The evolution of environmental tolerance and range size: A comparison of geographically restricted and widespread Mimulus

The geographic ranges of closely related species can vary dramatically, yet we do not fully grasp the mechanisms underlying such variation. The niche breadth hypothesis posits that species that have evolved broad environmental tolerances can achieve larger geographic ranges than species with narrow environmental tolerances. In turn, plasticity and genetic variation in ecologically important traits and adaptation to environmentally variable areas can facilitate the evolution of broad environmental tolerance. We used five pairs of western North American monkeyflowers to experimentally test these ideas by quantifying performance across eight temperature regimes. In four species pairs, species with broader thermal tolerances had larger geographic ranges, supporting the niche breadth hypothesis. As predicted, species with broader thermal tolerances also had more within-population genetic variation in thermal reaction norms and experienced greater thermal variation across their geographic ranges than species with narrow thermal tolerances. Species with narrow thermal tolerance may be particularly vulnerable to changing climatic conditions due to a lack of plasticity and insufficient genetic variation to respond to novel selection pressures. Conversely, species experiencing high variation in temperature across their ranges may be buffered against extinction due to climatic changes because they have evolved tolerance to a broad range of temperatures.

近缘物种的地理分布范围差异往往十分显著，但目前学界尚未完全明晰此类差异背后的作用机制。生态位宽度假说（Niche Breadth Hypothesis）提出：演化出宽泛环境耐受能力的物种，其地理分布范围要大于环境耐受范围较窄的物种。反之，生态关键性状的表型可塑性（Phenotypic Plasticity）与遗传变异，以及对环境异质性区域的适应，可促进宽泛环境耐受能力的演化。本研究以五对北美西部猴面花（Monkeyflower）为实验材料，通过量化其在八个温度梯度下的生长表现，对上述假说开展实验验证。在四对物种中，热耐受范围更广的物种地理分布范围更大，该结果支持了生态位宽度假说。正如预期，相较于热耐受范围较窄的物种，热耐受范围更广的物种其种群内部的热反应规范（thermal reaction norms）遗传变异水平更高，且在其地理分布范围内经历的温度异质性也更强。由于缺乏表型可塑性，且应对新选择压力的遗传变异不足，热耐受范围较窄的物种可能更容易受到气候变化的冲击。反之，地理分布范围内温度异质性较高的物种，因已演化出对宽泛温度范围的耐受能力，或可抵御气候变化带来的灭绝风险。