Coevolution that must go on between predators and prey, for example, between the seastars and mussels on the rocky shores of the Pacific Northwest. This is an antagonistic coevolution. There is a winner and a loser, so it is a +/- relationship. All predator/prey relationships are like this. The predator runs for its supper, and the prey runs for its life. Over time, one would predict that both the redator and the prey would evolve to improve their adaptations. This is sometimes called an "arms race": each side gets better equipment, but if they remain equally matched, there is still selection to get better.
Such coevolution can go on through geological time. One study showed a counterintuitive result.
You might predict that if snails are drilling holes through clam shells, then those clams that happen to have thicker shells will survive better than those that do not: so as natural selection takes its course, clams with thicker shells will have more offspring, and clams will evolve thicker shells. If the snails were stupid, this would probably happen.
The snails are subject to natural selection too: those that make a clever decision on which clams to attack will gain more food and will have more offspring.
Clever snails strike just the right balance between clams to drill. As a result, the clams evolve in a counterintuitive way. They do not grow meaty enough to make themselves an obvious target for a snail. They grow a little body in a thin shell (cheap), and they begin to reproduce early, when they are still small. That way they have offspring, and if the snail drills them later, when they are bigger, they have already passed on their genes. So, over geological time, we see a trend in shells that says the coevolution between drilling snails and clams produces smaller, thinner-shelled clams, just the opposite of what we might have expected before we really began to analyze it.
As horses evolved larger, longer teeth with more enamel, they had to have bigger jaws to place the teeth in, and larger muscles for chewing. The horse face grew longer and stronger. Also, the horses out in the plain could not hide easily, so the taller, faster ones survived and reproduced better than the shorter, slower ones. The "evolution of the horse" involved a many-million year increase in size, in running ability, in chewing capacity, that has become famous.
Another reaction of grasses to grazing pressure cannot be seen in the fossil record, though we know from veterinary reasearch that it exists. Some grasses produce cyanide if they are heavily grazed, and of course this does no good to the animals that are doing the grazing.
Page last revised January 7, 2000.
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