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June 15, 2000.
How Finches and Tortoises Contributed to the Theory of Evolution
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This is Roger Payne speaking to you from the Odyssey, as we sail among the Galapagos Islands.

As we have seen, when Darwin landed here the colonization by plants and animals had not been going on very long in terms of geological time-the oldest islands are only five to ten million years old, meaning that they have relatively few species of animals and plants. Though Darwin could not have known it, this is one reason that the Galapagos offered such a clear example of how colonization and speciation progress. The finches and tortoises show clearly that species are not stable, they change. This was perhaps the key fact that enabled Darwin to break out of the mental straightjacket in which people of his age were held-the notion that species come from acts of special creation and that once created they do not change.

Perhaps the most surprising (certainly the biggest) animals on the islands were the giant land tortoises. Darwin wrote extensively about them but somehow failed to notice while he was here that every island had its own species. Had he focussed on that single fact it would have given him a strong clue as to what he first called his theory of the "transmutation of species" (i.e. evolution). But in his account of the trip, "The Voyage of the Beagle," he only refers in a passing way to the possibility that there might be a different tortoise species on each island. Recent readings of his dairies by scholars show that when the vice governor of the Galapagos pointed out to Darwin that each island had its own unique species of tortoise he was surprised. He also left the Galapagos believing that the various Galapagos finches he had collected were just varieties, not distinct species, a belief that lay uncorrected until after he had returned to England. When he found out that the Galapagos finches were true species he pondered long and hard trying to fit that fact into his developing theory on the "transmutation of species. " How he did it we shall see next time.

So that's all from me tonight, more tomorrow.

2000 - Roger Payne

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