Adaptive Radiation

Abstract

Adaptive radiation is the simultaneous diversification of a lineage into numerous sublineages and specializations (Simpson, 1953a). All of the species of a radiation constitute a monophyletic group, and they often share some innovative trait or set of traits (sometimes called a key innovation—Liem, 1973) that is thought to have allowed the lineage to undergo a major transition, moving it into a previously unoccupied zone of opportunity exploited in different ways by different branches A radiation is a proliferation of variations on a phenotypic theme, accompanied by a proliferation of species. Adaptive radiations are a major feature of the evolution of life. Biologists describe radiations at all phylogenetic levels—the radiation of the eukaryotes, the radiation of multicellular organisms, the radiation of vertebrates, of birds, of ungulates, of dung beetles, of a particular species complex of fish. The concept of adaptive radiation usually includes both lineage branching (speciation) and phenotypic diversification (Schluter, 2000). Speciation and phenotypic diversification are obviously related phenomena: the genetic isolation associated with speciation promotes phenotypic divergence between populations (Mayr, 1963; Futuyma, 1987), and phenotypic divergence within populations (polymorphism and polyphenism) can promote speciation, as discussed in chapter 27, so the two phenomena must often occur together. It does not follow, however, that adaptive radiation and speciation are inevitably linked (see Turner, 1999). Adaptive radiation can occur without speciation, as in the multiple individual trophic specializations of Cocos Island finches (see below) or in the diversity of human careers. And speciation can occur with little adaptive (ecological) diversification. Polistes, a cosmopolitan genus of social wasps (Vespidae), contains more than 300 species that differ primarily in their social and sexual behavior, with relatively little interspecific variation in trophic morphology and behavior, diet, size and shape, or habitat (reviewed in Turillazzi and West- Eberhard, 1996). The possibility of socially or sexually selected radiations with little ecological diversification is a neglected topic that deserves attention, but I do not pursue it here.