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).
Since Darwin's study of the Galapagos, archipelagos have played a central role in understanding how new species evolve from existing ones (speciation). Islands epitomize allopatric speciation, where geographic isolation causes individuals of an original species to accumulate sufficient genetic differences to prevent them breeding with each other when they are reunited.
Current day Martinique in the eastern Caribbean Sea is composed of several ancient islands that have only recently coalesced into a single entity. The phylogeny and geology show that these ancient islands have had their own tree lizard (anole) species for about six to eight million years.
Capitalizing on the islands' meeting, researchers genetically tested the lizards for reproductive isolation from one another. In using selectively neutral genetic markers, researchers observed that these anoles are freely exchanging genes and therefore not behaving as separate species. Indeed, there is more genetic isolation between conspecifics from different habitats than between those lizards originating from separate ancient islands.
"This replicated population genetic study robustly and consistently suggests that, across a range of opportunities and conditions, there is pronounced introgression after allopatry and that even a very substantial amount of time in spatial isolation does not, on its own, necessarily allow for the development of reproductive isolation and speciation," the authors conclude.
Citation: Thorpe RS, Surget-Groba Y, Johansson H, 'Genetic Tests for
Ecological and Allopatric Speciation in Anoles on an Island
Archipelago', PLoS Genet 6(4): e1000929; doi:10.1371/journal.pgen.1000929