The Ecology of Nonecological Speciation and Nonadaptive Radiations

Growing evidence for lineage diversification that occurs without strong ecological divergence (i.e., nonadaptive radiation) challenges assumptions about the buildup and maintenance of species in evolutionary radiations, particularly when ecologically similar and thus potentially competing species co-occur. Understanding nonadaptive radiations involves identifying conditions conducive to both the nonecological generation of species and the maintenance of co-occurring ecologically similar species. To borrow MacArthur’s [1] (Challenging Biological Problems 1972;253–259) form of inquiry, the ecology of nonadaptive radiations can be understood as follows: for species of type A, in environments of type B, nonadaptive radiations may emerge. We review purported cases of nonadaptive radiation and suggest properties of organisms, resources, and landscapes that might be conducive to their origin and maintenance. These properties include poor dispersal ability and the ephemerality and patchiness of resources. Nonadaptive radiations result from nonecological speciation and the absence of character displacement. There are properties of organisms and ecosystems that make nonadaptive radiations more or less likely. As our understanding of cryptic species increases, we are likely to become aware of more nonadaptive radiations. Nonadaptive radiations can give rise to functional redundancy in ecosystems, which is likely to be correlated with resilience and/or resistance to perturbation. Understanding the ecology of why competitive exclusion is mitigated is essential for the conservation of biodiversity.