Even if conditions on either side of the barrier are identical, two geographically separated gene pools of the same species will eventually drift apart from one another, to the point where they can no longer interbreed when the geographical isolation eventually comes to an end. Random changes in the two gene pools will gradually build up, to the point where, if a male and a female from the two sides meet, their genomes will be too different to combine to make a fertile offspring. Whether by random drift alone, or with the aid of differential natural selection, once the two gene pools have reached the point where they no longer need the geographical isolation to stay genetically separate, we call them two different species. In our hypothetical case, perhaps the island population changed more than the mainland population, because of the lack of predators and the switch to a more vegetarian diet. So, a zoologist of the time might have recognized that the island population had become a new species and given it a new name, say Protamnio saurops, while the old name, Protamnio darwinii, might have continued to serve for the mainland population. In our hypothetical scenario, perhaps it was the island population that was destined to give rise to the sauropsid reptiles (that’s everything we call reptiles today plus birds), while the mainland population eventually gave rise to the mammals.
Once again, I must stress, the details of my little story are pure fiction. It could equally well have been the island population that gave rise to the mammals. The ‘island’ could have been an oasis surrounded by desert, rather than land surrounded by water. And of course we haven’t the faintest idea whereabouts on the Earth’s surface this great divide took place – indeed, the world map would have looked so different that the question scarcely means anything. What is not fiction is the major lesson: most, if not all, of the millions of evolutionary divergences that have populated the Earth with such luxuriant diversity began with the chance separation of two sub-populations of a species, often, though not always, on either side of a geographical barrier such as a sea, a river, a mountain range or a desert valley. Biologists use the word ‘speciation’ for the splitting of a species into two daughter species. Most biologists will tell you that geographical isolation is the normal prelude to speciation, although some, especially entomologists, may chime in with the reservation that ‘sympatric speciation’ can also be important. Sympatric speciation, too, requires some kind of initial, incidental separation to get the ball rolling, but it is something other than geographic separation. It could be a local change in microclimate. I won’t go into the details, but will just say that sympatric speciation seems to be especially important for insects. Nevertheless, for simplicity’s sake, I shall in the rest of this chapter assume that the initial separation that precedes speciation is normally geographical. You may remember that, in Chapter 2’s treatment of domestic dog breeds, I likened the effect of the rules imposed by pedigree breeders to the creation of ‘virtual islands’.
‘ONE MIGHT REALLY FANCY . . .’
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.
Darwin’s map of the Galapagos islands with English names, now seldom used
This disparity, between tens of miles as the distance between islands within the archipelago, and hundreds of miles as the distance of the whole archipelago from the mainland, leads the evolutionist to expect that the different islands might house species that are pretty similar to each other but more different from their counterparts on the mainland. And that is exactly what we do find. Darwin himself put it well, coming tantalizingly close to evolutionary language, even before he had properly formulated his ideas. I have placed the key clause in italics, and shall repeat it throughout this chapter in different contexts.
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. In a like manne
r it might be fancied that a bird, originally a buzzard, had been induced here to undertake the office of the carrion-feeding Polybori of the American continent.
The last sentence is a reference to the Galapagos hawk, Buteo galapagoensis, another species that is found only on Galapagos, but which somewhat resembles species on the mainland, especially Buteo swainsoni, which annually migrates between North and South America and could well have been blown off course on one or two freak occasions. Nowadays, we should refer to the Galapagos hawk and the flightless cormorant as ‘endemic’ to the islands, meaning that this is the only place where they are found. Darwin himself, who had not yet fully embraced evolution, used the then current phrase ‘aboriginal creations’, which meant that God had created them here and nowhere else. He used the same phrase of the giant tortoises, which then abounded on all the islands, and also of the two species of iguana, the Galapagos land iguana and the Galapagos marine iguana. The marine iguanas are truly remarkable creatures, quite different from anything seen anywhere else in the world. They dive to the sea bottom and graze seaweed, which seems to be their only food. They are graceful swimmers, although not, in Darwin’s outspoken view, beautiful to look at:
It is a hideous looking creature, of a dirty black colour, stupid,* and sluggish in its movements. The usual length of a full-grown one is about a yard, but there are some even four feet long . . . their tails are flattened sideways, and all four feet partially webbed . . . When in the water this lizard swims with perfect ease and quickness, by a serpentine movement of its body and flattened tail – the legs being motionless and closely collapsed on its sides.
