I've recently finished reading The Ancestor's Tale
by Richard Dawkins, which contains an abundance of fascinating insights into evolutionary biology. One of the more interesting ones concerns the salamanders of California's Central Valley.
Salamander habitat consistes of the mountains around the valley, not the valley floor itself. That is, the area where salamanders are found is an elongated ring
of uplands, surrounding an internal area of non-salamander-habitable territory (the valley floor).
If you look just at the southern end of the ring, you will find that there are two distinct varieties of salamander which can easily be distinguished on sight. To the east are salamanders whose hides are patterned with irregular patches of black and yellow, while those to the west are an even brown in color. The salamanders themselves are well aware of the difference -- there is a small area where their ranges overlap, but they do not interbreed with each other. By this criterion, they would normally be considered two separate species.
The interesting part comes when you head north along the two separated sides of the valley. Let's take the eastern side. There, as expected, you see only salamanders of the black-and-yellow patchy type. But as you head further north along the eastern foothills, the black-and-yellow patches get less and less distinct. By the time you get to the north end of the valley, there has been a smooth transition to salamanders with mostly-brown skin and indistinct lighter patches. Then, as you head down the western foothills of the valley -- the other side of the "ring" -- you find salamanders with fainter and fainter light patches as you go south. By the time you're back at the southern end of the valley where you started, they are
the plain brown western salamanders you originally saw.
In the south, there are two distinct species. But around the "ring" of the valley, the one type gradually becomes
the other, with no noticeable discontinuity at any point.
Dawkins uses this to illustrate how the division of our successive proto-human ancestors such as the Neanderthals, Homo ergaster
, the australopithecines, etc. into successive species is misleading. If you were to meet a living Homo ergaster
, you would certainly classify it as a separate species from yourself, and you'd be right. But if you had a complete record of all the hundred thousand or so generations of intermediate descendants of that creature which lead forward in time to yourself, you would see only gradual change, no sudden discontinuity. Anthropologists sometimes argue vociferously about where exactly the line between two such successor species should be drawn, but it's actually a meaningless question. Cases like the California salamanders, where we see a gradual transition between two distinct species which live at the same time, are rare -- but between two species separated in time
, where one is the ancestor of the other, it's the norm.
It occurs to me that the salamander has a lesson for us in another area: human races. Of course, no serious anthropologist would argue that human racial differences are analogous to differences between species. Humans populations from different parts of the world can, and routinely do, interbreed when they come into prolonged contact -- despite efforts, in some cases, to prevent it by law and custom. All humans are the same species; genetically speaking the racial differences are superficial, and they are likely of recent origin (how they arose is a fascinating question which, unfortunately, I don't have space for here). Nevertheless, those differences are very noticeable
, and throughout history many humans have attached importance to them, to the disastrous misfortune of humanity as a whole.
We all know, for example, of the horrors which have happened in our own country because so many people thought
there were significant innate differences between light-skinned humans of European descent and dark-skinned humans of African descent. But the United States is like the southern end of the Central Valley. If you were to start out in, say, Scandinavia, and travel through western Russia, the Balkans, Turkey, the Fertile Crescent, Egypt, the Sudan, the Horn of Africa, and finally Central Africa, you would find that the human populations around you changed only gradually in skin color and facial features, with very little in the way of discontinuities. The same would be the case if you set out from Europe and traveled across Russia and Central Asia to China.
Even aside from the hundreds of millions of "mixed-race" people born in regions where migration has brought previously-distinct populations together, human racial variation has always been a matter of gradual gradation over geographical distance, not sharply-bounded categories. If more people had been aware of this throughout history, humanity might have avoided a great deal of nonsense and cruelty.