by Richard Dawkins
I am writing this on a boat (called the Beagle, as it happens) in the Galápagos archipelago, whose most famous inhabitants are the eponymous (in Spanish) giant tortoises, and whose most famous visitor is that giant of the mind, Charles Darwin. In his account of the voyage of the original Beagle, written long before the central idea of The Origin of Species condensed out of his brain, Darwin wrote of the Galápagos Islands:
“Most of the organic productions are aboriginal creations, found nowhere else; there is even a difference between the inhabitants of the different islands; yet all show a marked relationship with those of [South] America, though separated from that continent by an open space of ocean, between 500 and 600 miles in width. The archipelago is a little world within itself … Considering the small size of the islands, we feel the more astonished at the number of their aboriginal beings, and at their confined range … we seem to be brought somewhat near to that great fact – that mystery of mysteries – the first appearance of new beings on this earth.”
True to his pre-Darwinian education, the young Darwin was using “aboriginal creation” for what we would now call endemic species – evolved on the islands and found nowhere else. Nevertheless, Darwin already had more than a faint inkling of that great truth which, in his mighty maturity, he was to tell the world. Writing of the small birds now known as Darwin’s finches, he said: “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.”
He could as well have said the same of the giant tortoises, for he himself was told by the vice-governor, Mr Lawson:
” … that the tortoises differed from the different islands, and that he himself could with certainty tell from which island any one was brought. I did not for some time pay sufficient attention to this statement, and I had already partially mingled together the collections from two of the islands. I never dreamed that islands, about 50 or 60 miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted … ”
And he said the same kind of thing about the iguanas, both marine and land, and the plants.
With the benefit of hindsight, Darwinian hindsight, we post-Darwinians can piece together what happened. In every one of these cases – and this is typical of the origin of species everywhere – it is islands that constitute the vital, though accidental ingredient. Without the isolation provided by islands, sexual intermingling of gene pools nips species divergence in the bud. Any aspiring new species would be continually flooded by genes from the old species. Islands are natural workshops of evolution. A barrier to sexual intermingling is what you need, to allow that initial divergence of gene pools which constitutes the origin of species, Darwin’s “Mystery of Mysteries”.
But islands don’t have to be land surrounded by water. To a highland-breeding giant tortoise, each of the five volcanoes along the length of the big island of Isabela (Albemarle to Darwin, who used the traditional English names) is an island of green habitability surrounded by inhospitable lava desert. Most of the Galápagos Islands are single volcanos. But the big island, Isabela, is a necklace of five volcanos, spaced from each other at approximately the same distance as the single volcano on the neighbouring island of Fernandina which, from one point of view, might as well be a sixth volcano on Isabela. To a tortoise, Isabela is an archipelago within an archipelago. Not just to a tortoise. The same is true of some plants, moths, land and sea iguanas and other things.
Both levels of isolation have helped the giant tortoises to diverge. All the Galápagos giant tortoises are related to a particular mainland species of land tortoise, Geochelone chilensis, which is smaller than any of them. At some point during the few million years that the islands have existed, one or a few of these mainland tortoises inadvertently fell into the sea and floated across. How could they have survived the long and doubtless arduous crossing? Surely most of them didn’t. But it would have only taken one female to do the trick. And tortoises are astonishingly well-equipped to survive the crossing.
The early whalers took thousands of giant tortoises from the Galápagos Islands away in their ships for food. To keep the meat fresh, they were not killed until needed, but they were not fed or watered while waiting to be butchered. They were simply turned on their backs, sometimes stacked several deep, so they couldn’t walk away. I tell the story not in order to horrify (although I have to say that such barbaric cruelty horrifies me), but to make a point. Tortoises can survive for weeks without food or water, easily long enough to float in the Humboldt Current from South America to the Galápagos Islands. The record without food or water is more than one year, by a tortoise in the 19th century. And tortoises do float.
Having reached and multiplied upon their first Galápagos island, the tortoises would with comparative ease have – again accidentally – island-hopped the much shorter distances to the rest of the archipelago by the same means. And they did what many animals do when they arrive on an island. They evolved to become larger: the long-noticed phenomenon of island gigantism (confusingly, there is an equally well-known phenomenon of island dwarfism). If the tortoise story had followed the pattern of Darwin’s famous finches, they would have evolved a different species on each of the islands. Then, after subsequent accidental driftings from island to island, they would have been unable to interbreed (that’s the definition of a separate species) and would have been free to evolve a different way of life uncontaminated by genetic swamping.
You could say that the different species’ incompatible mating habits and preferences constitute a kind of genetic substitute for the geographic isolation of separate islands. Though they overlap geographically, they are now isolated on separate islands of mating exclusivity. So they can diverge yet further. Most of the Galápagos Islands have the Large, the Medium and the Small Ground Finch, which specialise in different diets. These three species originally diverged on different islands and have now come together on several islands where they coexist as different species, never interbreeding and each specialising in a different kind of seed diet.
The tortoises did something similar, evolving distinctive shell shapes on the different islands. The races on the larger islands have high domes. Those on smaller islands have saddle shaped shells with a high-lipped window at the front for the head. The reason for this seems to be that the large islands are wet enough to grow grass, and the tortoises there are grazers. The smaller islands are mostly too dry for grass, and the tortoises resort to browsing on cactuses. The high-lipped saddle shell allows the neck to reach up to the cactuses which, on their side, grow higher in an evolutionary arms race against the browsing tortoises.
The tortoise story adds to the finch model the further complication that, for tortoises, volcanos are islands within islands. They provide high, cool, damp, green oases, surrounded by dry lava fields at low altitude which, for a giant tortoise, constitute hostile deserts. Most of the islands have but a single volcano and each has its own single species (or sub-species) of giant tortoise (except in those few islands that have none at all). The big island of Isabela has five major volcanoes, and each volcano has its own species (or sub-species) of tortoise. Truly, Isabela is an archipelago within an archipelago: a system of islands within an island. And the principle of islands in the literal geographical sense setting the stage for the evolution of islands in the metaphorical genetic sense of species has never been more elegantly demonstrated than here in the archipelago of Darwin’s blessed youth.