All organisms are adapted to their environment to a greater or lesser extent. If the abiotic and biotic factors within a habitat are capable of supporting a particular species in one geographic area, then one might assume that the same species would be found in a similar habitat in a similar geographic area, e.g. in Africa and South America. This is not the case. Plant and animal species are discontinuously distributed throughout the world.
Even greater differences can be found if Australia is taken into consideration, though it occupies the same latitude as much of South America and Africa. Marsupials like the kangaroo, the wallaby, and the wombat make up over 80 percent of Australia’s indigenous mammal population. By contrast, marsupials are totally absent from Africa and are only represented by the opossum in South America and the Virginia Opossum in North America.
The main groups of modern mammal arose in Northern Hemisphere and subsequently migrated to three major directions:
- To South America via the land bridge in the Bering Strait and Isthmus of Panama; a large number of families of South American marsupials became extinct as a result of competition with these North American counterparts.
- To Africa via the Strait of Gibraltar.
- To Australia via South East Asia to which it was at one time connected by land.
The shallowness of the Bering Strait would have made the passage of animals between two northern continents a relatively easy matter, and it explains the present-day similarity of the two faunas. But once they had got down into the southern continents, they presumably became isolated from each other by various types of barriers.
- The submersion of the Isthmus of Panama: isolates the South American fauna.
- The Mediterranean Sea and the North African desert: partially isolate the African fauna.
- The submersion of the original connection between Australia and South East Asia: isolates the Australian fauna.
11.3.2 Continental drift
The same kinds of fossils are found from areas known to have been adjacent to one another in the past but which, through the process of continental drift, are now in widely divergent geographic locations. For example, fossils of the same types of ancient amphibians, arthropods and ferns are found in South America, Africa, India, Australia and Antarctica, which can be dated to the Paleozoic Era. At that time these regions were united as a single landmass called Gondwana. Sometimes the descendants of these organisms can be identified and show unmistakable similarity to each other, even though they now inhabit very different regions and climates.
The combination of continental drift and evolution can sometimes be used to make predictions about what will be found in the fossil record.