Many shades of green

Abstract

The origin of species, despite the title of Darwin's key book, has remained an intriguing question for biologists. With growing concern about biodiversity and man's increasing threat to it, issues of species richness are of growing interest and urgency. A new study has looked at the distribution of green lizards of the Anolis genus in Cuba. Although Cuba is large compared with many other islands in the Caribbean, it has an extraordinary range of green lizards – more than 60 species in total.A new study reported in the Proceedings of the Royal Society series B (published online) has investigated the origin of this Cuban diversity. A team led by Richard Glor at Washington University, St Louis, describes work on the Cuban anole lizards.Researchers have described two main mechanisms for speciation. Sympatric speciation is often considered as the main mechanism in species-rich isolated ecosystems such as lakes or islands. It is thought that populations may have adapted to different features of the habitat or that sexual selection has fragmented populations into different species. Alternatively, allopatric speciation is thought to result from the physical separation of populations which then evolve along diverging routes.View Large Image | View Hi-Res Image | Download PowerPoint SlideWest Indian anoles represent a classic example of a species-rich adaptive radiation within several relatively small, geographically isolated areas. On Cuba, where anole diversity is highest, more than 60 species occur over an area of only 110,000 square kilometres. Most anole species on Cuba and other Greater Antillean islands result from within-island processes leading some researchers to believe that sympatric speciation has played a role in anole diversification. However, the authors claim, several lines of evidence suggest that geographical processes have played a dominant role in intra-island anole speciation.On Cuba, the authors believe one overlooked mechanism of speciation may have been the geographical separation of regions of the island during historically higher sea levels. Reconstructions suggest that this island consisted of three distinct archipelagos separated by deep water channels throughout much of the Cenozoic era before being permanently reunited in the Pliocene. Evidence for fragmentation of Cuba during the Miocene is compelling, the authors believe.The researchers therefore carried out sequence analyses of mitochondrial and nuclear DNA from samples taken from more than 300 individuals of two species at 54 locations throughout Cuba.Their results support the hypothesis that partial submersion of Cuba during the Miocene initiated speciation among populations of green anoles. Despite their limited geographical range, historical barriers to gene exchange separated lineages that have maintained their evolutionary distinctness through long periods of evolutionary time.If this pattern is general, accumulation of allopatric speciation over many millions of years of evolutionary history has probably been important for Anolis species diversification and sympatric speciation processes are not required to explain the group's high species richness.The generality of these conclusions should be tested using other taxa from Cuba, the authors believe. The lesson appears to be that in the face of any case of species diversity, a deep study of geographical history may help throw light on the present situation.