Biogeography as Evidence of Evolution
Understanding the Discontinuity of Species Distribution
Jun 11, 2009
It is clear that major isolated land areas and island groups often evolved their own distinct plant and animal communities. Data about the presence or absence of species on various continents and islands (biogeography) can provide evidence of common descent and shed light on patterns of speciation.
Isolation Leads to Unique Species
Before humans arrived 40,000 to 60,000 years ago, Australia had more than 100 species of kangaroos, koalas, and other marsupials but none of the more advanced terrestrial placental mammals such as wolves, lions, bears, horses. Land mammals were entirely absent from the even more isolated islands that make up Hawaii and New Zealand. However, each of these places had a great number of plant, insect, and bird species that were found nowhere else in the world.
The most likely explanation for the existence of Australia's, New Zealand's, and Hawaii's mostly unique biotic environments is that the life forms in these areas have been evolving in isolation from the rest of the world for millions of years.
Continental Biogeography
All creatures are adapted to the abiotic and biotic factors of their habitat. Thus 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 around the planet.
This discontinuity of distribution can be seen in African and South American fauna. Africa has short-tailed (Old World) monkeys, elephants, lions and giraffes while South America has long-tailed monkeys, cougars, jaguars and llamas.
The flora of North and South America show a similar discontinuity. Deserts in North and South America have native cacti, but deserts in Africa, Asia, and Australia have succulent native euphorbs that resemble cacti but are very different, even though in some cases cacti have done very well (for example in Australian deserts) when introduced by humans.
Explanation for Continental Discontinuity of Species
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; and
- 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.
Once isolated, the animals in each continent have shown adaptive radiation to evolve along their own lines.
Island Biogeography
Biogeography divides islands into two categories. Continental islands are islands like Great Britain, and Japan that have at one time or another been part of a continent. Oceanic islands, like the Hawaiian islands and the Galapagos islands, on the other hand are islands that have formed in the ocean and never been part of any continent.
Oceanic islands have distributions of native plants and animals that are unbalanced in ways that make them distinct from the flora and fauna found on continents or continental islands. Oceanic islands do not have native terrestrial mammals (other than bats and seals), amphibians, or fresh water fish. In some cases oceanic islands have terrestrial reptiles but most do not.
Starting with Charles Darwin, many scientists have conducted experiments and made observations that have shown that the types of animals and plants found, and not found, on such islands are consistent with the theory that these islands were colonized accidentally by plants and animals that were able to reach them. Such accidental colonization could occur by air, such as plant seeds carried by migratory birds, or bats and insects being blown out over the sea by the wind, or by floating from a continent or other island by sea.
Many of the species found on oceanic islands are endemic to a particular island or group of islands, meaning they are found no where else on earth. However, they are clearly related to species found on other nearby islands or continents.
Evidence of Evolution
Biogeography is one of several lines of evidence that demonstrate the ongoing and dynamic nature of evolution. Other evidence is found in comparative anatomy, the fossil record, and the molecular record.
