Data from: Making pore choices: repeated regime shifts in stomatal ratio

Ecologically important traits do not evolve without limits. Instead, evolution is constrained by the set of available and viable phenotypes. In particular, natural selection may only favour a narrow range of adaptive optima constrained within selective regimes. Here, I integrate data with theory to test whether selection explains phenotypic constraint. A global database of 599 plant species from 94 families shows that stomatal ratio, a trait affecting photosynthesis and defence against pathogens, is highly constrained. Most plants have their stomata on the lower leaf surface (hypostomy), but species with half their stomata on each surface (amphistomy) form a distinct mode in the trait distribution. A model based on a trade-off between maximizing photosynthesis and a fitness cost of upper stomata predicts a limited number of adaptive solutions, leading to a multimodal trait distribution. Phylogenetic comparisons show that amphistomy is the most common among fast-growing species, supporting the view that CO2 diffusion is under strong selection. These results indicate that selective optima stay within a relatively stable set of selective regimes over macroevolutionary time.

具有生态重要性的性状并不会无限制地演化，而是受限于可获取且具生存适应性的表型（phenotype）集合。具体而言，自然选择仅会青睐局限于选择模式（selective regime）范围内的少量适应性最优解。本研究将数据与理论相结合，旨在检验选择作用是否可解释表型约束现象。一项涵盖94个科、599种植物的全球数据库分析表明，气孔比率（stomatal ratio）——一种影响光合作用（photosynthesis）与病原体（pathogen）防御的性状——受到极强的演化约束：多数植物的气孔仅分布于叶片下表皮（下生气孔型，hypostomy），而气孔均匀分布于叶片上下表皮的物种（两面生气孔型，amphistomy）则在该性状的分布中形成了一个独立的模态。一项基于最大化光合作用与上表皮气孔带来的适合度成本之间权衡关系的模型预测，适应性最优解的数量十分有限，这将导致该性状的分布呈现多峰模式。系统发育比较分析（phylogenetic comparison）结果显示，两面生气孔型在速生植物类群中最为普遍，这支持了二氧化碳（CO₂）扩散过程受到强烈选择作用的观点。上述研究结果表明，在宏演化（macroevolution）时间尺度下，适应性最优解始终局限于一组相对稳定的选择模式范围内。