Evolution and coexistence: current theory and the future
It is possible for evolutionary change to make coexistence more or less likely. The predominant view seems to be that the former outcome occurs much more often. This maybe true, but it is premature to reach a conclusion.
If a new species is in the process of excluding a resident, the two species must start with relatively similar competitive abilities for evolution to be rapid enough compared to the population decline to save the inferior species (‘evolutionary rescue'; see e.g., Klausmeier et al. 2020b). Cases of evolution promoting coexistence are more likely to come in the form of making the species less likely to be competitively excluded in the future when the environment changes in a manner that decreases its fitness relative to that of its competitors. However, as noted above, some evolutionarily favoured traits increase competition (e.g., relative capture rates of different essential resources), so they may make a species more susceptible to extinction. The traditional response of divergence under competition for substitutable resources is often associated with a lower range of resources being utilized. Increased specialization increases the probability that the remaining utilized resources of that consumer may all suffer poor conditions simultaneously. Therefore, the effect of evolutionary divergence on long-term coexistence may be negative.As with a good deal of other recent competition theory, the last few years have seen a focus on how evolutionary responses to competition affect the two requirements for coexistence stressed by Chesson (2000a): niche differences, and differences in general competitive ability. Pastore et al. (2021) was the most recent of these works at the time that this chapter was written. One problem with this two-way classification of coexistence requirements, explained in earlier chapters, is that many traits affect both requirements, and agreement on how to make the bipartite classification in such circumstances is lacking.
Another problem is that ‘general ability' traits will be selected for in the presence or absence of the competitor, so they are not expected to change significantly because of the addition or removal of a competitor. The fact that a trait influences the relative capture rates of different resources does not allow one to deduce the directional effect of a competitor on the trait unless one knows the nutritional effects of, and interactions between, the resources.Assume that ‘traits' are defined as being the three basic foodweb parameters; capture rates, conversion efficiencies, and mortality/metabolic loss rates. Most commonly measured consumer traits in an animal (e.g., speed or size of mouthparts) are likely to affect both general exploitation ability and the relative abilities to take up different resources. Both general and specific capture rates include the possibility that selectively favoured traits will decrease population size by increasing resource overexploitation (Abrams 2002, 2019). This possibility is almost never considered in analyses of coevolution. In traditional simple models with two or more resources, only the last of the three basic parameters (mortality) is treated as being independent of the niche, and that is due to the often-questionable assumption that mortality is independent of resource intake. A future theory of competitive coevolution will hopefully deal with the consequences of the interconnected, multi-parameter effects of most measurable traits, and will consider their effects on population size.
Given the relative paucity of empirical work on the evolution of competitors, the first order of business for future empirical work should be to understand the direction of trait responses to competition for a wider variety of types of consumers and traits. Evolutionary change is easiest to quantify in species with very short generation times. Studies of phytoplankton or microbes seem particularly promising, given the ability to manipulate resources and dynamic rates in the laboratory.