An Isolated Case? Geographical Speciation

When Charles Darwin first set foot on the Galápagos Islands in September 1835, they must have seemed like paradise to him. He had spent the austral winter braving snowstorms and earthquakes in the barren mountains of the Andes, so the 26-year-old naturalist must have welcomed the tropical flora and fauna that the oceanic archipelago offered to him. In the five weeks that he and the crew of HMS Beagle spent there, Darwin took to unbounded collecting and experimentation. He was fascinated by the extreme tameness of the birds. In his report he wrote: "A gun is here almost superfluous: for with the muzzle, I pushed a hawk off the branch of a tree." The sea iguana appalled him on account of its being "hideous-looking," "stupid," and "sluggish," and he even spent some time repeatedly hurling a particularly unlucky individual into the waves to see if it would eventually flee to sea (it never did, but doggedly kept returning to the shore).

Darwin was also most amused with the animals that give the archipelago its name, the gigantic galápagos or tortoises that roamed the islands, and which Darwin rode, ate, and - when he was very thirsty - drank the urine of ("only a very slightly bitter taste.) When the vice-governor of the Galápagos, a Mr. Lawson, told him that the tortoises from each island were different, and that he could readily tell by their looks from which island they were brought, Darwin would not believe him at first. "I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other . . . would have been differently tenanted," he wrote. In fact, Darwin had already lumped the specimens he had collected, without labeling them as to the exact island from which they came. Later he was able to confirm Lawson's claim. Some islands harbored smaller tortoises, some larger ones (on a particular island they grew so large that it took six to eight men to lift a single male). Certain islands had smooth-shelled tortoises, whereas on others the shells had ridges and furrows. The ones from Pinta Island had a shell with front edges upturned.

Being forewarned, Darwin was more careful with the herbarium he put together. He systematically noted the native island for each plant that he collected and dried. Ten years later, when his botanist friend Joseph Dalton Hooker had worked up his collection, he was able to see that what was true for tortoises was true for plants on a much more massive scale. Of the 71 different plant species that he brought home from Isabela Island, for example, 30 were exclusive to Isabela, and not found on any of the other islands, which had their own aboriginal species. On average, 40% of the flora of each island were unique to that place. The same phenomenon was visible among the birds of the Galápagos - the well-known Darwin's finches and the less-famous mockingbirds (which Darwin called "mocking-thrushes.") Altogether, Darwin wrote, "several of the islands possess their own species of the tortoise, mocking-thrush, finches, and numerous plants, these species having the same general habits, occupying analogous situations, and obviously filling the same place in the natural economy of this archipelago. That strikes me with wonder." And: "We seem to be brought somewhat near to that great fact - that mystery of mysteries - the first appearance of new beings on this earth."

When he wrote these words, in the 1840s, Darwin had an insight that in later years would prove to be very important in understanding how new species may come about - by geographical isolation. But Darwin never really put his heart into this idea. Instead, he later thought that new species could just as well be formed without isolation, by natural selection alone, a subject that we will return to later in this book. He left his earlier hunch to be picked up by some of his followers.

One of those followers was the German naturalist and explorer Moritz Wagner, who at the same time that Darwin was circumnavigating the world on the Beagle, had been traveling in Asia, Africa, and America. In 1837, while collecting insects in Algeria, the young Wagner was struck by the fact that each time he crossed a river, he would find new species of the wingless, blundering darkling beetle Pimelia. Just like Darwin in the Galapagos Islands, Wagner had found an example of species that were separated from their relatives by geographical barriers (rivers in Algeria that wingless beetles could not cross, stretches of sea in the Galapagos that 300-kilo tortoises would not swim).

Wagner seized on this idea and in 1868 he wrote a book about the subject, in which he proposed that "the formation of . . . incipient species can succeed in nature only where some individuals can cross the previous borders of their range and segregate themselves for a long period from the other members of their species." Wagner envisaged that a group of animals or plants would become cut off by some geographical barrier and start following their own evolutionary path: if the expatriates evolved a new characteristic, this would not be able to pass into the ancestral population, and vice versa. During a long period of separation, enough evolutionary changes could accumulate on either side of the barrier to transform the two populations into different species.

Wagner corresponded with Darwin on the subject, but the Englishman was not impressed. In a later edition of On the Origin of Species, Darwin wrote: "I can by no means agree with this naturalist, that migration and isolation are necessary elements for the formation of new species." And, in 1875, he privately brandished a recent paper by Wagner as "Most Wretched Rubbish." But Wagner was undaunted and towards the end of his life he expanded his theory further. In 1889, it appeared posthumously as Die Entstehung der Arten durch räumliche Sonderung ("The Origin of Species by Spatial Separation"). Hidden under this obscure German title, the theory lay dormant for more than half a century until it was revived in the 1940s by the legendary Ernst Mayr.