Adaptive Radiation: Innovations and Insights

Givnish, T.J. & Sytsma, K.J. ( 2000 ) Molecular evolution and adaptive radiation . Cambridge University Press , Cambridge . xvii + 621 pp, figs, tables, index. Paperback: Price £24.95, US$39.95 , ISBN 0 521 77929 4 . Adaptive radiation is one of the most fundamental concepts in evolutionary biology, and yet perhaps one of the least well understood. The phenomenon in the Galapagos finches spawned the classic studies of Darwin, and subsequently Lack and Grant. Implicit in the concept is the formation of a suite of closely related species adapted to exploit different habitats or life styles. The phenomenon presents an unusual opportunity for comparative studies on the processes of speciation and selection in natural populations. However, because adaptive radiation is usually rapid, and strongly ecologically driven, traditional phylogenetic studies have been impeded by the tendency of morphological synapomorphies to be few and frequently dominated by convergence. The advent of molecular systematics allows independent assessment of phylogeny, and the framework can be used to examine the evolution of morphological, ecological, behavioural, and physiological adaptations, and the circumstances under which they have allowed species’ proliferation. Based on the promise of molecular systematics to studies of adaptive radiation, Givnish and Sytsma organized an international symposium on Molecular Evolution and Adaptive Radiation, held at McGill University in Montreal during June 1995, upon which this book is based. The introduction of molecular systematics has revolutionized the study of adaptive radiation, illustrated by the diverse array of studies herein. The hardback version of this book was published in 1997 (ISBN 0 521 57329 7). The book provides an excellent compendium of some of the now classic studies of adaptive radiation in the light of recent molecular work. In particular, the ‘state-of-the-art’ (at least, as it was in 1995) is presented for the African cichlids (Reinthal & Meyer), sticklebacks (Taylor et al.), Caribbean anoles (Jackman et al.), and the Hawaiian silversword alliance (Baldwin) and contrasting Argyranthemum radiation in the Canaries (Francisco-Ortego et al.). Kambysellis and Craddock provide fascinating new insights into the diversification of Hawaiian Drosophila. Other papers are more preliminary, but suggest some fascinating new avenues for research, for example Givnish et al. on the Brocchinia of the South American tepuis, and Sakai et al. on Hawaiian Alsinodendron. Other very striking chapters were those that examine a key innovation of known adaptive significance. As Hodges notes ‘imperative to testing a key innovation hypothesis is determining how the key innovation may affect processes important in species diversification’. For Aquilegia, Hodges uses the known importance of nectar spur morphology together with sister species’ comparisons to make a strong argument for the basis of adaptive radiation. Likewise, Hapemann & Inoue examine the diversification of Platanthera orchids in terms of detailed knowledge of the pollination biology of the group. For readers that are looking for some resolution as to (1) the nature of an adaptive radiation, and (2) the relative use of molecular vs. morphological characters, this book will not provide the answer. First, there is a good deal of discrepancy on how adaptive radiation should be defined. Givnish gives an excellent and comprehensive review of the topic. However, some of the chapters view adaptive radiation simply as a form of disruptive selection, and ‘ecological dichotomies between ponds and lakes would likely create divergent selection pressures similar to those that have provoked [insular] radiations’ (Colebourne et al.). Also, it is not clear at what level to study adaptive radiation, as the ecological diversification initially occurs at the species level. When adaptive radiation is inferred to have occurred in the distant geological past, two assumptions must be made: (1) the ecological environment at the time is known, allowing inference of selection pressures that the lineage may have encountered; and (2) the pattern of adaptive radiation has not been obscured by long-separated evolutionary histories and differential extinction. These assumptions appear to be at least partially met in the case where ancient radiations can be linked to ecological changes in the environment. For example, colonization of the African savannas and the diversification of ungulates and bovids, and the adaptive radiation of marsupials in Gondwana (Springer et al.). However, when much of the ‘evidence’ for an adaptive radiation is based on emergent properties of intergeneric ecological diversity of a lineage (Colebourne et al.; Horovitz & Meyer; Barrett & Graham; Smith & Littlewood), arguments as to the nature of the adaptive radiation must often rely on identification of a ‘key innovation’. As Vogler and Goldstein point out, ‘topology is no more capable of distinguishing key adaptation’ from straightforward synapomorphy than it is from distinguishing adaptive from other character changes’. The second area of discrepancy between chapters was the use of molecular vs. morphological characters: Givnish discusses the ‘particularly insidious problem…. [of] concerted convergence’ in which characters converge independently on similar forms, and the necessity of using characters ‘independently involved in that radiation’. Also, Givnish and Sytsma state that ‘&… if morphological and molecular data separately generate quite incongruent phylogenetic relationships, we see … considerable danger in their indiscriminate combination’ (p. 89). However, a few chapters later, Horovitz and Meyer argue that ‘different data sets reflect a shared history, so the phylogenetic information they contain should be the same, even if it is obscured by homoplasy’. The final chapter by McCune is a nicely refreshing study of rates of diversification among lacustrine fish. She compares the rate of speciation of fish in each individual lake with rates inferred for other species’ radiations from the archipelagoes of Hawaii and the Galapagos, using the age of the oldest island in the case of the archipelagoes. For such a comparison, it might have been more appropriate to use the age of the African lake system (approximately 5 mya, Sturmbauer, 1998) for the cichlids, in which case there is probably little difference in speciation rates between many groups that have radiated. However, this chapter does provide a very stimulating new approach to studying adaptive radiation. Overall, this book is an essential text for anyone interested in evolutionary biology. Many questions are left unanswered. Some are answered ‘too often’ and appear to contradict each other. However, if all the answers were there, why would we be so fascinated by the study of adaptive radiation? Not surprisingly, since 1995 many additional studies have applied molecular systematics to the study of adaptive radiation (Gillespie et al., 2001), including that of the classic Galapagos finches (Petren et al., 1999; Sato et al., 1999). Givnish and Sytsma’s book has set the groundwork for this burgeoning and exciting, developing, field of biology.

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