Trends in Ecology & Evolution
Opinion
Adaptive radiation, nonadaptive radiation, ecological speciation and nonecological speciation
Section snippets
Contributions of ecological and nonecological speciation to young and old radiations
Adaptive radiations are generally recognized as a pattern of ecological differentiation among a group of related, sympatric, species 1, 2 (see Glossary). Classic contemporary radiations, such as Darwin's finches in the Galápagos [3], or Anolis lizards of the Caribbean [4], contain species that are differentiated in body size and microhabitat, with up to 10 (Darwin's finches) or 11 (Anolis) species being found in some localities. Routes to adaptive radiation lie along a continuum where, at one
Nonadaptive radiations
Gittenberger [10] described nonadaptive radiation as ‘evolutionary diversification from a single ancestor, not accompanied by relevant niche differentiation.’ Here, we define ‘relevant’ as ecological differences that are associated with coexistence in sympatry. Thus, we define a nonadaptive radiation as a collection of related ecologically similar species that are allopatric or parapatric replacements of one another (‘allospecies’ [11]). Such groups of species are common (e.g. in birds [7],
Adaptive radiation and ecological speciation; nonadaptive radiation and nonecological speciation
Ecological speciation is defined as ‘the process by which barriers to gene flow evolve between populations as a result of ecologically based divergent selection’ 5, 6. Divergent sexual or natural selection on one trait, and/or multifarious, divergent selection pressures on multiple traits can generate reproductive isolation as a correlated response 29, 30. Ecological speciation can be rapid, occurring on the order of thousands of years [31], and has been identified in young adaptive radiations
Adaptive differentiation following nonecological speciation
Ecological opportunity might also lead to rapid ecological differentiation after nonecological speciation has occurred. Consider a series of allospecies (Figure 3) and imagine environments change such that multiple niches become available. These species might then spread into each other's range, accompanied by ecological character displacement (i.e. divergence to occupy different parts of niche space; Figure 3d). Because allospecies are already reproductively isolated, they appear to be prime
Conclusion
Given the prominence that has recently been given to the role of ecology in the process of speciation 5, 6, 29, 31, it is important to ask how commonly speciation precedes significant ecological differentiation. This should be generally testable as we gain more knowledge about the history of the Earth. The timing of lineage-splitting events (associated with barrier formation among populations) might be distinguishable from the timing of ecological opportunity (e.g. climate change), promoting
Acknowledgements
We thank R. Gillespie, B. Holland and M. Hadfield for information on their study organisms and for photos, D. Wake for granting figure permission, and D. Liittschwager and S. Middleton for granting permission to use additional spider photos. We thank J. Losos, D. Schluter, J. Weir and two anonymous reviewers for criticism of earlier drafts.
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