Genomic signals of diversification along ecological gradients in a tropical lizard

Studies of rainforest diversification that simultaneously consider the effects of genetic drift and natural selection are rare. We use Amplified Fragment Length Polymorphism genome scans of the African rainforest lizard Trachylepis affinis from Cameroon to examine the spatial patterns and environmental associations of both neutrally evolving loci and those thought to be under selection. Bayesian selection scans revealed that approximately 7% of the genome may be under divergent selection. Using non-linear environmental modelling techniques, we fit patterns of genetic differentiation recovered from the pooled neutral data and from individual loci showing a signature of natural selection. Neutral differentiation occurred along a cline from coastal lowland rainforest inland toward the gallery forests-savanna mosaic (ecotone), and was associated with both geographic distance and changing precipitation patterns. Loci under selection were differentiated predominantly along the forest-ecotone gradient-in concordance with morphological divergence in traits related to fitness. A second set of these loci was differentiated between lowland and montane habitats. A third set of loci was indicative of divergent selection between rainforest refugia. Niche models and demographic signals in mitochondrial sequence data support a population expansion out of a core rainforest area into savanna since the last glacial maximum. Our findings indicate adaptive diversification in T. affinis may be taking place along the forest-ecotone gradient during range expansions or contractions, and that refugial isolation augmented by divergent adaptation to different rainforest environments appears to play a less significant role.