Today, the watchword is Anolis. Anolis, the anoles, is an enormous assemblage of lizards found in warmer parts of the Americas: with well over 350 species, it is the largest genus generally recognised among the amniotes. Species within the genus have received a lot of attention for their ecological diversity. In some parts of Cuba, there may be fourteen or fifteen Anolis species found in a single locality, each occupying their own distinct niche (Thomas et al. 2009). Examples of anole ecotypes include crown-giants, up to and above half a metre in length, that live in the forest canopy; slender, short-legged twig anoles, that creep along narrow branches; and also slender, but much longer-legged, grass-bush anoles that are found in dense undergrowth (Losos 2009).
Some species of anole have even been the subjects of direct experimental studies on evolutionary processes. Hind limb length in different anole species has been observed to correlate with substrate usage: those species that prefer wider branches have longer hind legs than those utilising smaller branches. In order to test whether natural selection played a direct part in determing leg length, the brown anole Anolis sagrei was introduced in 1977 and 1981 to fourteen small islands, of varying vegetation type, in the Bahamas that had been previously uninhabited by anoles. After ten years had passed, measurements were taken of anoles on each of the islands where they had persisted and compared back to the source population. It was demonstrated that (a) many of the experimental populations were statistically significantly different from the source population after ten years, and (b) the degree of difference between populations was correlated with the degree of difference in the vegetation oof the two localities (Losos et al. 1997). However, later studies of brown anoles in the laboratory reared in cages with different-sized available substrates indicates that leg length in anoles is, to some extent, phenotypically plastic depending on environmental pressures (Losos et al. 2000). Nevertheless, a selective component to variation was supported by experiments involving translocation between montane and lowland habitats of the Dominican anole Anolis oculatus (Thorpe et al. 2005).
Being such a huge genus, it is not surprising that attempts have been made to break Anolis down to more manageable units, with varying success. Hillis (1996, as quoted in Poe 2004) described anoles as "a huge group where all the species look virtually the same": a quite unfair aspersion (see above) but nevertheless expressive of the difficulties in establishing relationships between species (ecological convergence, for instance, is rampant). Anoles have been divided into two major subgroups, the Alpha and Beta anoles, on the basis of the presence (Beta) or (Absence) of transverse processes on the caudal vertebrae, and some authors have proposed recognising the Beta anoles as a separate genus Norops. Recent phylogenetic analyses have agreed that the Beta anoles are monophyletic, but nested deeply in the Alpha anoles (Poe 2004; Nicholson et al. 2005). Because of difficulties in defining usual subgroups among the Alpha anoles, recent authors have therefore continued to maintain a super-sized Anolis*.
*Which just highlights again how the binomial system can force false dichotomies. The nested position of 'Norops' within 'Anolis' means that one must either (a) subdivide Anolis, perhaps impractically, (b) sink Norops and obscure that group's distinctiveness, or (c) recognise a paraphyletic Anolis, obscuring the closer relationships between some Anolis and Norops species. Three suboptimal choices, but you must pick one because you can't have species without genera. Surely it would be better overall if one could just sidestep the question by recognising a clade Norops within a clade Anolis?
Also of interest are the biogeographic patterns within Anolis that phylogenetic analysis has revealed. Both the Alpha and Beta anoles have species on the Caribbean islands as well as continental South America (a little less than half the currently recognised Anolis species are found on Caribbean islands). The South American Alpha anoles form a clade (sometimes recognised as a separate genus Dactyloa) that, together with the Anolis roquet group of species found in the southern Lesser Antilles, forms the sister group of most or all of the remaining Anolis species (Poe 2004; Nicholson et al. 2005). Most lineages within the remaining anoles are Caribbean; the South American Beta anoles also form a single clade whose nested position among Caribbean taxa indicates a relatively rare demonstrable case of dispersal from an island to a continent (as opposed to the other way around, the more usual expectation in biogeography). Most of the Greater Antillean islands are home to multiple lineages of anoles; the exception is Jamaica which, except for the recently arrived Anolis sagrei, is inhabited by a single clade of Beta anoles (the sister group to the South American Beta anoles). The Carolina anole Anolis carolinensis of the southeast United States is also of Caribbean origin, being nested among a clade of Cuban species. It is worth noting that this last continental colonisation is, in a way, currently repeating itself: though probably brought by human agents, a number of Caribbean anole species are now known in the wild from Florida.
REFERENCES
Losos, J. B. 2009. Lizards in an Evolutionary Tree: Ecology and adaptive radiation of anoles. University of California Press.
Losos, J. B., D. A. Creer, D. Glossip, R. Goellner, A. Hampton, G. Roberts, N. Haskell, P. Taylor & J. Ettling. 2000. Evolutionary implications of phenotypic plasticity in the hindlimb of the lizard Anolis sagrei. Evolution 54 (1): 301-305.
Losos, J. B., K. I. Warheit & T. W. Schoener. 1997. Adaptive differentiation following experimental island colonisation in Anolis lizards. Nature 387: 70-73.
Nicholson, K. E., R. E. Glor, J. J. Kolbe, A. Larson, S. B. Hedges & J. B. Losos. 2005. Mainland colonization by island lizards. Journal of Biogeography 32: 929-938.
Poe, S. 2004. Phylogeny of anoles. Herpetological Monographs 18: 37-89.
Thomas, G. H., S. Meiri & A. B. Phillimore. 2009. Body size diversification in Anolis: novel environment and island effects. Evolution 63 (8): 2017-2030.
Thorpe, R. S., J. T. Reardon & A. Malhotra. 2005. Common garden and natural selection experiments support ecotypic differentiation in the Dominican anole (Anolis oculatus). American Naturalist 165 (4): 495-504.
Why no link to the premier blog on Anolis biology: Anole Annals?
ReplyDeleteYou mean, this link: http://www.anoleannals.org/;
Delete