Every species is a cousin of every other. Any two species are descended from an ancestral species, which split in two. For example, the common ancestor of people and budgerigars lived about 310 million years ago. The ancestral species split in two, and the two strands went their separate ways for the rest of time. I chose human and budgie to make it vivid, but that same ancestral species is shared by all mammals on one side of that early divide, and all reptiles (zoologically speaking, birds are reptiles, as we saw in Chapter 6) on the other side. In the unlikely event that a fossil of this ancestral species was ever found, it would need a name. Let's call it Protamnio darwinii. We don't know any details about it, and the details don't matter at all for the argument, but we won't go far wrong if we imagine it as a sprawling lizard-like creature, scurrying about catching insects. Now, here's the point. When Protamnio darwinii split into two sub-populations they would have looked just the same as each other, and could have happily interbred with each other; but one lot were destined to give rise to the mammals, and the other lot were destined to give rise to the birds (and dinosaurs and snakes and crocodiles). These two sub-populations of Protamnio darwinii were about to diverge from each other, over a very long time and in a very big way. But they couldn't diverge if they kept on interbreeding with each other. The two gene pools would continually flood each other with genes. So any tendency to diverge would be nipped in the bud before it could get going, swamped by gene flow from the other population.
What actually happened at this epic parting of the ways, nobody knows. It happened a very long time ago, and we have no idea where. But modern evolutionary theory would confidently reconstruct something like the following history. The two sub-populations of Protamnio darwinii somehow became separated from each other, most likely by a geographical barrier such as a strip of sea separating two islands, or separating an island from a mainland. It could have been a mountain range that separated two valleys, or a river separating two forests: two 'islands' in the general sense I defined. All that matters is that the two populations were isolated from one another for long enough so that, when time and chance eventually reunited them, they found they had diverged so much that they couldn't interbreed any more. How long is long enough? Well, if they were subject to strong and contrasting selection pressures, it could be as little as a few centuries, or even less. For example, an island might lack a voracious predator that prowled the mainland. Or the island population might have shifted from an insectivorous to a vegetarian diet, like the Adriatic lizards of Chapter 5. Once again, we can't know the details of how Protamnio darwiniisplit, and we don't need to. The evidence from modern animals gives us every reason to think that something like the story I have just told is what happened in the past, for every one of the divergences between the ancestry of any animal and any other.
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'.
Was this article helpful?