Biogeography
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[edit] Definition
Biogeography is the study of which species live where, and why. As such, it requires a synthesis of what we know about geography, the history of geography (i.e. plate tectonics); the present distribution of flora and fauna; their past distribution as evidenced by the fossil record; the theory of evolution; and our knowledge of how plants and animals spread from place to place (see the main article on Means of Dispersal).
[edit] Evolution and biogeography
When we find two members of the same species living in different places, we conclude, from the fact that they are the same species, that they had a common ancestor, which must necessarily have lived in some particular place, and that there was necessarily a means of dispersal from the place where the common ancestor lived to the places they are now (one of which may be identical with the home of the common ancestor).
Now, the study of evolution tells us that ancestry must go much deeper than that. For example, all mammals have a common ancestor. Hence since there are (non-flying, non-marine) mammals in isolated Australia, and also on the other continental landmasses, we have to conclude that there was once a possible means of dispersal: they walked (the arguments against rafting as a means of dispersal are somewhat involved and technical, so we omit them). This tells us that Australia was once part of a larger landmass, and that this state of affairs did not end until after the origin of mammals; as geologists confirm.
In general, then, the theory of evolution predicts that what we learn about the paleogeography of the Earth from consideration of common descent and of possible means of dispersal, will agree with what we learn about paleogeography from the study of geology and plate tectonics.
We can also consult the fossil record to learn about the distribution of species in the past; again, we have the prediction that this will correspond to the picture of paleogeography as revealed by geology.
We can use this in two ways. First, if we are confident of common descent and the taxonomic tree, then we can use the biogeographical information to confirm and refine our understanding of paleogeography.
Second, if we wish to put evolutionary to the test, we can check that the two pictures of paleogeography given by geology and by biogeography do indeed correspond. We would know that our picture of either geology or evolution was seriously flawed if, for example, geologists told us that Australia had always been isolated; or if they said that South America, with its unique mammal fauna, had always been isolated until its meeting with North America; or if Hawaii, which geologists tell us rose out of the ocean by volcanic action, had the same fauna as North America; or if the identical species of flightless pigeon was found on two widely separated islands; or any one of hundreds of like hypotheses. Such observations would certainly falsify our picture of either geology or evolution; of course, it would then be necessary to find out which.
Hence we can use evolution to provide evidence for the details of paleogeography, or the details of paleogeography to provide evidence for evolution. It would of course be wrong to do both at the same time: the confirming evidence for evolution must come exclusively from the agreement with the independent geological picture of paleogeography.
[edit] Some examples
We have said that biogeography requires a synthesis of evolution, geology, geography, and our knowledge of means of dispersal. We shall give a few examples which we find interesting.
For example, you might wonder why the species in the warm, shallow Mediterranean are more similar to those in the deep, cold Atlantic Ocean than they are to those in the warm, shallow Red Sea. The answer is that when the Mediterranean was refilled from the Atlantic, about 5 million years ago[1], Atlantic species could get into the Mediterranean and adapt to the new conditions, whereas the marine organisms of the Red Sea could not. It is interesting to note that since the construction of the Suez Canal, more than 120 organisms native to the Red Sea have successfully colonised the Mediterranean.[2].
Another example comes from the distribution of small forest-dwelling mammals in the cool, temperate forests that top the mountains that rise from the Great Basin Desert in the southern United States[3]. Their nearest relatives --- indeed, the same identical species --- are to be found in the cool temperate forest north of the desert. Common sense, observation, and the mathematics of biogeography show that these species cannot, do not, and would have no inclination to, trek across miles of arid sagebrush in the Great Basin Desert.
So now you should be wondering how on earth they got up there in the first place. This question is easily resolved when you reflect that geology and climatology show that North America was recently in the grip of a glacial period, during which the southern United States were cool and forested. When climatic conditions changed, the species suited to these conditions survived either by going up mountains or by following the cooler climates north.
As a third example, consider why oceanic islands are so often the homes of flightless birds. The dodo is a good example. According to molecular phylogeny and according to morphological studies, the dodo was a sort of pigeon, which explains how its ancestors got to Mauritius --- they were birds which could fly. On the other hand, the natural enemies of birds, such as cats, are not really suitable for any form of dispersal to Mauritius, being neither able to swim that distance, nor to be borne on the wind, nor suitable for the other methods of dispersal that we have listed.
For this reason, the pigeons on Mauritius had no selective pressure on them to be able to fly out of the reach of predators, whereas they had the usual pressures on them to be plump and sturdy. They lost, not only the capacity to fly, but, as history records, the capacity to be afraid of potential predators.
One more example. Biogeography tells us that we ought to expect different ecosystems on islands which are continental (that is, produced by either erosion or plate tectonics splitting an island from the mainland) and islands which are raised from the sea by tectonic mechanisms such as volcanic action or uplift caused by the collision of continental plates.
For if an island breaks off from a continent, then until the point at which it breaks off, mammals can get from the mainland onto the island-to-be just by walking; whereas if an island rises out of the sea, then their means of dispersal are more limited. Hawaii is a case in point. Being produced entirely by volcanic action, it must once have been as barren as Krakatoa after its eruption. As a consequence, its only native mammals are the exceptions that prove the rule --- the Hawaiian Hoary Bat and the Hawaiian Monk Seal, which dispersed by flying and swimming respectively.
We could continue with examples --- the distribution of all species across an entire planet is, after all, a large subject. For further reading about modern developments in biogeography, and its application to the conservation of endangered species, we recommend David Quammen's popular work, The Song of the Dodo.
[edit] Creationists and biogeography
However, that is not the case. This might be because they recognize the truth of the results of biogeography ... or just possibly, they may have some particular reason for wishing to avoid the subject (see picture, right).
