June 27, 2000.
How Finches Explained Tortoises to Darwin
This is Roger Payne speaking to you from the Odyssey, as we sail among the Galapagos Islands.
As Darwin was pondering the question of how all the different finches he found in the Galapagos Islands had come to pass he was already confident that the Galapagos archipelago had been built by volcanoes that emerged from the sea, and that these volcanic islands must therefore have been sterile at first. He reasoned that they must have received a few birds from the South American mainland as immigrants. Arriving in a region with few or no competitors (i.e. in an area in which all of the niches, or ways of life, were unoccupied) the ancestors of these first arrivals must have spread across all the islands by slowly changing, through the accumulation of small differences in their bills and in their behaviors, into a suite of species that could occupy the same kinds of niches Darwin had observed on continents: the niches of seed eaters, insectivores, flower feeders, and woodpeckers. To say nothing of a finch that appears to have crossed classes to become more like a vampire bat (a mammal) than a bird (there are, after all, no vampire bats in the Galapagos with which the vampire finch's lifestyle had to compete). Darwin could see that species are not individually made by separate acts of creation, but are formed by changes that gradually accumulate through the process of natural selection. And it was through the example of the Galapagos finches that Darwin saw that species evolve.
Once this theory was in place it was easy to incorporate such surprises as the fact that the giant tortoises were divided into separate indigenous species on each island. Although some of these steps in reasoning were made after Darwin's lifetime it is clear that once the first tortoise progenitors arrived in the islands descendants must have slowly spread to adjacent islands. The changes in form that then accumulated on each island would have been different either because the conditions on each island were different, or because the mutations occurring on each island were random. The fact that all islands had similar conditions but different species of giant tortoises suggests that random mutations are the rule and therefore that there is no way each island's population of tortoises could end up looking the same. Instead, the accumulation of random differences would slowly change each tortoise population until a time came where the accumulated differences were so great that individuals from different populations could no longer reproduce. New species must therefore result from the isolation of populations that lasts long enough for the accumulated differences to change the resulting individuals so much that when and if they ever get back together they will be so different they can no longer reproduce.
That's all from me tonight, more tomorrow.
© 2000 - Roger Payne