Abstract
Comparisons between closely related radiations in different environments provide a unique window into understanding how abiotic and biotic factors shape evolutionary pathways. Anolis lizards have radiated extensively in the West Indies, as well as in mainland Central and South America. In the Caribbean, similar communities of anole species specialized for different habitats (ecomorphs) have evolved independently on each Greater Antillean island. We examined ecological and morphological data on 49 Anolis species (33 Caribbean, 16 mainland) to investigate whether the same set of ecomorphs has arisen in mainland regions. More generally, we investigated whether the relationship between ecology and morphology was similar among anoles in the two regions.
Radiations in the two regions
are very different. The majority of mainland anole species exhibit
morphological characteristics unlike any Caribbean ecomorph. Furthermore,
relationships between ecology and morphology are very different between
the two sets of anole species. Among mainland anole species, toepad
size is positively correlated with perch height, whereas tail length is
negatively related to perch diameter. In contrast, among Caribbean
anole species, both forelimb length and body mass are positively associated
with perch diameter, and tail length and hindlimb length are negatively
related to perch diameter. Biomechanical considerations provide a
functional basis for some of these correlations, but much variation remains
to be explained. These findings demonstrate that factors that caused
anole species to converge repeatedly in the West Indies are not present
in mainland regions, and that environmental factors can strongly influence
the shape of evolutionary radiations.
Non-Technical Summary
Considerable research has been conducted on the evolutionary radiations of Anolis lizards in the Caribbean. On islands of the Greater Antilles (Cuba, Hispaniola, Jamaica, and Puerto Rico), a remarkable case of convergent evolution has occurred. On each island, species have diversified to adapt, both behaviorally and morphologically, to different microhabitats. Across islands, the same set of habitat specialists, termed ecomorphs, have evolved on each island (with several exceptions; see Losos et al., 1998).
Despite the great body of research devoted to the study of all aspects of Caribbean anole natural history, the majority of anole species do not occur in the Caribbean. Rather, most are found in Central and South America (currently, about 400 species of anoles are known, only 140 of which occur in the Caribbean). Moreover, just as in the Caribbean, communities on the mainland are composed of many (up to 10 or more) sympatric species differing in habitat use, behavior, and morphology.
We set out to ask two questions:
1. Do members of the Caribbean ecomorph classes occur on the mainland?
2. Does a relationship between morphology and habitat use exist among mainland anoles and, if so, is the relationship the same as in the Caribbean?
To conduct these analyses, we amassed data on habitat use and morphology for 33 Caribbean species from the Greater Antilles and 16 mainland species from Brazil, Ecuador, Nicagarua, and Panama.
We found:
1. For the most part, species on the mainland did not correspond to any of the Greater Antillean ecomorph classes, although A. auratus shows great similarity to grass-bush anoles and A. frenatus is moderately similar to crown-giants.
2. The relationship between morphology and habitat use varied substantially between mainland and Caribbean anoles. For example, among island anoles, limb length is strongly correlated with diameter of perch supports. Conversely, the relationship between number of subdigital lamellae and perch height (see Glossip and Losos, 1997) was substantially stronger for mainland anoles than for island anoles.
These findings indicate that the course of adaptive differentiation has been quite different in the two areas. First, in the Caribbean, anole species have adapted to similar ecological "niches" in similar ways. On the mainland, these niches--the base of trees, grassy areas--would seem to be available, but anole species either are not occupying them, or, if they do, they adapt to them in different ways. This latter possibility is supported by the fact that the relationship between morphology and habitat use differs in the two areas.
Many factors could be responsible for these differences, but one that seems most germane is biotic differences in the two areas. In particular, in the Caribbean, anoles are essentially the only arboreal, diurnal insectivorous lizards present. Bird diversity and density is relatively low and the number of predators are low. Consequently, anoles attain high population densities and the major selective factor--for many species and in many localities--is intraspecific competition. By contrast, on the mainland there are many other ecological competitors, both lizards and other types or organisms. In addition, predation pressure is substantially higher. As a result, anole densities are much lower. Moreover, anole lifestyles are much different, particularly because avoiding predation is a much greater concern. Consequently, the ways in which anoles adapt to a particular habitat in the Caribbean are probably very different from those favored in the mainland, where avoiding being detected by and escaping from predators are probably of paramount importance. More detailed studies of mainland taxa are needed to further explore these intriguing patterns.