Digest: Fitness gains in guppies support evolution's role in coexistence*

Abstract

Understanding processes that drive the assembly of communities is not a new goal in ecology; researchers have long studied what leads to the coexistence of species. However, the study of rapid evolution as a driving process of community assembly is an emerging frontier in evolutionary-ecology research.Stable coexistence requires that the effect of intraspecific interactions on fitness is greater than the effect of interspecific interactions. One way evolution can promote coexistence is through character displacement, where one or both of the competitors evolve to use less overlapping niches, thus reducing competition. Coevolved competitors would both have higher fitness than historically separated competitors when paired together.Coexistence between immigrants and residents may be mediated by an evolutionary response in one or both species to the traits of their competitor. Testing for unidirectional or reciprocal adaptation to the other competitor's presence requires a comparison of the fitness of each species in experimentally paired sympatric populations and paired allopatric populations.For decades the mountain streams of Trinidad have provided researchers an opportunity to witness rapid evolution in two fish competitors: guppies, Poecilia reticulata, and killifish, Rivulus hartii. In response to changes in killifish presence, the guppies rapidly evolve differences in color, behavior, and life-history traits (see, e.g., Reznick and Endler 1982). Similarly, killifish lifehistory traits repeatedly diverge between natural ponds with and without guppies (Walsh and Reznick 2009).Using this model system, Bassar et al. (2016) seek to address whether the observed changes in guppies and killifish promote * This manuscript corresponds to Bassar, R. D., Simon, T., Roberts, W., Travis, J., and Reznick, D. N. 2016. The evolution of coexistence: reciprocal adaptation promotes the assembly of a simple community. Evolution.