Evolution and Geographical Distribution


On the Voyage of the Beagle, Charles Darwin was struck by the different varieties of animals which were found at different locations.
 

Galapagos Islands

The Galapagos are a group of islands in north-western South America.

 

Darwin noted that there were many types of mocking-birds and finches, and they differed from island to island within the archipelago.  In evolutionary terms this can be explained very easily.  Originally there was a common ancestor, which then adapted to fit in with their environments.  Some finches evolved long beaks if the fruits on that island were hard-shelled, some finches didn’t, because they ate insects, etc.

To quote Darwin himself:

“I have stated, that in the thirteen species of ground-finches [in the Galapagos Islands], a nearly perfect gradation may be traced, from a beak extraordinarily thick, to one so fine, that it may be compared to that of a warbler. I very much suspect, that certain members of the series are confined to different islands; therefore, if the collection had been made on any one island, it would not have presented so perfect a gradation. It is clear, that if several islands have each their peculiar species of the same genera, when these are placed together, they will have a wide range of character. But there is not space in this work, to enter on this curious subject.”

An even clearer example can be drawn from tortoises.  Some Galapagos tortoises evolved shells and necks suitable for reaching up at bushes and trees.  Other islands had grass, and so the tortoises were grazers, therefore did not need to evolve in that direction.

Compare the two tortoises below. One was taken from an island with trees and bushes - but no grass; the other was taken from an island with grass:

Immediately we see the importance of geographical barriers (islands) in evolution.  Because they are separated, the species could evolve independently, and given time, can become two distinct species altogether.
 

Continental Distributions

We have seen before how evolution worked on small islands such as the Galapagos.  Geographical barriers however can be much bigger in size.  We see differences between Australian fauna, and South-east Asian fauna, and African fauna, and South American fauna.  Oceans, rivers and mountains separate these continents, creating geographical barriers.  Compare Australian fauna (left) with African fauna (right).  The animals have evovled independently. 

We must, of course, when studying continental distributions, also take into consideration the theory of Continental Drift.

In conclusion, geographic distributions can explain why we see the variety of species that exists today.  Different geographic settings mean different criteria for natural selection.  Distributions like these also ensure speciation, ie. macroevolution.  In light of evolution, it is very easy to explain why animals appear on some ‘islands’ but not on others.  A creationist would have to say something like “God liked it there better” or “God works in mysterious ways” or “Who are we to question God’s judgement?”… Which is clearly not very scientific at all, right?

Right.

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