A second report in two weeks about research from the Caribbean (specifically, Martinique) that challenges common assumptions about how evolution works, from scientificblogging.com:
A new study of island lizards suggests that geographical isolation may not be as important to evolution as previously thought.
The new research, published in PLoS Genetics, shows that even those lizards that have been geographically isolated for many millions of years have not evolved into separate species as predicted by conventional evolutionary theory.
The findings reject allopatric speciation in a case study from a system thought to exemplify it, the researchers say, and suggest the potential importance of speciation due to differences in ecological conditions (ecological speciation).
The research has also received coverage from Science magazine:
The Caribbean island of Martinique has a lot to offer: beautiful beaches, tropical weather, and a perfect place to challenge notions about how new species arise. When anole lizards (Anolis roquet) arrived on Martinique more than 8 million years ago, there were four separate, smaller islands. Over time, this physical isolation should have allowed the lizards to evolve into different species, according to conventional evolutionary wisdom. But that’s not what happened.
One of the best ways to split a species is allopatric speciation. That’s when some sort of physical barrier separates individuals from the same population, causing them to eventually become so genetically distinct that they can no longer interbreed. A famous example is Darwin’s finches, which started out as a single species but adapted over millions of years to the various environments of the Galápagos Islands, eventually becoming 13 species.
Not so for Martinique’s lizards. Evolutionary biologist Roger Thorpe of Bangor University in the United Kingdom and colleagues analyzed the DNA of individual lizards from all over the island to uncover how much they had diverged during isolation. Using a molecular clock—short segments of DNA that accumulate mutations at a steady rate—the researchers identified four genetic sequences corresponding to unique lizard populations on Martinique’s island predecessors. The analysis, reported today in PLoS Genetics, shows that the isolated lizard populations diverged for 8 million years after they arrived on the precursor islands. But they didn’t accumulate enough mutations to become separate species before volcanic flows connected the islands together and the populations were reunited.
The complete journal article, by Roger S. Thorpe, Yann Surget-Groba, and Helena Johansson, is available online: Genetic Tests for Ecological and Allopatric Speciation in Anoles on an Island Archipelago.
The previous report on research into evolution in the Caribbean focussed on hamlet fish, and found that ecology might be more important to speciation than was previously thought, a conclusion that seems to be supported by the findings of the Martinican lizard research.