Éva Kisdi, Helene C. Weigang, and Mats Gyllenberg (Jan 2020)
The evolution of immigration strategies facilitates niche expansion by divergent adaptation in a metapopulation model
“The evolution of immigration strategies facilitates niche expansion by divergent adaptation in a structured metapopulation model”
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The vicious circle broken: Habitat choice can broaden habitat use
Via Darwinian evolution, organisms adapt to the habitats where they live. Yet once adapted to a particular set of environmental conditions, it is pretty hard to broaden this "niche" of environments where the organism thrives, because adapting to a new environment usually comes at the cost of diminishing success in the old habitat. Since most individuals survive and reproduce in the old habitat, sacrificing reproduction in this habitat is too high a price for improving the chances of the few who wither in the new habitat. As a result, no change occurs and the niche is conserved.
Many animals are able to choose actively where they live. Habitat choice appears to aggravate niche conservatism; as individuals choose favorable environments to which they are already adapted, they will not be exposed to new habitats. When not exposed to new habitats, there is no need to adapt to them. Live where you succeed, then there is no need to change how you live.
This vicious circle can however be broken. The model of Kisdi et al. highlights the range of habitats that are marginally favorable, i.e., favorable if empty but unfavorable if crowded. In these habitats, a delicate balance evolves, where the local population size is just on the edge between making the habitat favorable or unfavorable. Once two strains of the organism are present (representing a minimum of genetic variability), this delicate balance is upset. No matter how little is the initial difference between the strains, each marginal habitat will be preferentially used by the strain that is better adapted there. With the two strains using different habitats, they start to adapt to different environmental conditions, which drives their habitat preferences further apart. As a result, the niche expands.
Abstract
Local adaptation and habitat choice are two key factors that control the distribution and diversification of species. Here we model habitat choice mechanistically as the outcome of dispersal with non-random immigration. We consider a structured metapopulation with a continuous distribution of patch types, and determine the evolutionarily stable immigration strategy as the function linking patch type to the probability of settling in the patch upon encounter. We uncover a novel mechanism whereby coexisting strains that only slightly differ in their local adaptation trait can evolve substantially different immigration strategies. In turn, different habitat use selects for divergent adaptations in the two strains. We propose that the joint evolution of immigration and local adaptation can facilitate diversification, and discuss our results in the light of niche conservatism versus niche expansion.