July 16, 2003 Despite social notions of race, human populations around the world are genetically so similar that geneticists find no different sub-species among them. The genetic continuity of human populations is the exception rather than the rule for most animal species, however.
Richard Glor, graduate evolutionary biology student in Arts & Sciences at Washington University in St. Louis, has found extensive genetic differentiation among populations of numerous Anolis lizard species inhabiting single Caribbean islands. While to the naked eye the lizards appear to be uniform, these lizards from the islands of Cuba, Puerto Rico, Hispaniola and Jamaica all show a surprising amount of genetic diversity. Glor goes to the islands and collects lizard samples to study morphology, or body features, and color patterns and then sequences DNA from the different species.
"The levels of differentiation we're seeing genetically with anoles completely blows away any kind of variation in humans," Glor said. "We've found an unanticipated dimension of biodiversity, far greater than ever thought to exist. If you look at DNA in any widespread species, it suggests that several species may actually be present."
The variation that Glor has found startles evolutionary biologists and challenges researchers to understand what is causing the DNA evolution, said Jonathan Losos, Ph.D., Washington University professor of biology, and Glor's co-adviser.
"What's so exciting about the variation Rich has discovered is that it's completely unexpected," said Losos, who has studied Caribbean lizards for more than 15 years. "These lizards have been a model system for understanding evolutionary diversification for 30-plus years,including by a number of famous scientists, yet Rich was the first to discover this. He's uncovered a whole different layer of speciation and diversification in these species. It's possible that one group is not just one species but represents maybe six or eight species. At the very least, it shows within species there is a lot of genetic diversity that we had been clueless about beforehand."
Glor's other adviser is Allan Larson, Ph.D., professor of biology at Washington University. Part of the results of the Anolis study will be published in a forthcoming issue of Evolution.
Glor has found significant geographic differentiation in 11 of 12 widespread Anolis species that he's analyzed. He has focused on two widely distributed species for each island. Two common ones to all four islands are what are known as a trunk-ground species -- lizards that live at the bottom of tree trunks and forage on the ground – and a trunk-crown species, which live at the treetop and forage in the foliage found there. Glor and Larson's analyses show that a trunk-crown species in Hispaniola diverged millions of years ago from those in Cuba, and different trunk-crown species on Hispaniola are genetically different from other populations in different regions of the island.
"With each species, there are forms that in one area are greatly different genetically from what we thought was the same species in another area," Glor said.
Glor and his collaborators intend to formulate and test theories on what is driving the genetic variation. Geographic events – the formation of a mountain range, the rising and falling of sea levels, the creation of river valleys – are potential factors. Ecological heterogeneity is another possibility. Species whose ranges extend across a range of different habitats may diverge from one another; for instance, natural selection may drive populations from a dry coastal area on an island to diverge from adjacent populations in wet inland forests. To determine this, the researches will have to analyze the DNA and develop phylogenies –trees that represent evolutionary relationships and incorporate a time factor. This will help them see the patterns of species diversification over time. "What's so good about anoles is that they are so abundant and what's so good about these islands is that the same ecologies have evolved independently on all the islands," Glor said.
Another research thrust will be an effort to determine if the size of the island or other factors such as species ecology has an impact on the rate of fragmentation and speciation. Cuba is the largest island, followed by Hispaniola, Jamaica and Puerto Rico. Does speciation occur differently if the playing field is larger?
"Having at least two species from each island and all of the islands varying in size allows us to probe these kinds of questions, " Glor said.
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