For Darwin and his heirs, the Galapagos islands, 600 miles over the western horizon from South America, are the Islands of the Blest. It's controversial whether Darwin's visit to that remarkable archipelago really was the seminal experience of his life, but it should have been. If you were to set aside a few million years to design the ideal natural experiment to give the game away about evolution and spark the idea in the mind of a travelling naturalist, Galapagos is exactly what you would dream up. Imagination could not outperform this reality.
The key to the Galapagos islands is that they are young. All are volcanic, and none has ever been in contact with the mainland, so every endemic animal and plant has evolved there in extremely recent times. On the evolutionary timescale, Galapagos is the Country of the Young. Even the oldest of the islands (Espanola near the eastern end of the archipelago) is only just over three million years old, while the youngest (Fernandina, in the west) is probably less than half a million. It is no accident that the islands get younger as you move from south-east to north-west.
The (also very young) theory of plate tectonics makes all clear. The Nazca plate is moving, with a somewhat jerky and erratic course, in a south-easterly direction towards South America, where it is slowly being subducted under the continental shelf. As it moves, it passes over a volcanic "hot spot", which periodically punches through to raise a new volcano: in some cases a new island. Some of the islands have but a single large volcano. Isabela, the largest island, is a chain of five. Old islands are those that have been carried furthest away from the hot spot. Young ones, such as Fernandina and Isabela, are closest to the hot spot today: indeed, Fernandina is right above it. On the oldest island, Espanola, the single volcano is the last survivor of a previously larger island. There were even older islands that have now sunk out of sight altogether, as the plate inches its way under South America. The Nazca plate, with its volcanic hot spot underneath, serves as a conveyor belt for the manufacture and subsequent destruction of islands.
We know that every single animal and plant in the archipelago arrived recently – by geological standards, not by human standards, of course. You cannot say the same of Madagascar or New Zealand, which are fragments of the ancient continent of Gondwana. The faunas and floras of these primeval islands are a complicated mixture of Gondwanan originals together with the descendants of immigrants, which have had many tens of millions of years to go their separate evolutionary ways.
Galapagos evolutions are not all as young as might be suggested by the approximately three million years of the oldest extant islands. This is because their evolutionary radiation could have begun on the now-sunk older islands. But the maximum time available for the unique Galapagos fauna to have evolved is still very short. Continents and older islands like Madagascar have become palimpsests of history, the clarity of the story ravaged and distorted by time and repeated invasions of immigrants. But in Galapagos, we are privileged to see evolution in its burgeoning youth.
A young archipelago like Galapagos is a natural laboratory of evolution, a bustling workshop for the manufacture of new species, again as if on a conveyor belt. Speciation is the name we give to the divergence of one ancestral species into two: Darwin's "origin of species".
For speciation to occur, there must be some initial separation – otherwise sexual cross-breeding will continually mix the gene pool of the original species with that of the budding new species, and prevent them from diverging.
Sometimes the barrier is imposed from without, as when an earthquake, say, changes the course of a river so that it bisects the range of a previously united species. At other times the barrier is already there, wide enough to be difficult to cross but not totally impossible. This means that crossings occur just often enough to initiate speciation, but not so often as to allow enough sexual mixing ("gene flow") to stop the species diverging. Archipelagos are speciation factories, and Galapagos is the pick of the bunch.
Speciation requires an optimal spacing of islands, not too far and not too close. The actual magnitude of the optimal distance will depend on the mobility of the animals concerned. To a wide-ranging bird like a tropicbird, the whole archipelago might as well be a single continent. To a Galapagos finch, the 600-mile distance to the mainland constitutes a major barrier, which was bridged perhaps only once.
The distances between individual islands – of the order of tens of miles rather than hundreds – are just small enough to allow occasional island-hops, but too large to allow gene swamping. These are ideal conditions for speciation. Galapagos might have been designed for the origin of species. Darwin, in The Voyage of the Beagle, enunciated the principle in a dawning of recognition: "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." Darwin was here speaking of the finches that now bear his name, but he was first alerted to the principle by the Floreana mockingbird. He had already noticed, on San Cristobal, that the local mockingbirds resembled those on the mainland but were nevertheless different. The next island the Beagle visited was Floreana, and that was where Darwin's penny dropped. The Floreana mockingbird was different again.
And then other islands turned out to have their own species of mockingbird. The same is true of the giant tortoises, of the lava lizards, and of many of the plant species. But the Floreana mockingbird was Darwin's epiphany. It is of great significance in the history of ideas, and it is at present one of the most endangered species in the world.
The extinction of any species is a tragedy. The plight of the Floreana mockingbird – inspiration of Darwin's youth – moved the actress and artist Lalla Ward to accede to the Gerald Durrell Foundation's suggestion that she should prepare a special exhibition on the theme of, and in aid of, Galapagos wildlife.
Lalla Ward's exhibition is at the Chris Beetles Gallery, 8-10 Ryder Street, London SW1, 10.30 am - 5.30 pm, 21-28 March (except Sunday).
Pictures are for sale in aid of the Durrell Wildlife Conservation Trust's campaign for the Floreana mockingbird and other wildlife of Galapagos.
This article is an extract from Richard Dawkins' introduction to the exhibition's catalogue. The full introduction can be seen at http://richarddawkins.net/article,3640,To-Save-a- Mockingbird, Richard-Dawkins , or via http://tinyurl.com/ckvu32
Comments