Adaptive radiation: does the evidence support the theory?

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The Ecology of Adaptive Radiation. By Dolph Schluter. Oxford University Press, Oxford. 2000. 288 pp. Price £19.95. ISBN 0 19 850523 X (hbk), 0 19 850522 1 (pbk).

The process of adaptive radiation is a fundamental building block of the process of speciation and equally important to those concerned with variation at the species level, for adaptive radiation is first manifested as variation among populations. Dolph Schluter has done an excellent job of assembling what evidence exists for adaptive radiation and comparing it to theoretical predictions. An enormous amount of data and analysis has been compressed into a relatively short space without sacrificing clarity. This book is not simply a review of the relevant literature, though it does this, but a detailed analysis of the data, presenting new and novel analyses. Thus this book marks an important step in research, providing a firm basis from which future studies can proceed.

Chapter 1 is a short introductory chapter setting out the scope of the book. What is an ‘adaptive radiation’? According to Schluter it is (p. 2) ‘the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage’ and ‘It occurs when a single ancestor diverges into a host of species that use a variety of environments and that differ in traits used to exploit those environments’. This definition does not specify whether the group under consideration consists of species within the same genus or groupings at higher taxonomic levels. Schluter concentrates on lower taxonomic levels but does not restrict his analysis to one particular level.

Chapter 2 sets out definitions in more detail, provides a very useful table summarizing experiments testing the ‘fit’ of species to their diverse environments, and gives four examples (Galapagos finches, West Indian anoles, Hawaiian silverswords, and columbines).

Chapter 3 addresses the long-standing hypothesis that radiation leads to specialization. Schluter questions the theoretical basis of the hypothesis and presents persuasive evidence that radiation can commence from either a generalist or a specialist ancestor. A general pattern that does emerge is that the same pattern of radiation can be repeated, demonstrating that ‘historical contingency’ is not overwhelming.

Chapter 4 examines the ecological basis of the theory, which requires that divergence results from diverse habitats and competition leading to divergent selection. This last process is taken up in chapter 6, which commences with a discussion of the concept of the adaptive landscape and devotes the remaining part of the chapter to an assessment of the evidence for divergent selection. In chapter 6, Schluter assembles data to test the hypothesis that interspecific competition occurs and leads to character displacement. As in the previous chapter, the survey of the evidence is extensive and convincing.

An alternative route for adaptive radiation is the presence of ecological opportunity, as for example when fish invade lakes in which there are no competitors. This possible route is described in chapter 7, and though there is some evidence, it is not overwhelming and Schluter concludes that its general importance requires further study. Adaptive radiation can lead to divergent populations but the theory posits that the process described above will lead to speciation.

Chapter 8 considers the logic and empirical support for this theory. In part, because speciation is not a process that can be observed in ‘experimentally tractable’ time periods, we have much more theory and anecdotal evidence than convincing data. Nevertheless, some data are available, such as evidence for divergent sexual selection between the pied and collared flycatchers, that do provide tentative support at least for components of the theory.

Chapter 9 takes a different tack and discusses the role of quantitative genetic analysis for an understanding of adaptive radiation. One of the important messages of this chapter is that evolution trajectories are not unconstrained but will tend to follow pathways that deviate from the direction of steepest ascent to the most fit trait combination. How much this contributes to adaptive radiation depends upon whether the world is at or close to equilibrium. At present there are certainly too few data to answer this question.

Chapter 10 provides an overall summary and concludes: ‘Although the theory needs to be expanded in parts, none of its most significant claims have yet been overthrown’.

Because of the clarity with which this book is written it should be a suitable text for advanced undergraduates, graduate students and other researchers either interested in or studying adaptive radiation. It was a pleasure to read and I highly recommend it.

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