The role of rivers as geographical barriers in shaping genetic differentiation and diversity of Neotropical primates

We quantitatively tested the riverine barrier hypothesis and its influence on biogeographical distributions and molecular variation in New World monkeys (Parvorder: Platyrrhini). Using mitochondrial markers (cytochrome oxidase subunit II and cytochrome b), we analyzed taxonomic differences and the effects of geographical barriers on molecular patterns across Central and South America. Nearly 80% of described species are separated by geographical barriers. River width exhibited a positive correlation with molecular dissimilarity in adjacent taxa for both molecular markers. Streamflow also showed a positive association, although not statistically significant, likely due to limited sample size. Several presently described taxa were not supported based solely on these molecular phylogenetic markers, including Saimiri, Mico, Cebus, Sapajus, and Cherecebus. These taxonomic issues are far more common where river barriers do not exist. In conclusion, we found a significant correlation between river width and molecular divergence in adjacent taxa, indicating that wider rivers were associated with greater molecular dissimilarity for two commonly used mitochondrial genes. Species boundaries were predominantly found at river interfaces, and in the absence of discernable geological barriers, adjoining species were more likely to exhibit molecular similarity. Our findings suggest that both river and mountain barriers significantly restrict gene flow for the majority of neotropical taxa, with geological formation of river barriers coinciding with estimated speciation events. Additionally, river width proves to be a valuable tool for estimating molecular divergence in adjacent taxa, particularly in regions with limited sampling.