Abstract
The tenet that ecological opportunity drives adaptive diversification has been central to theories of speciation since Darwin, yet no widely accepted definition or mechanistic framework for the concept currently exists. We propose a definition for ecological opportunity that provides an explicit mechanism for its action. In our formulation, ecological opportunity refers to environmental conditions that both permit the persistence of a lineage within a community, as well as generate divergent natural selection within that lineage. Thus, ecological opportunity arises from two fundamental elements: (1) niche availability, the ability of a population with a phenotype previously absent from a community to persist within that community and (2) niche discordance, the diversifying selection generated by the adaptive mismatch between a population's niche-related traits and the newly encountered ecological conditions. Evolutionary response to ecological opportunity is primarily governed by (1) spatiotemporal structure of ecological opportunity, which influences dynamics of selection and development of reproductive isolation and (2) diversification potential, the biological properties of a lineage that determine its capacity to diversify. Diversification under ecological opportunity proceeds as an increase in niche breadth, development of intraspecific ecotypes, speciation, and additional cycles of diversification that may themselves be triggered by speciation. Extensive ecological opportunity may exist in depauperate communities, but it is unclear whether ecological opportunity abates in species-rich communities. Because ecological opportunity should generally increase during times of rapid and multifarious environmental change, human activities may currently be generating elevated ecological opportunity – but so far little work has directly addressed this topic. Our framework highlights the need for greater synthesis of community ecology and evolutionary biology, unifying the four major components of the concept of ecological opportunity.
Highlights
Ecological opportunity underlies adaptive diversification of species and may represent the primary environmental driver of phenotypic evolution, determining the rate and magnitude of lineage radiations
Ecology and Evolution published by John Wiley & Sons Ltd
We develop a mechanistic definition for ecological opportunity; one in which we restrict its meaning to environmental conditions that, when encountered by a lineage, directly cause divergent selection, and in which speciation, when it occurs under such conditions, produces ecologically diversified species
Summary
Ecological opportunity underlies adaptive diversification of species and may represent the primary environmental driver of phenotypic evolution, determining the rate and magnitude of lineage radiations. Niche discordance refers to diversifying selection generated by an adaptive mismatch between a focal population’s niche-related traits and the environment’s ecological conditions In this context, “diversifying” selection describes selection for increased phenotypic variance within a lineage (or between newly diverging lineages), and may occur by a broadening of the selective surface (niche expansion) or disruptive selection within a population, or by divergent selection acting between spatially segregated subpopulations. The lack of diversifying selection (i.e., no niche discordance) in both of these cases means that selection does not favor increased phenotypic variance within the lineage, and no ecological opportunity exists These scenarios may involve considerable anagenetic change or allopatric speciation, even perhaps as a response to similar selection pressures (Langerhans and Riesch 2013), these outcomes do not derive from ecological opportunity, and do not entail new ecological diversity generated within the lineages. Phenotypic plasticity may readily evolve in sympatry as a response to disruptive selection without a constraining role for gene flow (Doughty and Reznick 2004; Dudley 2004; Martin and Pfennig 2010; WestEberhard 1989)
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