Climate and Land Cover Analysis Suggest No Strong Ecological Barriers to Gene Flow in a Natural Baboon Hybrid Zone

Admixture between diverging taxa has made, and continues to make, an important contribution to primate diversity and evolution. However, although naturally occurring hybrids have now been documented in all major primate lineages, we still know relatively little about the factors that shape when and where admixture occurs. Baboons (genus Papio), in which multiple natural hybrid zones are well described, provide a valuable system to investigate these factors. Here, we combined Geographic Information Systems and weather station data with information on genetically characterized populations in southern Kenya to investigate if ecological variables present a potential barrier to gene flow between anubis baboons and yellow baboons in the region. Specifically, we asked if altitude, seasonal temperature, or seasonal precipitation differ for weather stations in anubis, yellow, or hybrid ranges in southern Kenya, and if land cover or altitude covary with population ancestry near the hybrid zone. Our analyses suggest that the range of yellow baboons in Kenya is climatically distinct from the range of anubis baboons, with hybrids in intermediate regions. However, we identified no clear pattern of climate or land cover differentiation near the hybrid zone itself. Thus, when yellow baboons and anubis baboons come into contact, our data suggest that the resulting population composition is not consistently predicted by the ecological variables we considered. Our results support the designation of baboons as highly flexible generalists, and suggest that more fine-grained analyses (e.g., relative success in ecologically stressful years) may be necessary to detect clear signals of ecological barriers to gene flow.