Challenges to the Traditional Theory of the Great American Interchange

Evolution -- Great American Interchange

The natural Panamanian bridge adjoining early North America with early South America is believed to have occurred 3 -- 4 million years ago. By studying evolutionary changes in animal species in North America, Central America and South America, experts formed the theory of the Great American Interchange, a mutual migration of Northern species to South America and of Southern species to North America. The evolutionary changes that came from these migrations are at least partially attributed to a Great American Biotic exchange. Experts traditionally believe that Northern species that migrated to South America were more successful in surviving and evolving because of prior migrations from greater land masses and easier adaptation to the climate of South America. However, as experts make more and more discoveries in the field, there are questions and controversies about the number of migrations from each continent and migrations from sources other than the Panamanian bridge. The raised questions and controversies seem to expand our knowledge about the evolutionary events in pre-History.

2. Body: How the Great American Interchange Affected Animal Evolution

a. Important Results from recent primary literature articles and how they shape our current understanding of the topic

The Great American Interchange was the migration of North American (Nearctic) species to South America and South American (Neotropic) species to North America due to the rise of the natural Panama land bridge between North America and South America 3 -- 4 million years ago during the Pleistocene Period (Smith and Klicka 334). The Great American Interchange resulted in a Great American Biotic Interchange, resulted in the evolutionary diversity of several species, both the North and South Americas, with different origins and different evolutionary paths (Jimenez, Gardner and Navone 1167). The Nearctic species that migrated to South America were more successful in evolving and surviving than were the Neotropic species, as evidenced by fossil finds in North America, Central America and South America (MacFadden 162). Some experts maintain that approximately 17 New World species evolved, 12 of which remain (Jimenez, Gardner and Navone 1168). The 5 New World species who suffered extinction did so because of competition for survival sources, climate and the impacts of human migrations (MacFadden 164). This traditional theory supports a far neater scenario than the possible explanations, questions and challenges created by new discoveries.

b. Controversies in the Field (what is the evidence for and against various theories?)

There are several controversies in the field regarding the effects of the Great American Interchange on the evolution of animals. The controversies appear to involve the question of multiple migrations only from North to South, multiple migrations from both directions, additional migrations from other avenues such as water, and ultimately the reasons for the Northern or Nearctic species' apparently greater success after migrating to South America.

The prevailing traditional belief is that the migrations North and South were basically even, according to an equilibrium theory (Jablonski and Sepkoski 1367), and that Northern animals succeeded in their new Southern geographic area better than Southern animals succeeded in their new Northern geographic area for 2 reasons: because they were used to more vigorous evolutionary competition over a larger land mass from prior Eurasian migrations; and because they were better able to adjust to the South American climate (Jablonski and Sepkoski 1367). However, other experts believe that the greater survival rate of Nearctic animals in South America was due to a greater number of migrations from North to South (Perini, Russo and Schrago 657), rather than to genetic superiority or outright size. This belief in several Northern-to-Southern migrations at different times is supported by the discovery of 2 new fossils of procyonids in Venezuela, outside and north of Argentina, which are similar to Procyonid fossils found in the southern part of South America (Forasiepi, Soibelzon and Suarez Gomez). If the theory that there were multiply more Northern-Southern migrations than Southern-Northern migrations is true, the sheer number of migrating Northern animals may at least partially account for their evident success in South America.

The traditional equilibrium migration theory is also challenged by other experts who are that there is evidence of several migrations from South America to North America, concluded from tracing the movements of parasites in and around animal fossils from South to North (Jimenez, Gardner and Navone 1170). Other experts who agree with the multi-migration theory believe that diversity in phylogenetics of these species is traceable to different diversification patterns and different migrations (Davies and Buckley 2415). If these theories are true, then the evolutionary impact of the Panamanian bridge is far more complex and layered than the traditional theory states.

A third challenge to the traditional equilibrium theory is the belief in other sources of migration, which would lessen the impact of the Panamanian bridge on the evolution of animals by placing it in the context of several types of migration. For example, some experts believe some of the diversification may have actually been caused through migration over water through "rafts" or floating rather than or in addition to the natural Panamanian bridge (MacFadden 164). This is because rodent and primate fossils that originated outside of South America prior to the Great American Interchange have been found in South America (MacFadden 164). If this theory is true, then some of the evolutionary impact on species in South America would come from the sea and some of that impact would come prior to 3 -- 4 million years ago, rather than exclusively from the Panamanian bridge 3 -- 4 million years ago. The questions and controversies challenging the traditional theory of the Great American Interchange are not necessarily fatal to the traditional theory; rather, they expand, deepen and place that traditional theory in context as we amass more knowledge about pre-History.

3. Conclusion

The Great American Interchange was a migration of Northern or Nearctic species to South America and migration of Southern or Neotropic species to North America. According to traditional theory, it was accomplished due to the rise of a natural Panamanian bridge between North and South America 3 -- 4 million years ago. The migrations are traditionally believed to be essentially equal, according to the equilibrium theory. In addition, the traditional theory finds that Northern species evolved and survived in greater numbers after migrating to South America because of prior migrations from larger land masses and easier acclimation to South America's climate.