Partial island submergence and speciation in an adaptive radiation: a multilocus analysis of the Cuban green anoles

Richard E. Glor () 2 Matthew E. Gifford 2 Allan Larson 2 Jonathan B. Losos 2 Lourdes Rodrguez Schettino 1 Ada R. Chamizo Lara 1 Todd R. Jackman 0 0 Department of Biology, Villanova University , Villanova, PA 19085 , USA 1 Instituto de Ecologi 2 Department of Biology , Campus Box 1137 , Washington University , St Louis, MO 63130-4899 , USA Sympatric speciation is often proposed to account for species-rich adaptive radiations within lakes or islands, where barriers to gene flow or dispersal may be lacking. However, allopatric speciation may also occur in such situations, especially when ranges are fragmented by fluctuating water levels. We test the hypothesis that Miocene fragmentation of Cuba into three palaeo-archipelagos accompanied species-level divergence in the adaptive radiation of West Indian Anolis lizards. Analysis of morphology, mitochondrial DNA (mt DNA) and nuclear DNA in the Cuban green anoles (carolinensis subgroup) strongly supports three predictions made by this hypothesis. First, three geographical sets of populations, whose ranges correspond with palaeo-archipelago boundaries, are distinct and warrant recognition as independent evolutionary lineages or species. Coalescence of nuclear sequence fragments sampled from these species and the large divergences observed between their mtDNA haplotypes suggest separation prior to the subsequent unification of Cuba ca. 5 Myr ago. Second, molecular phylogenetic relationships among these species reflect historical geographical relationships rather than morphological similarity. Third, all three species remain distinct despite extensive geographical contact subsequent to island unification, occasional hybridization and introgression of mtDNA haplotypes. Allopatric speciation initiated during partial island submergence may play an important role in speciation during the adaptive radiation of Anolis lizards. 1. INTRODUCTION Speciation is the least understood aspect of adaptive radiation (Schluter 2000). Classic models of allopatric divergence are often challenged by adaptive radiations occurring in small, geographically homogeneous areas where barriers to gene flow or dispersal may be lacking. In some such radiations, non-allopatric speciation is suggested: sympatric speciation, for example, is considered common in African rift-lake cichlids (Schliewen et al. 1994, 2001; Seehausen & van Alphen 1999; Shaw et al. 2000). However, allopatric speciation also occurs in cichlids when large lakes become fragmented into smaller ones during historically dry periods (Sturmbauer & Meyer 1992; Ru ber et al. 1998; Sturmbauer et al. 2001). We test the hypothesis that a similar mechanism contributes to speciation in adaptive radiation of Anolis lizards. West Indian anoles represent a classic example of a species-rich adaptive radiation within several relatively small, geographically isolated areas (Losos & Schluter 2000). On Cuba, where anole diversity is highest, more than 60 species occur over an area of only 110 000 km2 (Rodrguez-Schettino 1999). Most anole species on Cuba and other Greater Antillean islands result from within-island processes (Losos & Schluter 2000), leading some authors to suggest that sympatric speciation plays a part in anole diversification (Shaw et al. 2000; Thomas et al. 2003). However, several lines of evidence suggest that geographical processes play the dominant role in intraisland anole speciation. Much of this evidence comes from small (area of less than 2000 km2) Lesser Antillean islands where anoles often exhibit extraordinarily high levels of geographical genetic structure (Malhotra & Thorpe 1991, 1993, 1994, 1997a,b, 2000; Schneider 1996; Thorpe & Malhotra 1996; Ogden & Thorpe 2002; Stenson et al. 2002; Thorpe & Stenson 2003). Although anole speciation is not known to have occurred on these small islands, probably owing to their small size (Losos & Schluter 2000; Stenson et al. 2002), the same geographical processes acting there may result in speciation on the larger Greater Antillean islands (Stenson et al. 2002). Indeed, recent studies reveal high levels of mitochondrial DNA (mtDNA) geographical structure within several species of Greater Antillean anoles ( Jackman et al. 2002; Glor et al. 2003). More generally, groups of closely related allopatrically or parapatrically distributed species, which are common on the large Greater Antillean islands, suggest an important role for geographical processes in intra-island anole speciation (Losos 1996). One overlooked mechanism for allopatric differentiation in the Greater Antilles is partial submergence, or historical fragmentation of islands by over-water barriers (Stenson et 15 Hispaniola al. 2002), a pattern analogous to fragmentation of African rift lakes by lower water levels. Reconstructions of historically emergent areas of Cuba, for example, suggest that this island consisted of three distinct archipelagos separated by deepwater channels throughout much of the Cenozoic before being permanently united in the Pliocene (IturraldeVinent & MacPhee 1999; MacPhee et al. 2003; Graham 2003). Evidence for fragmentation of Cuba during the Miocene is compelling and includes the discovery of fossil marine vertebrates in central Cuba (MacPhee et al. 2003). Several lines of evidence also suggest that the Miocene was an important period of Anolis diversification: ecologically specialized anoles occur in Early to Mid-Miocene Dominican amber (de Queiroz et al. 1998) and molecular analyses of albumin, allozymes and mtDNA suggest that anoles arrived in the West Indies 1637 Myr ago and that most species diverged more than 10 Myr ago (Hedges 1996; Jackman et al. 2002; Glor et al. 2003). We test the hypothesis that partial submergence of Cuba contributed to speciation of the canopy-dwelling green anoles (carolinensis subgroup). This subgroup includes three morphologically distinct populations whose distributions closely mirror the boundaries of the palaeo-archipelagos (figure 1). Anolis allisoni , a large species whose males have striking blue heads, is common across the area corresponding to the large central palaeo-archipelago. Anolis porcatus, which has a smaller body size, includes two geographically disjunct and morphologically distinct populations (Ruibal & Williams 1961) whose ranges correspond to the eastern and western palaeo-archipelagos (figure 1). Anolis porcatus also extends into Central Cuba but is rare where its range overlaps with the larger A. allisoni (Ruibal & Williams 1961; Rodrguez-Schettino 1999). Character displacement in body size observed in A. porcatus suggests that interspecific competition may explain this pattern (Schoener 1977). The partial submergence hypothesis makes three specific predictions. First, all three morphologically distinguishable populations (eastern and western A. porcatus and A. allisoni ) should represent distinct evolutionary lineages that diverged prior to the unification of Cuba in the Pliocene. Second, phy (...truncated)