Since marine iguanas are so good at swimming, it might be supposed that they, rather than the land iguanas, made the long crossing from the mainland and subsequently speciated, in the archipelago, to give rise to the land iguana. This is almost certainly not the case, however. The Galapagos land iguana is not greatly different from iguanas still living on the mainland, whereas the marine iguanas are unique to the Galapagos archipelago. No lizard with the same marine habits has ever been found elsewhere in the world. We are nowadays confident that it was the land iguana that originally arrived from the South American mainland, perhaps carted on driftwood like the modern ones from Guadeloupe that were blown to Anguilla. On Galapagos, they subsequently speciated to give rise to the marine iguana. And it was almost certainly the geographical isolation permitted by the spaced-out pattern of the islands that made possible the initial separation between the ancestral land iguanas and the newly speciating marine iguanas. Presumably some land iguanas were accidentally rafted across to a hitherto iguana-free island, and there adopted a marine habit, free from contamination by genes flowing in from the land iguanas on the original island. Much later, they spread to other islands, eventually returning to the island from which their land ancestors had originally hailed. By now they could no longer interbreed with them, and their genetically inherited marine habits were safe from contamination by land iguana genes.
In example after example, Darwin noticed the same thing. The animals and plants of each island of Galapagos are largely endemic to the archipelago (‘aboriginal creations’), but they are also for the most part unique, in detail, from island to island. He was especially impressed with the plants in this respect:
Hence we have the truly wonderful fact, that in James Island [Santiago], of the thirty-eight Galapageian plants, or those found in no other part of the world, thirty are exclusively confined to this one island; and in Albemarle Island [Isabela], of the twenty-six aboriginal Galapageian plants, twenty-two are confined to this one island, that is, only four are at present known to grow in the other islands of the archipelago; and so on . . . with the plants from Chatham [San Cristobal] and Charles [Floreana] Islands.
He noticed the same thing with the distribution of mockingbirds over the islands.
My attention was first thoroughly aroused, by comparing together the numerous specimens, shot by myself and several other parties on board, of the mocking-thrushes, when, to my astonishment, I discovered that all those from Charles Island belonged to one species (Mimus trifasciatus); all from Albemarle Island to M. parvulus; and all from James and Chatham Islands (between which two other islands are situated, as connecting links) belonged to M. melanotis.
So it is, all over the world. The fauna and flora of a particular region are just what we should expect if, to quote Darwin on the finches that now bear his name, ‘one species had been taken and modified for different ends’.
The Vice-Governor of the Galapagos Islands, Mr Lawson, intrigued Darwin by informing him
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 fifty or sixty 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.
All the Galapagos giant tortoises are similar 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 in 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 Galapagos islands away in their ships for food. To keep the meat fresh, the tortoises 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 does horrify me), but to make a point. Tortoises can survive for weeks without food or fresh water, easily long enough to float in the Humboldt Current from South America to the Galapagos archipelago. And tortoises do float.
Having reached and multiplied upon their first Galapagos island, the tortoises would with comparative ease – again accidentally – have 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. This is the long-noticed phenomenon of island gigantism (confusingly, there is an equally well-known phenomenon of island dwarfism).*
If the tortoises 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, remember, 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. The Large, the Medium and the Small Ground Finch originally diverged on different islands; the three species now coexist on most of the Galapagos islands, never interbreeding and each specializing in a different kind of seed diet.
The tortoises did something similar, evolving distinctive shell shapes on the different islands. The species on the larger islands have high domes. Those on the 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 t
o reach up to the cactuses which, for their part, 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, volcanoes are islands within islands. Volcanoes provide high, cool, damp, green oases, surrounded at low altitude by dry lava fields which, for a grazing giant tortoise, constitute hostile deserts. Each of the smaller islands has a single large volcano and its own single species (or sub-species) of giant tortoise (except those few islands that have none at all). The big island of Isabela (‘Albemarle’ to Darwin) consists of a string of 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 blest youth.*
The Greatest Show on Earth Page 26