Taxonomic Considerations
Several recent hypotheses have been presented regarding relationships among the living genera of large herbivorous lizards collectively known as iguanas (de Queiroz 1987; Frost and Etheridge 1989; Norell and de Queiroz 1991; but see Lazell 1989, 1992). As a working hypothesis, the family Iguanidae is here considered to be restricted to this radiation, which contains seven genera (Amblyrhynchus, Conolophus, Ctenosaura, Cyclura, Iguana, Dipsosaurus, Sauromalus; eight if Enyaliosaurus is considered distinct from Ctenosaura) indigenous to North and South America (de Queiroz 1987; Frost and Etheridge 1989), as well as the South Pacific genus Brachylophus. Much attention has been given to the behavioral and ecological aspects of iguana biology (Burghardt and Rand 1982), but until recently modern systematic studies have lagged behind (Etheridge 1982). Consequently, the biogeographic history and evolutionary relationships among these genera remain poorly understood and controversial. In addition to morphological and chromosomal data collected thus far, different classes of molecular data hold great promise for further resolving iguanid relationships. These approaches might also address some of the objections raised by Lazell (1989) regarding species and generic boundaries within the radiation. Research is needed to test independently the current definitions of species and genera recognized on the basis of morphological and skeletal characteristics, and to develop a well-corroborated phylogenetic hypothesis for the entire group.
About 30 species of iguanas are currently recognized (Burghardt and Rand 1982; de Queiroz 1987). Many are not well defined, especially within the genera Ctenosaura and Cyclura, and ideally all should be verified by several genetic markers. The radiation of rock iguanas throughout the Greater Antilles and the Bahamas is among the most diverse within the Iguanidae with respect to species numbers recognized on the basis of morphological criteria. Sampling with molecular markers to verify species boundaries is especially important in the West Indies because of the island distributions of rock iguanas and the extremely complex patterns of diversification and geological history in this region (Rosen 1985; Hedges et al. 1992).
The literature on species and their recognition continues to increase, with little apparent consensus on how to diagnose independently-evolving lineages (Wiley 1978; reviewed by Frost and Kluge 1995). However, a number of operational proposals have been offered which avoid the pitfalls of tree-based character methods (Doyle 1995), with specific guidelines for their implementation (Avise and Ball 1990; Davis and Nixon 1992; Templeton 1994; Templeton and Sing 1993; Mallet 1995). Under these methods, evaluations of species boundaries become testable hypotheses subject to verification or falsification, a practice that should become standard in future genetic studies of iguanas whenever species boundaries are an issue. A concern arises with regard to the use of mitochondrial DNA markers to address questions regarding species boundaries (Moritz 1994). Although mtDNAs are known to show substructuring in some geographically widespread iguanid lizards (Lamb et al. 1992), small inbreeding effective population size may render this approach inadequate for delimiting species boundaries.
Equally important to the identification of species for determining the taxonomic composition and diversity of the Iguanidae is the development of a well-corroborated phylogenetic hypothesis for all species and genera. This will provide not only an evolutionary and biogeographic context within which specialized ecological and behavioral adaptations can be evaluated, but also a measure of the genealogical distinctness of each species. The latter can contribute to the establishment of conservation priorities on the basis of overall measures of taxonomic diversity and evolutionary importance (Vane-Wright et al. 1991; Avise 1992; Brooks et al. 1992; Stiassny 1992). Sites et al. (1996) recently reconstructed the phylogenetic relationships among living Iguanidae on the basis of mtDNA sequence and morphological characters, and showed strong support for monophyly of all genera named above except Enyaliosaurus.
