A macrogenetic analysis of isolation mechanisms shaping genetic divergence in mammals

Understanding the processes that shape spatial genetic differentiation is essential for understanding how populations adapt to environmental change. By evaluating the influence of these processes, we can gain insights into evolutionary dynamics and the potential for species to respond to shifting landscapes. Three well-accepted processes that create spatial patterns in genetic variation are isolation-by-distance (IBD), where individuals are more genetically similar the closer they are geographically; isolation-by-environment (IBE), where gene flow is reduced due to selection against migrants in unsuitable ecological conditions; and isolation-by-resistance (IBR), where landscape features limit dispersal. We conducted a macrogenetic meta-analysis of single-nucleotide-polymorphism data to identify patterns shaping spatial genetic differentiation in 40 mammalian datasets globally. For each species, we built a species-distribution model and combined it with the global Human Footprint layer to create a composite resistance surface, revealing that habitat suitability and anthropogenic impact contribute roughly equally to resistance. Model selection tests found IBR was the mechanism most frequently retained in the best models and had the highest variable importance. IBD and IBE alone had little effect but were often selected alongside IBR, suggesting they may have secondary, context-dependent effects. However, the probability of finding IBD in the best model of divergence significantly increased as the number of populations sampled increased. Similarly, the probability of finding IBE increased with the spatial scale of the study. Our findings suggest that resistance is pervasive in shaping genetic variation in mammals worldwide, but that study design affects our ability to detect the presence of IBD and IBE.