How, then, do two populations of a species find themselves on opposite sides of a geographical barrier? Sometimes the barrier itself is the novelty. An earthquake opens up an impassable gorge, or changes the course of a river, and a species that had been a single breeding population finds itself severed in two. More usually, the barrier was there all along, and it is the animals themselves that cross it, in a rare freak event. It has to be rare, otherwise it doesn't deserve to be called a barrier at all. Before 4 October 1995 there were no members of the species Iguana iguana on the Caribbean island of Anguilla. On that date, a population of these large lizards suddenly appeared on the eastern side of the island. Fortuitously, they were actually seen arriving. They were clinging to a mat of driftwood and uprooted trees, some more than 30 feet long, that had drifted from a neighbouring island, probably Guadeloupe 160 miles away. The previous month two hurricanes, Luis on 4-5 September, and Marilyn two weeks later, had ripped through the area and could easily have uprooted the trees, complete with iguanas, which habitually spend time up trees. The new population on Anguilla was still going strong in 1998, and Dr Ellen Censky, who led the original study, informs me that they are flourishing to this day, seemingly even more so than the other species of iguana that lived on Anguilla before the new invaders arrived.
The point about such freak dispersal events is that they must be common enough to account for speciation, but not too common. If they were too common - if, say, iguanas drifted from Guadeloupe to Anguilla every year - the incipiently speciating population on Anguilla would be continually swamped by incoming gene flow and therefore could not diverge from the Guadeloupe population. By the way, please don't be misled by my use of a phrase like 'must be common enough'. It could be misunderstood to mean that steps of some kind were taken to ensure that the islands were just the right distance apart to facilitate speciation! Of course that puts the cart before the horse. It is rather that, wherever there happen to be islands (islands in the broad sense, as always) spaced out at an appropriate distance to facilitate speciation, there speciation will occur. And the appropriate distance will depend on how easy it is for the animals concerned to travel. The 160 miles that separate Guadeloupe from Anguilla would be child's play to any strong flying bird such as a petrel. But even a sea crossing of a few hundred yards might be difficult enough to midwife a new species of, say, frogs or wingless insects.
The Galapagos archipelago is separated from the mainland of South America by about 600 miles of open water, nearly four times as far as those iguanas sailed on their uprooted raft to Anguilla. The islands are all volcanic, and young by geological standards. None of them has ever been connected to any mainland. The entire fauna and flora of the islands must have travelled there, presumably from mainland South America. Even though small birds can fly, 600 miles is enough to make a crossing by finches a very rare event. Not so rare that it couldn't happen, however, and there are finches on Galapagos, whose ancestors, at some point in history, were presumably blown across, perhaps by a freak storm. These finches are all of a recognizably South American type, although the species themselves are unique to the Galapagos islands. Look at Darwin's map which I have adopted for sentimental reasons and because he used the magnificently naval-sounding English names for the islands, rather than the modern Spanish names. Notice that the 60-mile scale is about a tenth of the distance an animal would have had to travel to arrive on the archipelago from the mainland in the first place. The islands themselves are only tens of miles from each other, but hundreds of miles from the mainland. What a wonderful recipe for speciation. It would be too simple to say that the chance of being accidentally blown or rafted across a sea barrier to an island is inversely proportional to the width of the barrier. Nevertheless, there will clearly be some sort of inverse correlation between distance and probability of crossing. The difference between the average inter-island distance of a few tens of miles, and the 600-mile distance to the mainland, is so large that you would expect the archipelago to be a powerhouse of speciation. And so it is, as Darwin eventually realized, although not until after he had left the islands, never to return.
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