There are two primary kinds of evolutionary “arms races” — symmetric and asymmetric. A symmetric arms race is one in which two or more competitors are trying to do essentially the same thing. If we imagine a forest full of various trees, vines, and other plants, we can easily see this kind of arms race in action. All of the trees, regardless of their particular species, are interested in the same thing, namely sunlight. To that end, many of them “discover” the same path to their goal.
Imagine a primordial forest in which (for simplicity’s sake) there is one kind of tree that grows to approximately ten feet in height. So long as all the trees grow to the same height, everything will be stable, but the fact is, natural selection produces variation, so sooner or later, one of the trees is going to grow to eleven feet. Let’s assume that this tree has an umbrella like spread of leaves at the top, so that ten foot trees will receive less sunlight if an eleven foot tree is next to them. In a very few generations, eleven foot trees will dominate the landscape.
Only… natural selection produces variation, so soon, there will be a twelve foot tree. This is the nature of symmetrical arms races, and individual species are one of the best examples. All members of a species are competing for the same thing, and so they tend to push each other towards new innovations. Once we understand this dynamic, it’s easy to see that different species also have the same effect on each other. Elms and oaks and pines all want the same thing, and so they tend to evolve in very similar ways. Though elms, pines and oaks don’t have precisely the same leaf shape or root structure, they are still in the same business — getting to the sunlight and absorbing it through leaves. They’re all part of the same arms race.
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