The vagaries of history lead
to the prediction that repeated instances of evolutionary diversification
will lead to disparate outcomes even if starting conditions are similar.
We tested this proposition by examining the evolutionary radiation of Anolis
lizards on the four islands of the Greater Antilles. Morphometric
analyses indicate that the same set of habitat specialists, termed ecomorphs,
occurs on all four islands. Although these similar assemblages could
result from a single evolutionary origin of each ecomorph, followed by
dispersal or vicariance, phylogenetic analysis indicates that the ecomorphs
originated independently on each island. Thus, adaptive radiation
in similar environments can overcome historical contingencies to produce
strikingly similar evolutionary outcomes.
Approximately 140 species of Anolis lizards occur on islands in the Caribbean. More than 3/4 of these species occur on the four islands of the Greater Antilles (Cuba, Hispaniola, Jamaica, and Puerto Rico). Examination of the lizards on these islands reveals two patterns of interest. First, on any given island, a number of species (as many as 11 on Cuba) occur sympatrically. Examination of these species reveals that they differ ecologically, morphologically, and behaviorally in a way that makes sense. For example, one species occurs on the base of trees, where it sits with a characteristic head-down posture surveying the ground. It has long legs, which presumably are useful for jumping to the ground and running quickly in pursuit of insects or competitors. Similarly, twig are utilized by slender lizards whose short legs are useful in carefully navigating through their complex and precarious habitat. Other species occur in the canopy, the grass and the tree trunk.
Most interestingly, when one compares the lizards on different islands, one finds the same set of habitat specialists--termed "ecomorphs"--on each island. Thus, if one looked at the "trunk-ground" species (ecomorphs are named for the part of the habitat they normally utilize) on one island (say Puerto Rico), one could then go to any of the other islands and find a morphologically very similar species occurring in the same part of the habitat and behaving in pretty much the same way. The same is true for the other ecomorphs (with a few exceptions, such as the absence of a grass-dwelling species on Jamaica).
Two hypotheses could explain this pattern. First, it is possible that each of the ecomorph types arose only a single time. This could have resulted if all of the islands were once combined into a single landmass (Caribbean geology is surprisingly poor understood, but such a configuration has never been suggested) or if an ancestral species arose on one island and subsequently dispersed to the other islands. Alternatively, each of the habitat specialists could have evolved independently on each island.
These hypotheses can be easily distinguished by examination of a phylogeny. If each ecomorph evolved only once, then members of that ecomorph should be closely related, even if they occur on different islands. Conversely, if the members of an ecomorph class evolved convergently, then they should not be closely related.
To test these hypotheses, we sequenced 1400+ base pairs of mitochondrial DNA. Examination of the most parsimonious trees strongly supports the independent evolution hypothesis--members of the same ecomorph class on different islands are not closely related (with two possible exceptions). Overall, this phylogeny suggests that a minimum of 17 evolutionary transitions occurred from one ecomorph class to another. We used a variety of statistical approaches, including both parsimony and maximum likelihood methods, to investigate whether phylogenetic hypotheses requiring fewer such transitions could be statistically excluded. Phylogenies requiring 15 or 16 transitions were not statistically inferior to the tree requiring 17 changes, but all trees requiring fewer transitions were. Thus, we are confident in our conclusion that ecomorphs have evolved independently many times on the islands of the Greater Antilles.
Although convergence at the level of communities or faunas has often been suggested (e.g., comparisons between Australian mammals and North American eutherians), usually such comparisons are anecdotal and little quantified. To our knowledge, this is the first quantitative demonstration that the composition of communities (in terms of features of the organisms, rather than attributes of the community, such as species richness) really is highly similar and independently evolved.
Moreover, these results
are an interesting contrast to the claims that contingency has a paramount
role in evolutionary diversification. In some sense, the evolutionary
tape has been replayed four times in the Greater Antilles, yet the outcome
has been highly similar in each case. An interesting further point,
however, is revealed by closer examination of within-island diversification.
Although evolution of the ecomorphs seems little constrained, as evident
by their repeated evolution across islands, within island diversification
is a different story: few examples of multiple evolution of the same ecomorph
on the same island exist. Most likely, the constraining factor is
interspecific interactions: once an ecomorph "niche" is filled on an island,
it is very difficult for another lineage to subsequently evolve to also
use it. Given the extensive literature on interspecific competition
in anoles, this conclusion seems reasonable.