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Biogeography

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Introduction

Biogeography is the study of the distribution of species over the surface of the earth. It is important for fields/topics such as:

In the evolution/creation debate, it is an important aspect of:

History

In the 1400's, Europeans started journeying farther and farther from their home shores, and over the next few centuries, European explorers had traveled all over the globe. They found that flora and fauna typically had very restricted distributions -- and that this restriction of distribution was not due to ecological suitability. Cultivated plants and domestic animals could live and grow and reproduce successfully in distant lands, sometimes going feral.

There was, however, a certain difficulty with the story of Noah's Flood. Noah's Ark had come to rest on Mt. Ararat, in eastern Turkey, and all its animals had to travel from there to the rest of our planet. A common theory back then was that travelers had brought those animals to distant shores, but while that theory could work on docile animals like turtles, there are many animals that are hard to catch, with some being very dangerous. Who would enjoy sharing their ships with rattlesnakes or big cats or bears? Why weren't any of them left behind near Ararat?

This left-behind problem was, of course, much bigger than that. Why did all the kangaroos and wallabies get taken to Australia and New Guinea, with none being left behind? Why did all the wombats get taken to Australia and no marmots or woodchucks? Why did all the sloths get taken to South America, with none left behind slowly munching leaves in trees near Mt. Ararat?

A step away from Biblical literalism was taken in the middle of the eighteenth century by G.L.L Buffon who proposed that the continents and islands had had several "centers of creation", where their inhabitants' ancestors had been created. This successfully solved the biogeographical puzzles, though the number of centers of creation had to be large to accommodate oceanic islands, with their high rates of endemism (proportion of species found nowhere else).

Charles Darwin considered the conundrum of oceanic-island species, and he concluded that these were species whose ancestors could have traveled to those islands without much trouble. Plants could travel as seeds, and flying animals could cross large expanses of ocean. Land animals had more difficulty, though the smaller ones could drift in logs. Lizards and turtles could travel the farthest, because they can slow down during the voyage, and eat the islands' vegetation when they arrived. Small mammals would have more difficulty slowing down, though they could also eat vegetation. A half-starved snake would have a hard time finding prey on an oceanic island, and amphibians are freshwater-adapted -- they would die of thirst due to the osmotic pressure of seawater.

The sizes of many oceanic islands make it difficult for them to support large predators, so many island birds are remarkably tame. Attempting to flee possible predators would be a waste of their energy, so there would be no selection pressure for continuing to do so. Likewise, Darwin had noted that his celebrated Galapagos finches had performed an adaptive radiation, with some ancestral population splitting into several species, each one adapted to eating some different kind of food item.

Since then, biogeography has been an important part of evolutionary biology. And not only oceanic islands have been studied from that standpoint, but also their counterpart for aquatic organisms, inland lakes. To the Galapagos finches may be added the greatly-diversified and much-studied cichlid fishes of east-African lakes.

Continent-scale biogeography was an early clue to continental drift, though that was not usually considered strong-enough evidence. Land bridges were a common alternative explanation until mainstream geologists became convinced of continental drift for different reasons in the 1950's and 1960's.

Creationist Biogeography

Creationists have written remarkably little on biogeography, perhaps because it is such a glaring embarrassment for them -- YEC's have the post-Flood dispersion problems outlined above. However, some of them have risen to the challenge, proposing that the Earth's land formed the supercontinent Pangaea only a few thousand years ago. After the animals dispersed, that supercontinent broke up, with its fragments moving to their present positions in only a few thousand years. That hypothesis still has the problem of why species are present in some places and not others. Why did all the kangaroos and wallabies hop to Australia and New Guinea, and none to anywhere else? Why did all the rattlesnakes slither off to North and South America, and none to anywhere else? Etc.

At least one creationist response to the problem of biogeography in Australia invokes hyper-evolution and massive convergence. The reason that marsupials are found on Australia is then concluded to be that numerous placental mammals settled on Australia and then all spontaneously evolved into marsupials. [1] The existence of his response alone should demonstrate how serious the problem for creationism is.

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Comments

This page should aim at developing into the long-suggested-but-never written Biogeography FAQ for talkorigins.org. Adding ideas, an outline, contributions on specific topics, etc., are all welcome.

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