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（interspecific resource competition）通常会驱动物种演化出资源利用策略的分化，弱化种间竞争相较于种内竞争（intraspecific competition）的强度，进而促进物种种间的稳定共存。更宽泛地而言，由于种间竞争会降低物种的适合度（fitness），任何涉及种间竞争的机制都应产生选择压，青睐能够弱化种间竞争的性状。但与此同时，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。
本研究以两种豆象为实验对象：专食性的绿豆象（Callosobruchus maculatus）与广食性的四纹豆象（C. chinensis）。我们分别在异域饲养条件（allopatry，即两物种单独饲养）与同域饲养条件（sympatry，即两物种共同饲养）下，以红小豆（adzuki beans）与小扁豆（lentils）的混合基质作为食物开展饲养实验，并在演化至第4、8、12代时，测定二者的相互入侵性（mutual invasibility）。
与“竞争者的协同进化（coevolution）会弱化种间竞争”的预期相悖，同域组与异域组的相互入侵速率并无显著差异。与之相反，仅四纹豆象（C. chinensis）的入侵速率随演化时长增加而提升：四纹豆象能够适应小扁豆，且并未降低对红小豆的适应性，且这一效应不受绿豆象（C. maculatus）存在与否的影响。
本研究结果凸显出：针对种间竞争的演化响应，仅在实际操作中难以实现的特定条件下，才能推动物种种间的稳定共存。