One of the obstacles that some people face in understanding evolutionary theory is the natural tendency to think in excessively discrete terms, insisting on parsing the continuity of the world into distinct categories. Richard Dawkins, in his book The Ancestor’s Tale, addresses this problem — which he calls “the tyranny of the discontinuous mind” — and offers some examples of how the categories we see in the natural world are often not sharply bounded, but merge seamlessly into one another. I have promised to write about some of the fascinating ideas in this book, and this topic seems a good one to begin with.
One of our more stubborn concepts is that of the species. We look around us and see familiar and distinct types of creatures, different enough in their appearance and behavior that we feel justified in assigning them their own leaves on the taxonomic tree, and their own Latin names. After all, we can see that they are natural categories by the fact that different species, by definition, do not, and usually cannot, interbreed. Those whose religious views interfere with their acceptance of the fact of evolution even go so far as to argue that the species we see are distinct ideas of God.
But the truth is not so simple, and Nature cares not at all about making life easier for human taxonomists (or evangelical preachers), as we shall see.
In Britain are two species of seagulls called herring gulls and lesser black-backed gulls. They are easy to tell apart; as you might already have guessed, the black-backed gulls have darker backs, and as befits two distinct species, they don’t interbreed. But if you keep an eye on the black-backed gulls as you travel east across the northern latitudes, their coloration becomes lighter and lighter, until, when you get round to North America, they are an intermediate grey. They continue to become lighter and lighter, until we get all the way back round to Britain, where — lo and behold — they are now the herring gulls.
What we have, then, is a linear distribution of birds, shading continuously from light grey at one end to black at the other. At any intermediate point on the line, the birds can and do breed with their immediate neighbors. What makes this case interesting is that the two ends of the chain have wrapped all the way around the Earth, and in Britain, where they meet, enough difference has accumulated that the populations live as two immiscible groups. This is called a “ring” species, and gulls are not the only example. Dawkins calls our attention to a similar case involving salamanders that form a ring around California’s Central Valley, and there are other instances besides.
Dawkins goes on to make an even more provocative point. This idea — that small differences, forming a continuous gradient over a geographic range in such a way that any local differences are negligible, can nevertheless add up over sufficient distance into truly distinct species — is just as applicable to distribution in time as it is to space.
To understand what this means, think about our own lineage. Imagine leapfrogging back in time at thousand-year intervals. There would be no reason why a man from the 21st century would be unable to mate with a woman from the 11th; likewise she in turn would, most likely, be able to bear offspring with a man from the 1st century A.D., and so forth. If we were to continue this process long enough, eventually we would get all the way back to the ancient common ancestor (or “concestor”, to use Dawkins’s apt coinage) of both chimpanzees and humans. This concestor would, in turn, beginning the process in reverse, presumably be able to mate successfully with her great-grandchildren of a thousand years later in the chimpanzee line, and in this way we could work our way back up the other side of the loop — arriving, finally, at the present day chimpanzees. Here, once again, the two ends of the ring are brought together as two distinct species — just as the gulls and the salamanders are, but with the ring in this case being an ambit backwards and forwards in time, rather than through geographical space.
Yes, humans and chimps are quite distinct nowadays, and nobody is arguing here that it is an error to consider them separate species. But it is important for us to understand that the categories we create, the boundaries that we impose on the world’s continua, are, more often than not, simply artifacts that we create for our own convenience, to help us impose order on Nature’s blooming confusion. In this, at least, perhaps the fundamentalists have it right: we are the sons and daughters of Adam, whose first order of business was to give the animals names.