The Galápagos mockingbird was one of four species Darwin found on the islands. Why, he wondered, did a small group of similar islands produce so many forms of mockingbird.
All original photos this page copyright and courtesy Robert Rothman

Closely related and mysterious, the evolution of "Darwin's finches" can be seen in real time. Left: small tree finch; right: large cactus finch, with a chunky beak designed to break up large, hard fruits.

Flinch, finch
As we've seen, everybody knows it was finches that clued Darwin in to the notion of evolution by natural selection -- and everybody is wrong.

In that more naive era, Darwin shot finches and stuffed them in a bag. Not yet aware of the significance of geographic distribution, he failed to label which came from where.

Since Darwin's time, we've learned that the bird's beak evolved to cope with a wide variety of food sources, and simply looking at the beak tells you what the bird eats. And as ecological conditions change, so does the beak's shape.

Problem was, numerous species of similar finches lived on overlapping chunks of terrain in the Galápagos. In contrast, the four species of mockingbirds lived in a simpler pattern: three islands each housed one species, and one species lived on all the other islands.

Robert Rothman says Darwin found the mockingbirds far more tractable as a scientific puzzle. The islands resembled each other, he observes, and "Darwin wanted to know, why do you have four different species, when one species certainly seems good for all the islands."

Not mocking those birds
While sailing home through the Pacific Ocean, Darwin recognized that the diversity of mockingbirds-- and of the Galápagos giant tortoises he had watched, ridden, and eaten -- might provide clues to evolution. They might, in Darwin's words, "undermine the stability of species."

If there was one critical lesson in the Galápagos, however, Rothman says it was the fact that a rich assemblage of related species had arrived -- or developed -- after the islands emerged from the ocean.

If Darwin experienced a "eureka!" moment -- when his entire theory become clear for the first time -- it came not during the voyage -- when the seasick young naturalist probably had his hands full firing his shotgun, making his collections and scribbling in his journal-- but after his return to London in 1836.

Soon after distributing his bounteous collections to specialists, he learned that his finches comprised 13 species -- all new to science (the details of classification have since changed). The giant tortoises were all new to science. Those marine iguanas -- not seen on the mainland. The bushes and cacti, ditto.

In other words, the archipelago housed plants and animals seen nowhere else -- and the nature of those organisms often varied from one tiny island to the next, even though the basic volcanic geology and climate were essentially the same.

"To Darwin, all these species, marooned in their lonely archipelago, had diverged from their ancestral stocks and then gone right on diverging. They had broken the species barrier," concluded Jonathan Weiner (see p. 28, "The Beak..." in the bibliography).

Darwin called marine iguanas "imps of darkness." We call them the kind of weird life that evolves on islands.

And thus despite the fact that "just about everything Darwin had said about the birds was mistaken," as science historian David Hull tells us, further finch research did prove that the species are related, and that their evolution could be observed in real time.

That's one reason the Galápagos became the headquarters of evolutionary biology. And as Hull observes, there are "dozens or hundreds of places that would have worked as well, but they do not have this connection to Darwin."

Hidden history! Darwin was lucky to get a berth on HMS Beagle in the first place!