Abstract
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.
Highlights
Stable species coexistence requires that every species be able to increase when rare (Chesson, 2000)
If interspecific competition promotes coexistence by selecting for resource partitioning, when both species coevolve in sympatry each should become increasingly dominant on the resource on which it has a competitive advantage
After 12 generations of evolution, the invasion rate of C. chinensis that had evolved in sympatry did not differ significantly from those that evolved in allopatry
Summary
Stable species coexistence requires that every species be able to increase when rare (Chesson, 2000). Experimental studies of the evolution of competitive ability are limited because they mostly consider (1) initially similar mutant and wild-type strains of the same species, and (2) growth conditions that provide little to no opportunity for stable coexistence of different types (reviewed in Taper & Case, 1992). Providing two different resources (lentils and adzuki beans) provides an opportunity for evolutionary divergence in resource use, because the beetles exhibit an interspecific generalist-specialist trade-off in ability to use lentils and adzuki beans (they would not necessarily exhibit the same trade- off if grown on other resources) Both species can consume adzuki beans, for which the specialist C. maculatus is the superior competitor (Hausch, 2015; Utida, 1953). Improvement in the ability of C. chinensis to use lentils did not come at a cost to its ability to use mung beans, and so presumably increased its overall competitive ability
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