Continental Stock Exchange
Complex evolutionary dynamics ensue after isolated continents unite. Many South American (Neotropic) mammals went extinct during the Great American Interchange. North American (Nearctic) carnivores rapidly occupied South American predatory niches.
Geography favored the Nearctic invaders. Any species reaching Panama had to be able to tolerate humid tropical conditions. Species heading south would then encounter climates in South America that were not markedly different. Conversely, heading north meant heading into the cooler and/or drier conditions found in the Sierra Nevada volcanic mountain range that runs across central-southern Mexico.
This climatic asymmetry was particularly hard on Neotropic species, which had specialized for the tropical rainforests. They had little prospect for getting beyond Central America. Central America currently has about 40 mammal species originating from South America compared to 3 Nearctic species.
There were exceptions to Nearctic takeover. A few Neotropic immigrants prospered. Porcupines and opossums became conspicuously successful northward migrants. The naturally armored armadillo fared fairly well too.
A broader historical perspective also factors in on the lopsidedness of the Great American Interchange. During the Cenozoic era (65 MYA–10 TYA), North America was periodically connected to Eurasia via Beringia: the Bering land bridge between Siberia and Alaska, which was at times 1,600 kilometers wide.
Beringia allowed multiple migrations back and forth. In turn, Eurasia was connected to Africa, adding further to the mix of species into North America.
On the other end, South America was connected to Antarctica and Australia, 2 much smaller continents, only during the earliest part of the Cenozoic. Further, this land connection carried little traffic; no mammals save some marsupials and a few monotremes (egg-laying mammals).
In sum, Nearctic species were descendants of a more competitive arena: an ideal setup to quicken evolutionary pace. Nearctic animals tended to be smarter and more efficient; generally able to outwit and outrun their Neotropical counterparts. Neotropic ungulates (hoofed animals) and their predators were replaced wholesale by Nearctic invaders.
Many of the Nearctic mammals speciated as they populated South America. This diversification dynamic is typical. Hominids speciated as they spread into Eurasia from Africa, as did antelopes as they diffused and thrived throughout Africa.
Sea levels and glaciation also define biotic isolation or connection. Beringia appeared at various times during the Pleistocene epoch ice ages, 2.6 MYA to 11 TYA. Humans migrated from Asia into the Americas over Beringia 26–20 TYA.