locus with clear dominance, and heritable variation in height as a polygenic trait. Fruit phenotype and plant height were well-predicted by deer occurrence; plants in populations with resident deer were short (15.0 ± 1.1 cm) and mainly expressed wingless fruits (73.0 ± 4.0 %), whereas plants in populations without deer were 2.6 times taller (38.9 ± 5.3 cm) and only 9.0 ± 1.6 % expressed wingless fruits. Wingless fruits were less common in populations experiencing wetter conditions with more seasonal variability in temperatures, particularly in the absence of deer. In contrast, population isolation was unrelated to fruit phenotype, except in populations rarely exposed to deer, where plants expressed phenotypes more like those in populations without deer as isolation increased. Our results are consistent with the hypothesis that spatial variation in browsing by deer, or other factors correlated with it, contributes to population-level variation in fruit phenotype and plant height in P. congesta, and that climate leads to a modest spatial gradient in plant height.

气候与植食作用均可驱动植物性状相关的自然选择，但在跨岛屿种群开展调查时，二者的交互作用可能会催生性状分布的差异化格局。此类差异化格局的产生，往往缘于有蹄类植食动物的分布在群岛间存在显著差异，进而可能导致自然选择的方向或强度出现强烈且突变的空间异质性。与之形成对比的是，气候的变化通常较为平缓，因此更易引发性状值的渐变性渐变群。种群隔离同样可能影响性状值：一方面，在缺乏基因流的情况下，随机遗传漂变可能固定等位基因或性状；另一方面，具有相似或相反选择压力的种群间的基因流，可能强化或抵消选择的作用效应。

本研究针对北美西部77个岛屿样点与44个陆地样点的285个密花泽兰（Plectritis congesta）种群，估算了鹿类、气候与种群隔离对其果实表型及株高的独立与交互效应。密花泽兰是一种适口性优良的冬性一年生植物，其果实表型由一个具备明确显性关系的单孟德尔位点控制，存在两种表型；而株高作为多基因性状，存在可遗传的变异。

研究发现，鹿类的存在可较好地预测果实表型与株高：栖息有定居鹿类的种群中，植株株高较矮（15.0 ± 1.1 cm），无翅果实的表达比例高达73.0 ± 4.0%；而无鹿类定居的种群中，植株高度是前者的2.6倍（38.9 ± 5.3 cm），无翅果实的表达比例仅为9.0 ± 1.6%。在气候更湿润、温度季节波动更强的种群中，无翅果实的占比更低，这一效应在无鹿类定居的种群中尤为显著。与之相反，种群隔离与果实表型并无显著关联，仅在极少接触鹿类的种群中，随着隔离程度的升高，植株的果实表型会愈发趋近于无鹿类种群的表型。

本研究结果支持如下假说：鹿类啃食的空间变异，或与其相关的其他因素，导致了密花泽兰种群间果实表型与株高的分化；同时，气候会促使株高形成适度的空间渐变格局。