Vicariance

Vicariance is the process by which a continuous species range is split into disjunct populations by the emergence of a geographical or environmental barrier. Unlike dispersal, which involves the active movement of organisms across existing barriers, vicariance is a passive process: the barrier moves, and the organisms are left behind. The resulting disjunct populations then evolve independently, potentially diverging into new species through the mechanisms of natural selection and genetic drift.\n\nVicariance is the central concept of the vicariance paradigm in biogeography, which holds that the primary cause of disjunct distributions is the fragmentation of ancestral ranges by geological or climatic events rather than the long-distance dispersal of organisms across barriers. The paradigm gained strength in the 1960s and 1970s when the acceptance of plate tectonics provided a rigorous mechanism for continental fragmentation. The splitting of Gondwana, the rise of the Isthmus of Panama, and the drying of the Sahara are all classic vicariance events that have shaped modern biogeographical patterns.\n\nThe vicariance paradigm is not merely an alternative to dispersal; it is a claim about the priority of historical process over ecological mechanism. Where dispersal biogeography asks how organisms overcome barriers, vicariance biogeography asks how barriers were created and what the ancestral range looked like before the split. The two approaches are now recognized as complementary, but their historical opposition shaped the theoretical development of biogeography for a century.\n\nVicariance is not merely an alternative to dispersal; it is a claim that geological history has priority over ecological process in explaining disjunct distributions. But this priority is itself a historical contingency. The vicariance paradigm rose to prominence because plate tectonics provided a rigorous mechanism for continental fragmentation, not because vicariance is intrinsically more important than dispersal. In a world without plate tectonics, dispersal would dominate biogeographical explanation. The lesson is that theoretical paradigms in biogeography do not reflect the relative importance of processes in nature; they reflect the availability of rigorous mechanisms within the explanatory framework of the day. Vicariance and dispersal are not competing truths; they are competing model systems, and their relative dominance tells us more about the state of geological theory than about the state of biological distribution.\n\n