Lizards in an Evolutionary Tree: Ecology and Adaptive Radiation of Anoles

14. Speciation and Geographic Differentiation

291 14 SPECIATION AND GEOGRAPHIC DIFFERENTIATION Adaptive radiation involves both multiplication of species from a single ancestor and ecological and phenotypic diversification of these species, with the end result that communities are composed of multiple species adapted to different niches. The focus of the last several chapters has been on the second of these two aspects, but the first, the manner in which one ancestral species gives rise to many descendant species, is equally important. Anoles have speciated prolifically, and in the Greater Antilles most of this speciation has occurred within islands, rather than resulting from cross-island colonization and subsequent divergence (Chapter 6). Despite the great extent of this withinisland speciation, surprisingly little research has addressed the means by which it occurs, much less the role that speciation plays in anole adaptive radiation. SPECIATION AND ADAPTIVE DIVERGENCE At the extreme, two views could be taken on the relationship between speciation and adaptive divergence (reviewed in Schluter, 2000, 2001). On one hand, the two could be completely unrelated. During the process of speciation,349 differentiating populations might not diverge adaptively, with the result that speciation would produce the raw 349. By “speciation process,” I refer to cladogenetic speciation in which one ancestral species gives rises to two descendant species, rather than anagenetic speciation, in which an ancestral species transforms into a different descendant species. losos_ch14.qxd 4/11/09 9:32 AM Page 291 material—multiple reproductively isolated entities—for adaptive radiation, but the adaptive component would come later in the process, perhaps as the result of ecological interactions as discussed in previous chapters. This scenario might be particularly likely if speciation occurs in allopatry, followed by ecological differentiation of descendant species when they secondarily come into sympatry. Selection and adaptive divergence are not necessary in several types of speciation, such as speciation resulting from founder effects, polyploidy, or genetic drift in large populations. For example, allopatric speciation in many groups (e.g., salamanders, snails) often produces geographically isolated species that exhibit few or no adaptive differences (Gittenberger, 1991; Kozak and Wiens, 2006; Wake, 2006); such speciation might result from adaptively neutral processes such as genetic drift or some forms of sexual selection (reviewed in Schluter, 2000; Rundle and Nosil, 2005). On the other hand, at the opposite extreme, the processes of speciation and ecological divergence might be intimately interrelated. Disruptive selection could lead to ecological differentiation occurring within an ancestral species at a single locality. As the subpopulations differentiated ecologically, selection might favor the evolution of reproductive isolation to prevent interbreeding, thus avoiding the production of offspring ecologically intermediate and unfit for either niche; in turn, the evolution of reproductive isolation would avoid the homogenizing effect of genetic exchange, thus allowing further ecological differentiation. This of course is the highly controversial process of sympatric speciation, in which adaptive differentiation and speciation are causally related and occur simultaneously (reviewed in Coyne and Orr, 2004). Repeated instances of sympatric speciation could lead to a multitude of ecologically differentiated species—an adaptive radiation—all produced in situ. Two intermediate possibilities exist between the extremes of sympatric speciation and nonadaptive speciation in allopatry: adaptively-driven divergence in allopatric populations resulting in speciation, and speciation among parapatric populations arrayed along an ecological gradient. ADAPTIVE DIVERGENCE IN ALLOPATRY Populations that speciate in allopatry may also diverge phenotypically, and some of this divergence may reflect adaptation to different environmental conditions. In fact, laboratory and field studies make clear that allopatric populations diverging to adapt to different environmental situations are much more likely to evolve barriers to successful interbreeding than are allopatric populations living in similar environments (Rice and Hostert, 1993). This result is in agreement with Dobzhansky’s (1937) view that reproductive isolation often results as an incidental by-product of evolutionary change, rather than being selected for directly. Consequently, the divergence that occurs in allopatric speciation may contribute to the extent of phenotypic diversification that occurs in an adaptive radiation. 292 • S P E C I A T I O N A N D G E O G R A P H I C D I F F E R E N T I A T I O N losos_ch14.qxd 4/11/09 9:32 AM Page 292 Assuming that populations diverge adaptively in allopatry, what happens when they come back into sympatry? Basically, three possibilities exist. If complete reproductive isolation has already evolved, the populations will interact as different species, and...