Biogeography: The Geographic Distribution Of Species

Darwin thought up his Theory Of Common Descent because he had found biogeographic evidence. He thought that that evidence was much stronger than the fossil evidence. Scientists still think so.

Basically, some species have suspicious resemblances to supposedly different species that just happen to live nearby. Often, it would be better design for them to instead resemble some further-away species. And, this is the norm. There are a huge number of good examples.

The trees on the remote island of St. Helena are unlike the trees anywhere else on earth. Sunflowers are the closest relative to the strange gumwood tree and to the various cabbage-trees. And, the most closely related sunflower is the local sunflower. The scientific explanation is that this volcanic island was originally formed far away from any continent, and therefore started out with no land plants. Eventually, some sunflower seeds managed to get there. Since no one else was filling the role of "tree", the largest plants on the island - some of the sunflowers - took the job. Transformed by time and competition and by the demands of their role, they now look like trees.

Every other remote island has its own examples. In the Galapagos, the role of woodpecker is taken by a finch. Or rather, it's mostly a finch, but it has a beak specialized for the woodpecker role. Apparently, the only land bird which got to the Galapagos was a finch, so all the land birds there are modified finches. (DNA studies prove the relationship.)

In Hawaii, the role of woodpecker was taken by a modified honeycreeper bird. Again, every land bird in Hawaii is a modified honeycreeper. There's a honeycreeper "parrot" too.

Most remote islands had flightless birds, such as the Dodo. There is an obvious evolutionary explanation for why these existed on islands, but not on continents. Birds could get to islands, and ground animals couldn't. So, the role of ground animal was available, and on each island, some bird took on the role. From this explanation, we can predict that each island would have its own unique species of flightless bird. And that is a correct prediction.

Because of Australia, South America, and the various remote islands, it's easy to come up with a really long list. For example, South American tropical species are more closely related to South American desert species than they are to African tropical species. Australia's marsupials took on the roles of predator (Tasmanian Wolf), mouse, mole, wolverine (Tasmanian Devil), and anteater.

The point is that over and over again, evolution explains this very well. The process of species arrival has been observed on islands such as Krakatoa, which was wiped clean by a volcanic eruption in 1883. We know that the more remote islands receive very few species from the outside world. So, the lucky arrivees have a golden opportunity to take up many new lifestyles, and adapt into them. This is referred to as an evolutionary radiation into ecological niches, and it explains (for example) the 700 species of Hawaiian fruit fly.

Bird are the same on both sides of the Grand Canyon, but there are different rodents on each side. The obvious explanation is that the canyon isolates groups of rodents, but doesn't isolate birds.

Fossil biogeography reinforces this point. For example, the Atlantic Ocean was formed when a supercontinent broke up. Fossils that date to just before this breakup are the same in Europe as they are in North America. Fossils from after the breakup show progressively more and more differences between the land animals (and plants) of the two continents. The breakup made no difference to ocean creatures.

In short, species are mutable. And, they are not designed according to any master plan. The faux woodpeckers found on remote islands aren't as good at it as real woodpeckers. And, real woodpeckers are found in some treeless places. A good designer would have re-used his best designs, but instead we find nature has made do with whatever was at hand.

Last modified: 29 June 2000

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