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317 15 THE EVOLUTION OF AN ADAPTIVE RADIATION In Chapter 11, I defined adaptive radiation as the evolutionary divergence of members of a clade to adapt to the environment in a variety of different ways and presented three predictions made by a hypothesis of adaptive radiation: . Species interact ecologically, primarily by competing for resources. . As a result of these interactions, species alter their resource use. . As a result of shifts in resource use, species evolve appropriate adaptations. A combination of experimental, observational, and comparative data (reviewed in Chapters 11 and 12) strongly supports these predictions. Thus, Greater Antillean anoles meet the expectations of an adaptive radiation. In this and the next chapter, I will shift the focus from how adaptation occurs within species to the nuts and bolts of how an entire radiation unfolds. Gould (1989, 2002) argued for the predominance of contingency in evolution, suggesting that if we “re-ran the evolutionary tape” and started again from the same point, the outcome likely would be very different. However, the replicate adaptive radiations of anoles in the Greater Antilles challenge this view, a point on which I’ll focus in the next chapter. Before doing so, however, it’s worth looking in close detail at how anole diversification has proceeded. Can we trace the actual course of evolution, and has it occurred in the same way on each island in the Greater Antilles? What factors, if any, have instigated anole diversification, and have rates and patterns of evolution changed through time? losos_ch15.qxd 4/11/09 9:36 AM Page 317 In this chapter, I’ll focus primarily on the anoles of the Greater Antilles, finishing by examining the concept of adaptive radiation itself. In Chapter 16, I will examine the other four anole faunas (Chapter 4) to understand why their evolutionary diversification for the most part has taken a different path than that followed by the Greater Antillean ecomorphs. HISTORICAL INFERENCE OF PATTERNS OF ADAPTIVE RADIATION IN THE GREATER ANTILLES ECOMORPHOLOGY OF THE ANCESTRAL ANOLE We might wonder, for starters, what the progenitor of the anole adaptive radiations was like. Is there any way to infer the morphology and ecology of the species ancestral to the anole radiations on each of the Greater Antilles? Several lines of evidence provide somewhat conflicting insights. Initially, I attempted to infer the morphology of ancestral species by reconstructing their character states using parsimony (Losos, 1992a). This exercise resulted in hypothetical ancestral species on Jamaica and Puerto Rico that were intermediate in morphology between existing species, leading to the conclusion that the ancestral anoles were ecological generalists that presumably had relatively broad niches in the absence of competing species.383 However, as discussed in Chapter 5, reconstructing ancestral states for characters that evolve at relatively high rates is fraught with difficulty, and Schluter et al. (1997) showed that the confidence limits on reconstructions for the ancestral anole in Puerto Rico are enormous, overlapping the position of most ecomorphs in morphological space (Fig. 7.3; see also Fig. 7.4).384 Consequently, character reconstruction appears unable to provide much insight into the morphology of the ancestral anole of the Greater Antilles. An alternative approach to investigating the ancestral anole is to look at the species occupying one-species islands today (henceforth, “solitary anoles”) with the assumption that those species approximate the state of the ancestral anole. Consideration of solitary anoles suggests the hypothesis, in contrast to the results of phylogenetic reconstruction, that the ancestral species to the anole radiations may have been a trunk-crown anole. 318 • T H E E V O L U T I O N O F A N A D A P T I V E R A D I A T I O N 383. This is a fairly standard view for the ancestor of an adaptive radiation, implying that a generalist ancestor gives rises to specialized descendants. Schluter (2000) showed that the evidence for this pattern as a general scenario for adaptive radiation is not compelling. 384. Two notes on my analysis in the 1992 paper. First, the conclusion that the ancestral form was intermediate is not an artifactual outcome that resulted because the method for inferring ancestral traits averages phenotypes of descendant species. Although some algorithms to reconstruct ancestral states work in this manner (e.g., squared change parsimony [Huey and Bennett, 1987]), the method that I employed, linear parsimony, does not (Swofford and Maddison, 1987). Second, the phylogenies were patched together based on a variety of...

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