Coevolution of competing Callosobruchus species does not stabilize coexistence

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.

Experimental evolution of competing bean beetle species reveals long‐term reversals of short‐term evolution, but no consistent character displacement

“…Hausch, Fox, and Vamosi (2017) previously reported results of this experiment related to species coexistence. Here, we report results related to phenotypic trait evolution.…”

“…Here, we report results related to phenotypic trait evolution. Hausch et al (2017) reported all methodological details except those related to trait assays, but we repeat those details here for ease of reference. The relevant text is edited from Hausch et al (2017).…”

“…Comparing per‐bean densities of eggs versus emerging adults illustrates the intensity of competition among larvae within beans. Adzuki beans in established mixed cultures of C. maculatus and C. chinensis had an average of 48 eggs/bean oviposited on them, 89% of which hatched but from which only 6 adults/bean emerged on average (Hausch et al, 2017, S. Hausch, unpublished data). Lentils in established mixed cultures had an average of 14 eggs/bean, 79% of which hatched but from which an average of only 1.5 adults/bean emerged (Hausch et al, 2017, S. Hausch, unpublished data).…”

Effects of intraspecific phenotypic variation on species coexistence

“…We assayed these traits using a method combining the approaches of Mitchell (1990) and Taper (1990) that was optimized to allow multiple traits to be assayed simultaneously. Standard assays of larval competitive strategy (scramble vs. contest competition), and x-rays of beans, revealed no evidence for contest competition (Hausch 2015).…”

“…But the competitors of a focal species also can evolve, whereas the abiotic environment cannot, and selection need not favor traits that weaken interspecific competition (Lankau 2011). Experimental studies of competitive coevolution reveal cases in which coevolution stabilizes and destabilizes previously-stable coexistence, and numerous cases in which coevolution fails to rescue competitive exclusion (Taper and Case 1992b, Le Gac et al 2012, Hausch et al 2017. Elsewhere we show that after 12 generations of evolution in either the presence or absence of C. maculatus, 5-lineage populations of C. chinensis evolve improved ability to invade 5-lineage populations of C. maculatus, due to improved ability to use lentils (Hausch et al 2017).…”