Organisms often occur in the same habitat, and have evolved so that they have an intimate interaction. If they each have adaptations that are specifically linked to the biology of the other, then they are said to have co-evolved, in the process called coevolution. (Of course, it is just a particular sub-category of natural selection).

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.

Snails and Clams

Some marine snails drill holes in clams. To do this efficiently, they must make a decision on a cost-benefit to them. It takes time and energy to drill a hole through the clam shell: is the effort going to be worth it in terms of the food gained?

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.

Grassland and Horses

Here is another piece of coevolution. Horses once browsed soft leafy bushes, when they first evolved around 60 Ma. But the Earth's climate cooled, and woodland gave way to open grassland with scattered trees (savanna, prairie, steppe, veld, in different languages). Now some horses began to graze in open country, and that changed everything. The grasses responded to grazing by evolving little silica pieces in their leaves (phytoliths) that were essentially jagged little grains of sand. Over time, they wore away the dentine of the horse teeth. Once their teeth worre down, they could no longer eat. So the horses with more enamel and longer teeth survived better, and this coevolution of phytoliths and horse teeth continued for millions of years, drastically altering horse morphology as they continued this "arms race" against the grasses.

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.

Savanna Ecosystems

While we are on the subject of grazing, we can see a good example of parallel evolution, in which animals on different continents reacted in similar ways to the evolution of the savanna grasslands. In South America, for example, there was a set of grazing animals unlike those on other continents. Edgar Rice Burroughs used the real animal, the Miocene "thoat", Thoatherium, for an animal used for riding by his fictional Martian characters. The thoat had evolved to look very much like a horse, even down to the single toe, despite the fact that it had completely different ancestors and lived on a continent that was an island separated away from the rest of the world.

Page last revised January 7, 2000.

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