THE NATURE OF EVOLUTIONARY RADIATIONS: ABIOTIC FACTORS AS MECHANISMS

Taxic radiations are considered to be major features of the history of life and have been treated as both macro- and microevolutionary phenomena. Since G.G. Simpson popularized the idea of adaptive radiation, explanations for the processes behind the radiations have largely focused on ecological aspects—competition, new niche occupation, ecological differentiation, key innovations, etc. Rapid speciation, however, is still a basic element of any evolutionary radiation and we argue what causes it must be the core focus of study. Further, given that the prevailing mechanism of speciation is allopatry, the distribution and spatial aspect of an evolutionary radiation must be considered. We examine several case studies from the fossil record, including examples involving trilobites. Geographic complexity seems to be the major factor motivating the radiations; such complexity includes geologic and tectonic complexity overlaid with cycles of sea-level rise and fall. The latter provides multiple opportunities for distributional change, and in conjunction with geology and tectonics, allows taxa to become geographically isolated; if isolation is sustained, this triggers allopatric speciation. In an evolutionary radiation mediated by geographic complexity, there still may be a role for adaptation as natural selection might be the cause of divergence following allopatry. However, geographic isolation is the initiating factor. Further, the switch between evolutionary radiation and quiescence may involve declining geographic complexity. Studies of adaptive radiations should therefore focus more on historical aspects—such as the geology and climate of the region—the radiating clade occupies.