Adaptation to current and future climates can be constrained by trade‐offs between fitness‐related traits. Early seedling emergence often enhances plant fitness in seasonal environments, but if earlier emergence in response to seasonal cues is genetically correlated with lower potential to spread emergence among years (i.e., bet‐hedging), then this functional trade‐off could constrain adaptive evolution. Consequently, selection favoring both earlier within‐year emergence and greater spread of emergence among years—as is expected in more arid environments—may constrain adaptive responses to trait value combinations at which a performance gain in either function (i.e., evolving earlier within‐ or greater among‐year emergence) generates a performance loss in the other. All such trait value combinations that cannot be improved for both functions simultaneously are described as Pareto optimal and together constitute the Pareto front. To investigate how this potential emergence timing trade‐off might constrain adaptation to increasing aridity, we sourced seeds of two grasses, Stipa pulchra and Bromus diandrus, from multiple maternal lines within populations across an aridity gradient in California and examined their performance in a greenhouse experiment. We monitored emergence and assayed ungerminated seeds for viability to determine seed persistence, a metric of potential among‐year emergence spread. In both species, maternal lines with larger fractions of persistent seeds emerged later, indicating a trade‐off between within‐year emergence speed and potential among‐year emergence spread. In both species, populations on the Pareto front for both earlier emergence and larger seed persistence fraction occupied significantly more arid sites than populations off the Pareto front, consistent with the hypothesis that more arid sites impose the strongest selection for earlier within‐year emergence and greater among‐year emergence spread. Our results provide an example of how evaluating genetically based correlations within populations and applying Pareto optimality among populations can be used to detect evolutionary constraints and adaptation across environmental gradients.

中文翻译：

---

权衡驱动年内和年间出现时间的帕累托最优，以应对日益干旱的情况


对当前和未来气候的适应可能会受到与健康相关特征之间的权衡的限制。早期出苗通常会增强植物在季节性环境中的适应性，但如果响应季节线索的较早出苗在遗传上与年份之间传播出苗的较低潜力相关（即赌注对冲），那么这种功能权衡可能会限制适应性进化。因此，有利于较早出现的年内出现和在年份之间出现较大传播的选择（正如在更干旱的环境中所预期的那样）可能会限制对性状值组合的适应性反应，在这种情况下，任一功能的性能增益（即，在或内较早进化）更大的年间涌现）会导致另一方的绩效损失。所有这些不能同时针对两个功能进行改进的特征值组合被描述为帕累托最优，并共同构成帕累托前沿。为了研究这种潜在的出现时间权衡如何限制对日益干旱的适应，我们从加利福尼亚州干旱梯度种群内的多个母系中获取了两种草的种子，即白花针茅和雀麦，并在温室实验中检查了它们的表现。我们监测了出苗情况并分析了未发芽种子的活力，以确定种子的持久性，这是潜在的年内出苗传播的指标。在这两个物种中，具有较大比例持久种子的母系出现较晚，表明年内出苗速度和潜在的年内出苗传播之间存在权衡。 在这两个物种中，帕累托前沿上的种群由于较早出苗和较大的种子持久性比帕累托前沿以外的种群占据了更多的干旱地区，这与以下假设一致：更多的干旱地区对较早的年内出苗和更大的种子施加了最强的选择。年间出现传播。我们的结果提供了一个例子，说明如何评估种群内基于遗传的相关性并在种群之间应用帕累托最优性来检测跨环境梯度的进化约束和适应。