Why, if it is so easy to improve the teeth of rats by artificial selection, did natural selection apparently make such a poor job of it in the first place? Surely there is no benefit in tooth decay. Why, if artificial selection is capable of reducing it, didn't natural selection do the same job long ago? I can think of two answers, both instructive.
The first answer is that the original population that the human selectors used as their raw material consisted not of wild rats but of domesticated laboratory-bred white rats. It could be said that lab rats are feather-bedded, like modern humans, shielded from the cutting edge of natural selection. A genetic tendency to tooth decay would significantly reduce reproductive prospects in the wild, but might make no difference in a laboratory colony where the living is easy, and the decision on who breeds and who does not is taken by humans, with no eye to survival.
That's the first answer to the question. The second answer is more interesting, for it carries an important lesson about natural selection, as well as artificial selection. It is the lesson of trade-offs, and we have already adverted to it when talking about pollination strategies in plants. Nothing is free, everything comes with a price tag. It might seem obvious that tooth decay is to be avoided at all costs, and I do not doubt that dental caries significantly shortens life in rats. But let's think for a moment about what must happen in order to increase an animal's resistance to tooth decay. I don't know the details, but I am confident that it will be costly, and that is all I need to assume. Let us suppose it is achieved by a thickening of the wall of the tooth, and this requires extra calcium. It is not impossible to find extra calcium, but it has to come from somewhere, and it is not free. Calcium (or whatever the limiting resource might be) is not floating around in the air. It has to come into the body via food. And it is potentially useful for other things apart from teeth. The body has something we could call a calcium economy. Calcium is needed in bone, and it is needed in milk. (I'm assuming it is calcium we are talking about. Even if it is not calcium, there must be some costly limiting resource, and the argument will work just as well, whatever the limiting resource is. I'll continue to use calcium for the sake of argument.) An individual rat with extra strong teeth might well tend to live longer than a rat with rotten teeth, all other things being equal. But all other things are not equal, because the calcium needed to strengthen the teeth had to come from somewhere, say, bones. A rival individual whose genes did not predispose it to take calcium away from bones might consequently survive longer, because of its superior bones and in spite of its bad teeth. Or the rival individual might be better qualified to rear children because she makes more calcium-rich milk. As economists are fond of quoting from Robert Heinlein, there's no such thing as a free lunch. My rat example is hypothetical, but it is safe to say that, for economic reasons, there must be such a thing as a rat whose teeth are too perfect. Perfection in one department must be bought, in the form of a sacrifice in another department.
The lesson applies to all living creatures. We can expect bodies to be well equipped to survive, but this does not mean they should be perfect with respect to any one dimension. An antelope might run faster, and be more likely to escape a leopard, if its legs were a little longer. But a rival antelope with longer legs, although it might be better equipped to outsprint a predator, has to pay for its long legs in some other department of the body's economy. The materials needed to make the extra bone and muscle in the longer legs have to be taken from somewhere else, so the longer-legged individual is more likely to die for reasons other than predation. Or it may even be more likely to die from predation because its longer legs, although they can run faster when intact, are more likely to break, in which case it can't run at all. A body is a patchwork of compromises. I shall return to this point in the chapter on arms races.
What happens under domestication is that animals are artificially shielded from many of the risks that shorten the lives of wild animals. A pedigree dairy cow may yield prodigious quantities of milk, but its pendulously cumbersome udder would seriously impede it in any attempt to outrun a lion. Thoroughbred horses are superb runners and jumpers, but their legs are vulnerable to injury during races, especially over jumps, which suggests that artificial selection has pushed them into a zone that natural selection would not have tolerated. Moreover, Thoroughbreds thrive only on a rich diet supplied by humans. Whereas Britain's native ponies, for example, flourish on pasture, racehorses don't prosper unless they are fed a much richer diet of grains and supplements - which they would not find in the wild. Again, I'll return to such matters in the arms race chapter.
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