Another global influence on biodiversity relates to patterns of geographic connectivity and isolation between regions throughout Earth’s history. Recall that the Earth’s crust is not one uniform piece, but instead split into several pieces, called tectonic plates (Fig 2.5.1). These plates are in motion (represented by the arrows on the map) and so have not always been located in the same place they are today.
Antonio Snider-Pelligrini (a French geographer) and Alfred Wegener (a German geophysicist) were early proponents of the idea of continental drift, the concept that continental plates move. In 1858, Snider-Pelligrini mapped how the South American and African continents may once have fit together. Wegener and Snider-Pelligrini then added to this map the locations of certain fossil species, which were found in regions that today are very distant from one another (Fig 2.5.2). Today, these areas are too far apart to believe one species could have migrated between them; however, the researchers proposed that these continents may once have been connected, allowing organisms to easily migrate between them.
As continents drift apart, the species that inhabit them may experience vicariance, which occurs when a species’ geographic range is split into distinct regions separated by physical barriers the organisms cannot cross. In Figure 2.5.2, the species shown on the map were not able to cross the open ocean once the continents drifted apart. As we will see in future sections vicariance can play an important role in evolutionary change and speciation.
Patterns of vicariance throughout Earth’s history can have profound impacts on biodiversity. For instance, up until about 3 million years ago, North and South America were separated by water in the region that today is Panama. As sea levels dropped, the land was exposed and connected the two continents, leading to what scientists call the Great American Biotic Interchange (Fig 2.5.3a), in which South American species like glyptodons (Fig 2.5.3b) migrated north and North American species like mammoths (Fig 2.5.3c) migrated south. This connectivity between land masses led to a vast reassemblage of terrestrial species in both continents and also caused a vicariant event for marine species. Previously, marine organisms had been able to migrate easily between the Caribbean and the Pacific Oceans; however, once the land bridge formed, populations on either side were separated from one another.