Data from: Coevolution of competing Callosobruchus species does not stabilize coexistence

Interspecific resource competition is expected to select for divergence in resource use, weakening interspecific relative to intraspecific competition, thus promoting stable coexistence. More broadly, because interspecific competition reduces fitness, any mechanism of interspecific competition should generate selection favoring traits that weaken interspecific competition. However, species also can adapt to competition by increasing their competitive ability, potentially destabilizing coexistence. We reared two species of bean beetles, the specialist Callosobruchus maculatus and the generalist C. chinensis, in allopatry and sympatry on a mixture of adzuki beans and lentils, and assayed mutual invasibility after four, eight, and twelve generations of evolution. Contrary to the expectation that coevolution of competitors will weaken interspecific competition, the rate of mutual invasibility did not differ between sympatry and allopatry. Rather, the invasion rate of C. chinensis, but not C. maculatus, increased with duration of evolution, as C. chinensis adapted to lentils without experiencing reduced adaptation to adzuki beans, and regardless of the presence or absence of C. maculatus. Our results highlight that evolutionary responses to interspecific competition promote stable coexistence only under specific conditions that can be difficult to produce in practice.

种间资源竞争通常会驱动物种在资源利用策略上发生分化，使得种间竞争强度弱于种内竞争，进而促进物种间的稳定共存。从更广泛的视角来看，由于种间竞争会降低种群适合度，任何涉及种间竞争的机制都应产生选择压力，青睐能够弱化种间竞争的性状。然而，物种也可通过提升自身竞争能力来适应种间竞争，这一过程有可能破坏物种共存的稳定性。

我们以两种豆象为实验材料：专食性的Callosobruchus maculatus（四纹豆象）与广食性的C. chinensis（绿豆象），在赤小豆与扁豆的混合基质上分别设置异域培养（仅单一种群饲养）与同域培养（两种群共存饲养）两组实验，随后在演化至第4、8、12代时，对两组种群的相互入侵能力进行测定。

与"竞争者的共演化会弱化种间竞争"的经典预期相悖，同域培养与异域培养种群的相互入侵能力并无显著差异。进一步分析发现，仅C. chinensis（绿豆象）的入侵速率随演化时长增加而提升：这是因为C. chinensis（绿豆象）成功适应了扁豆资源，且并未降低对赤小豆的适应性，且这一结果不受C. maculatus（四纹豆象）是否存在的影响。

本研究结果凸显：针对种间竞争的演化响应，仅在实际中难以复现的特定条件下，才能推动物种间的稳定共存。