ADAPTIVE RADIATIONS IN THE CONTEXT OF MACROEVOLUTIONARY THEORY

Adaptive radiations are often invoked anytime clades show significant bursts of diversification, but it is important to not simply assume that any radiating clade constitutes an adaptive radiation. Here a macroevolutionary theoretic framework is developed, using specific examples from the fossil record, especially those involving trilobites, for identifying and defining various types of evolutionary radiations, including adaptive radiations. This framework builds on the fact that several highly relevant macroevolutionary concepts including the Turnover Pulse Hypothesis, the Effect Hypothesis, and species selection, have not previously been considered in the adaptive radiations literature; here these are more fully integrated into the theory of adaptive radiations. In addition, the relevance of whether radiations might have been driven by exaptations instead of adaptations is emphasized. Another important issue considered is the centrality of monophyly to the identification of an evolutionary radiation. For example, phenomena informally treated as evolutionary radiations, such as the Cambrian radiation, should in actuality be considered not as one but several separate evolutionary radiations: radiations where adaptation likely played little if any role in spurring diversification. Ironically the fossil record, the source of the initial hallmark examples of adaptive radiation, now appears to show little concrete support for this phenomenon.

Other types of evolutionary radiations are identified, including geographic radiations, which are driven by opportunities for allopatric speciation created by geologic or climatic change, or a taxon’s presence in a geographically complex region. Potential examples of adaptive radiation are here consigned to only a single quadrant of the various types of evolutionary radiation. Even within that quadrant there can be either exaptive or adaptive radiations. Special emphasis is also placed on considering the role that abiotic as opposed to biotic factors may play in motivating diversification during evolutionary radiations in general, and adaptive and exaptive radiations in particular.