Niche modeling within and between species in the genus Erythrophleum : integrating phylogenetic data and environmental niche models to explore speciation mechanisms

In this study we investigated the climatic niche between and within closely related tropical tree species of the Erythrophleum genus (Fabaceae, Caesalpinioideae): E. ivorense, E. suaveolens and E. africanum. Two major hypotheses, the refuge theory and the ecological gradient hypothesis, have been developed to explain the current distribution of tree clades across tropical Africa. To identify the speciation mechanisms, we used a combination of geographic data and environmental factors to quantify the degree of niche conservatism (or divergence). We used two sets of distribution data for the purpose of this study. Species distribution data for the whole of tropical Africa was gathered from herbarium records. Distribution data of the two sister species E. ivorense and E. suaveolens assigned to genetic cluster were available for the lowland tropical forests of western and central Africa. Using a Species Distribution Model approach based on MaxEnt algorithm we tested for the environmental differences between species and genetic clusters within species. At species level, the climatic niche significantly differed and only slightly overlapped, suggesting a parapatric speciation along a climatic gradient. Within the two sister species, the niche of the parapatric central African genetic clusters suggests mostly a secondary contact following the recolonization from different forest refugia.