Data from: Selection for seed size: the unexpected effects of water availability and density

1. Seed size is a functional trait with important fitness consequences that potentially extend throughout the life cycle of plants. Dithyrea californica experiences selection for larger seeds in post‐germination stages but it is still uncertain how environmental factors mediate the strength and the direction of natural selection on seed size. 2. D. californica represents a unique opportunity to investigate selection on seed size in natural conditions due to a persistent seed ring that stays attached to the root throughout the plant's life. This makes it possible to unearth plants at any stage and measure the size of the seed from which they originated. 3. We conducted a factorial experiment manipulating water availability and intraspecific competition using plants that naturally germinated in the wild. 4. Selection on seed size via survivorship was nil because all individuals survived to reproduce. The strength and the direction of selection on seed size via fecundity depended on water availability and conspecific density. 5. Contrary to our predictions, increasing conspecific density relaxed directional selection favoring larger seeds, but only in the wettest conditions and an increase in water availability strengthened it, but only at low density. A possible explanation of these counter‐intuitive results relies on the observed absence of survival selection and increased plant growth rates under high water and low density. 6. Larger seeds require more resources to construct and when this cost is taken into account; there is no overall fitness increase with seed size. This nicely follows the life history theory predictions for optimal seed size. At the evolutionary equilibrium, if seeds could be larger, per seed fitness would still increase, which is what we observed. But cost corrected fitness should be flat. Maternal fitness equals per seed fitness times seed number, so any increase to per seed fitness of making a bigger seed is balanced by the resulting cost to seed number. Our results indicate flat cost corrected fitness of seed size as theory predicts.1. 种子大小是一种功能性性状，具有重要的适合度（fitness）效应，其影响可能贯穿植物整个生命周期。加州双角荠（Dithyrea californica）在萌发后阶段会受到对大种子的选择压力，但目前仍不清楚环境因子如何调控种子大小相关自然选择的强度与方向。 2. 由于加州双角荠（D. californica）在整个生命周期中都会在根部附着宿存的种子环，该物种为在自然条件下研究种子大小相关选择提供了独特的研究契机。这使得研究者可以在任意生长阶段挖掘植株，并测量其起源种子的大小。 3. 本研究以野外自然萌发的植株为材料，开展了调控水分有效性与种内竞争的析因实验（factorial experiment）。 4. 通过存活率介导的种子大小选择效应为零，因为所有供试个体均存活至繁殖阶段。而通过繁殖力（fecundity）介导的种子大小选择的强度与方向，则取决于水分有效性与同种密度（conspecific density）。 5. 与我们的预测相悖的是，仅在最湿润的生境条件下，提高同种密度会减弱对大种子的定向选择（directional selection）；而仅在低密度条件下，提升水分有效性则会强化该选择效应。针对这些反直觉结果的一种可能解释是：在高水分、低密度条件下，未观测到存活选择，且植株生长速率加快。 6. 大种子构建需要消耗更多资源，若将该成本纳入考量，种子大小与整体适合度之间并不存在显著的正相关关系。这一结果完美契合生活史理论（life history theory）关于最优种子大小的预测。在进化平衡点上，即便种子可以更大，单粒种子适合度仍会提升，这与我们的观测结果一致。但经成本校正后的适合度应保持恒定。母本适合度等于单粒种子适合度乘以种子数量，因此，增大种子所带来的单粒种子适合度提升，会被种子数量减少所带来的成本所抵消。本研究结果表明，经成本校正后的种子大小相关适合度保持恒定，与理论预测相符。