 The Cozumel ThrasherHome
Natural history
Created by Robert L. Curry
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Origin of the Cozumel Thrasher |
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Original species descriptionPeninsular origin hypotheses
"The Cozumel Thrasher has had a relatively long independent evolutionary history. Given the small area of this island, and the frequent hurricanes of major environmental influence, it seems unlikely that T. guttatum has persisted for a long period solely on this small island. Rather, we hypothesize it to be a relict species. Maps of sea level at the height of the Pleistocene ... show that considerably more exposed land surrounded the Yucatan Peninsula 18,000 years ago than exists today, which might have provided more habitat for this species. The level of genetic distinctness shows that conservation of T. guttatum is important for the preservation of diversity in the genus." [emphasis added] Comparative observations lean against the peninsular origin hypothesis. Relatively rapid divergence in morphology and plumage is common in mimids believed to have colonized remote islands; examples include Galápagos mockingbirds (Mimus [Nesomimus] spp.), Socorro Mockingbird (Mimus graysoni), and Large-billed Mockingbird (Mimus [gilvus] magnirostris). In none of these cases, nor for Cozumel Thrasher, is there any direct evidence of a already-differentiated mainland ancestor from which an island colonist could have arisen. Island origin hypothesesAn alternative explanation posits that genetic and morphological differentiation of the Cozumel Thrasher occurred after the ancestor of T. guttatum colonized Cozumel. Natural selection favoring reduced body size on the island could account for divergence relative to an ancestor, presumably inhabiting the Yucatan Peninsula, closer in size to T. longirostre. Under this hypothesis, T. guttatum would still be considered a relict species, representing a small remnant population of a species presumably with a formerly broad distribution (on Cozumel and in the Yucatan), but with genetic and morphological features that evolved in situ. This hypothesis assumed that the thrasher has persisted long enough on Cozumel for changes to occur after colonization. Genetic drift associated with population bottlenecks, possibly caused by hurricanes, could have affected the speed of divergence. It is conceivable also that the original colonist of Cozumel came not from a now-extinct source population on the Yucatan mainland, but from a more distant source such as Florida. The fact that a Brown Thrasher (T. rufum), which is a breeding resident of the southeastern United States, once strayed as far as Curaçao in the Netherlands Antilles (K. Voous, 1983, Birds of the Netherlands Antilles), far south of Cozumel, suggests that colonization by a pair or group of thrashers from the mainland North America was possible, perhaps as a migration accident. The colonization of Bermuda by Gray Catbirds (Dumetella carolinensis) and subsequent evolution to non-mgratory behavior could represent a parallel example. If phylogenetic evidence suggesting that Long-billed Thrashers (T. longirostre) and Brown Thrashers are more closely related to each other than either is to the Cozumel Thrasher is correct, then the colonization of Cozumel presumably would have involved the common ancestor of all three taxa, prior to the split between the two northern species. Evolutionary relationshipsBehaviorEcology |
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