Chameleons, tree-living lizards found principally in Africa and Madagascar, are known for changing colors. Former Earthwatch-supported scientist Dr. Chris Raxworthy (American Museum of Natural History) suggests that chameleons may also change basic principles of biogeography, the study of the distribution of life.
In a recent article in Nature, "Chameleon radiation by oceanic dispersal," Raxworthy and his colleagues contend that dispersal across the Indian Ocean was responsible for the divergence of chameleon species from their origin in Madagascar. This finding runs counter to several theories and popular conception of the radiation of terrestrial animals in the region, which rely on the break-up of the ancient supercontinent Gondwana to explain modern species ranges.
"Dispersal origin theories became unpopular with the acceptance of plate tectonics, and vicariance biogeography," said Raxworthy, who was the principal investigator for Earthwatch's Madagascar Forest Life project. Vicariance biogeography explains the geographical distribution of species in light of historic geological events such as continents breaking apart or mountain ranges being thrust up. "But oceanic dispersal may still be an important factor in the evolutionary history for many groups."
The paper summarizes 10 years of collecting field data, including three seasons in Madagascar with Earthwatch volunteers. Raxworthy and his colleagues conducted a phylogenetic analysis of 52 chameleon species, examining more than 600 anatomical and molecular traits, the most comprehensive study on this animal group to date representing 10 years of field data. Their analysis used modern "cladistic" methods, which construct the ancestral relationships of species based on hundreds of equally weighted traits.
Their findings suggest that the two oldest lineages of chameleons are distributed in Madagascar, and that newer forms found in Africa, the Seychelles, the Comoros, and India were the result of several dispersal events across the ocean. These results are consistent with the fossil record, which only goes back to the Miocene, indicating that the first appearance of chameleons on Madagascar occurred millions of years after India and Madagascar broke away from Africa.
"Most researchers have tended to only think of species migrating into Madagascar," said Raxworthy. "I know of no other studies that show a diverse group of organisms that first evolved in Madagascar, and then spread out across the Indian Ocean and into Africa and Asia." His findings are further supported by two endemic chameleon species in the Comoros Islands, volcanic islands with no land connection to any other landmass.
How chameleons managed to cross hundreds of miles of ocean remains open to speculation, as they're mitten-like feet are adapted for climbing trees, not swimming. Raxworthy suggests it must have involved rafting, such as on a mat of vegetation or in a hollow tree trunk washed out from a flooding river. A single gravid female is all it would take, or even a nest of eggs, which may take up to a year to hatch. However chameleons dispersed across the Indian Ocean, Raxworthy's investigation has brought fresh perspective to their evolutionary history.
For more details, see Raxworthy, C. J., Forstner, M. R. J. & Nussbaum, R.
A. Chameleon radiation by oceanic dispersal. Nature, 415, 784 - 787 (2002).
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