A test of alternative models of diversification in tropical rainforests: Ecological gradients vs. rainforest refugia

Comparison of mitochondrial and morphological divergence in eight populatio ns of a widespread leaf-litter skink is used to determine the relative impo rtance of geographic isolation and natural selection in generating phenotyp ic diversity in the Wet Tropics Rainforest region of Australia. The populat ions occur in two geographically isolated regions, and within each region, in two different habitats (closed rainforest and tall open forest) that spa n a well characterized ecological gradient. Morphological differences among ancient geographic isolates (separated for several million years, judging by their mitochondrial DNA sequence divergence) were slight, but morphologi cal and life history differences among habitats were large and occurred des pite moderate to high levels of mitochondrial gene flow. A field experiment identified avian predation as one potential agent of natural selection. Th ese results indicate that natural selection operating across ecological gra dients can be more important than geographic isolation in similar habitats in generating phenotypic: diversity. In addition, our results indicate that selection is sufficiently strong to overcome the homogenizing effects of g ene flow, a necessary first step toward speciation in continuously distribu ted populations. Because ecological gradients may be a source of evolutiona ry novelty, and perhaps new species, their conservation warrants greater at tention. This is particularly true in tropical regions, where most reserves do not include ecological gradients and transitional habitats.