Natural selection favours drought escape and an acquisitive resource‐use strategy in semi‐arid Mediterranean shrubs

Natural selection is the major force driving adaptive evolution in natural populations, varying in strength, direction, and form through space and time, especially in highly variable environments such as Mediterranean ecosystems. Although a conservative resource-use strategy has been hypothesized to be adaptive in Mediterranean taxa, patterns of selection at the intraspecific level, i.e., the suite of traits determining individual fitness, are largely unknown.
Using a phenotypic selection experiment in natural semiarid conditions, we measured direct and indirect selection acting through two different fitness components (survival and reproduction), to assess the adaptive value of 20 ecophysiological traits on individuals of two gypsum endemic species, Centaurea hyssopifolia and Helianthemum squamatum, dwelling in environments with contrasting abiotic conditions (south- and north-facing slopes) during two climatically contrasting years (dry and mesic). This allowed quantifying the magnitude and direction of natural selection at different spatiotemporal scales.
Our results revealed that different abiotic conditions did not alter selection patterns, being the magnitude of selection more strongly affected by temporal environmental variation. Selection through reproduction indicated consistent selection for early phenology, low water use efficiency, high specific leaf area, low leaf dry matter content, and high leaf N across slopes and years in both species. In contrast, phenotypic trait variation was not linked to survival in either species. Furthermore, while individual reproductive output was higher or similar in environments with higher abiotic stress in both species and years, survival was similar across environmental conditions, and it was neither affected by plant size nor reproductive output.
Contrary to our expectations, natural selection via reproductive fitness consistently favoured a drought-escape, acquisitive resource-use strategy in Mediterranean semiarid plants, rather than a conservative resource-use strategy, even under conditions of higher abiotic stress (i.e., south slopes and dry year). Such acquisitive strategy could allow rapid development by maximizing resource assimilation and reproduction before the most limiting climatic conditions of mid-late summer. Our results shed light on adaptive functional strategies of Mediterranean taxa at the intraspecific level, providing insight on future responses to environmental change, and highlight remarkable differences in selection acting through different fitness components.

自然选择（natural selection）是驱动自然种群适应性演化的主要动力，其强度、方向与表现形式随空间与时间动态变化，在诸如地中海生态系统这类高度异质的环境中尤为显著。尽管保守型资源利用策略被假说认为在地中海类群中具有适应性，但种内（intraspecific）水平的选择模式——即决定个体适合度（fitness）的一系列性状组合——在很大程度上仍未被探明。
我们依托自然半干旱条件下的表型选择实验（phenotypic selection experiment），通过存活与繁殖两个不同的适合度组分（fitness component）衡量直接与间接选择作用，以评估两种石膏特有种——矢车菊属*Centaurea hyssopifolia*与鳞叶半日花*Helianthemum squamatum*——的20项生态生理性状（ecophysiological trait）的适应性价值。实验样地设置在非生物条件差异显著的南、北朝向坡地，并覆盖两个气候差异鲜明的年份（干旱年与湿润年）。借此得以量化不同时空尺度下自然选择的强度与方向。
研究结果显示，不同的非生物环境并未改变选择模式，而选择强度受时间环境变异的影响更为显著。通过繁殖组分介导的选择表明，在两个物种的所有坡地与年份中，均存在对物候提前、低水分利用效率（water use efficiency）、高比叶面积（specific leaf area）、低叶片干物质含量（leaf dry matter content）以及高叶片氮含量的一致性选择压力。与之相反，两个物种的表型性状变异均未与存活相关联。此外，尽管在两个物种的所有年份中，非生物胁迫更高的环境下个体繁殖输出更高或相近，但不同环境条件下的存活率并无差异，且存活率既不受植株体型也不受繁殖输出的影响。
与我们的预期相反，通过繁殖适合度介导的自然选择始终青睐地中海半干旱植物的“逃旱型”资源获取型策略，而非保守型资源利用策略，即便在非生物胁迫更高的环境中（即南坡与干旱年）亦是如此。这类获取型策略可通过在夏末最限制性气候条件出现前最大化资源同化与繁殖，实现快速生长发育。本研究阐明了地中海类群种内水平的适应性功能策略，为理解其对未来环境变化的响应提供了新视角，并凸显了通过不同适合度组分介导的选择作用存在显著差异。