EAGER: Does Adaptation Facilitate or Constrain Further Adaptation? Evaluating the Origins of Character Displacement.

A central goal of biology is to explain the origins of biodiversity. Darwin first proposed that a major cause of biodiversity is competition. The crux of his idea is that when organisms compete for scarce resources, natural selection should favor those individuals that are least like their competitors. Consequently, selection should cause these individuals to become more dissimilar over time, possibly becoming different species. However, species can compete for more than just resources: they can also compete by interfering with each other's ability to identify mates. Yet, few studies have evaluated how these two sources of competition interact with each other. The proposed work evaluates this possibility using a novel model system - spadefoot toads. The work is a significant step forward because the proposed experiments will provide insights into the origins of trait variation and its effects on adaptive evolution. The research is focused on character displacement which is a central evolutionary and ecological concept important to understanding the origins, abundance, and distribution of biodiversity. The work engages students in public outreach in North Carolina and beyond through work at the North Carolina Museum of Natural Sciences.

Character displacement takes two forms: ecological character displacement in which traits evolve in response to selection to minimize resource competition between species; reproductive character displacement in which traits evolve in response to selection to minimize deleterious reproductive interactions (e.g., hybridization) between species. Most previous studies of character displacement have focused on either ecological or reproductive character displacement, but not both. Yet, because species that are similar enough to compete for resources are also often similar enough to interact reproductively, the two processes might often interact. An untested hypothesis for how the two forms of character displacement interact is that one process alters trait variation in the alternative context in a way that either facilitates, or constrains, a population's response to selection in that alternative context. This project will use both field and laboratory experiments to test this hypothesis by experimentally mimicking 'first contact' between two species of spadefoot toads that have undergone both ecological and reproductive character displacement with each other. Specifically, the scientists will evaluate whether divergence in traits associated with one process generates novel trait variation on which the alternative process could act. In doing so, the research promises to have far reaching implications. In particular, because character displacement is central in the origins, abundance, and distribution of biodiversity, understanding its origins can shed light onto some of the most fundamental issues in evolutionary biology and ecology, including the causes of diversity, species coexistence, community assembly, and species distributions.