Development and evolution of character displacement

Abstract

Character displacement occurs when competition for either resources or successful reproduction imposes divergent selection on interacting species, causing divergence in traits associated with resource use or reproduction. Here, we describe how character displacement can be mediated either by genetically canalized changes (i.e., changes that reflect allelic or genotype frequency changes) or by phenotypic plasticity. We also discuss how these two mechanisms influence the tempo of character displacement. Specifically, we suggest that, under some conditions, character displacement mediated by phenotypic plasticity might occur more rapidly than that mediated by genetically canalized changes. Finally, we describe how these two mechanisms may act together and determine character displacement's mode, such that it proceeds through an initial phase in which trait divergence is environmentally induced to a later phase in which divergence becomes genetically canalized. This plasticity-first hypothesis predicts that character displacement should be generally mediated by ancestral plasticity and that it will arise similarly in multiple, independently evolving populations. We conclude by highlighting future directions for research that would test these predictions.

Figure 1

An experimental demonstration of how phenotypic plasticity might mediate ecological character displacement. Spadefoot toad tadpoles (Spea multiplicata and S. bombifrons) typically occur as (a) an omnivore morph, (b) which specializes on plant material and detritus, and (c) a morphologically distinctive carnivore morph, (d) which specializes on, and is induced by, anostracan fairy shrimp. (e) In allopatric populations, each species produces similar, intermediate frequencies of both morphs. However, in sympatry, S. multiplicata shift to producing mostly omnivores, whereas S. bombifrons shift to producing mostly carnivores; i.e., these two species have undergone character displacement in morph production. (f) Similar niche shifts can be experimentally recreated in the lab. When allopatric individuals are reared with a single conspecific and fed both shrimp and detritus, they produced similar proportions of both morphs (as in natural allopatric populations; see panel e). By contrast, when the two species are experimentally combined, S. multiplicata shift to producing mostly omnivores, whereas S. bombifrons shift to producing mostly carnivores (as in natural sympatric populations). These facultative niche shifts appear to reflect differences between species in ability to capture and consume shrimp, an environmental cue that induces carnivores.

Figure 2

How character displacement might evolve from an initial phase in which trait divergence is environmentally induced to one in which divergence is based on genetically canalized differences and thereby becomes expressed constitutively. (a) Initially, two interacting species may express plasticity in resource-use or reproductive phenotypes. In this case, two species of plants facultatively produce different-sized leaves as an adaptive response to different light levels. (b) When they come into sympatry and compete (e.g., for photons), each species may begin to facultatively express a different subset of the initial phenotypes. Here, Species 1 overtops Species 2, thereby gaining increased access to photons, which triggers the facultative production in Species 1 of small leaves only. By contrast, because it is shaded by Species 1, Species 2 facultatively produces larger leaves only (in each case, the expressed phenotype—and underlying genes—are shown in black, whereas the unexpressed ones are shown in gray). If individuals within the same population harbor genetic variation in the degree to which they respond to environmental cues, and if these reaction norm evolve in response to competitively mediated selection and thereby minimize competition between species, then such evolved environmentally induced shifts would constitute character displacement. (c) Over time, both species may lose this pre-existing plasticity and become fixed for a different alternative phenotype, possibly because of the loss (through selection or chance) of alleles or gene combinations underlying the non-expressed phenotype. Thus, character displacement might proceed through an initial phase in which trait divergence is environmentally induced to a later phase in which divergence becomes genetically canalized. Essentially, during character displacement, each species may evolve from expressing a wide range of phenotypes to becoming genetically canalized for a narrower range of phenotypes (in this case, as each species evolves from being polymorphic for leaf shape to being monomorphic).