Lots of things can be learned from biogeography. Far more than just what species live where. The evolutionary relationship of animals combined with our knowledge of biogeography can assemble a rather interesting interesting map of relationships. Before genetics was well established, biogeography played a large role in determining the relationships of extant closely related species (often not beyond the terrestrially bound kinds) such as reptiles, amphibians, and (excluding Chiroptera and Cetacea) mammals when morphology did not provide enough information, such as the Agkistrodons you should be familiar with by now, if you’ve read my blog at all. Interestingly, genetics alone does not precisely tell us about a fascinating little subspecies/species/whatever the hell taxonomists decide to call it known as Taylor’s cantil (A. bilineatus taylori/A. taylori). It is certainly a species in the sense of the Biological Species Concept (Mayr) and the Cohesion Species concepts (Hull) but not the Ecological Niche Concept (Simpson). More interestingly, where exactly this population fits within the Agkistrodon genus is not fully shown by the genetic data. This is because it appears to be a near-simultaneous three-way split between the subspecies/species of the A. bilineatus group (including A. taylori) and the A. piscivorus group. The Gulf Arc hypothesis postulates that a single interbreeding population of the ancestral A. bilineatus/piscivorus existed around the entire Gulf of Mexico and up the Pacific coast of Central American and Mexico (pretty much it’s present range if you fill in the gaps) and something caused gaps to form in the populations at (very slightly) different times resulting in species that are genetically as dissimilar to each other as they are to any other within the group, but also isolating them and further enhancing all of the distinguishing features which existed among each subpopulation within the entire Gulf Arc. What we know about the changing geography shows the formation of new features which could have resulted in the relatively recent speciation.
It requires both the genetic and geographical information to conclude such a model (genetics alone without any knowledge of the geography (or morphology, ecology, and behavior) would only be capable of indicating A. taylori is a reproductively distinct species from A. bilineatus and A. piscivorus and would be incapable (because of how recent this event was) of demonstrating when this occurred.