The Next Generation of Adaptive Radiation Studies in Plants

Adaptive radiation is an evolutionary process that has been promulgated in some clades as an explanation for species richness and disparity in morphological forms across ecological gradients. Studies designed to elucidate the mechanisms and causes of adaptive radiation have largely focused on animal systems, but plant clades have tremendous potential to answer elusive questions regarding adaptive radiations. The goals of this review are to (1) produce a synthetic understanding of adaptive radiations through studies that have investigated plants systems, (2) critically reflect on contemporary studies to highlight how approaches have been successful as well as limiting, and (3) outline gaps in our understanding of adaptive radiations while emphasizing that plants have ideal characteristics to answer future questions. Thirty-five adaptive radiation clades are highlighted, of which several are supported with multiple lines of evidence, such as the Hawaiian silverswords, Hawaiian lobeliads, and columbines. Plant adaptive radiation examples are commonly insular, diversified in the Miocene or Pliocene, are associated with dispersal-mediated ecological opportunities, are polyploids, and have experienced hybridization. From those studies, a general model of plant insular adaptive radiation is proposed. The limitations of the current reliance on phylogenetic comparative approaches to detect adaptive radiations are considered, and an integrative approach that includes phylogenetics, genomics, and evolutionary ecology is advocated. The review concludes with a call for additional studies that are needed before we are to fully understand adaptive radiations, and they include the following: (1) how do biological interactions influence adaptive radiations, (2) what role does environmental change play in generating ecological opportunity, (3) how does genetic evolution drive adaptive radiation, (4) do models adequately explain the adaptive radiation process, (5) what is the role of hybridization, and (6) why do some groups not undergo adaptive radiation after ecological opportunity?