From this viewpoint, trying to find physics-like laws of nature is like trying to track down the butterfly whose wing beat caused the typhoon.

Most of the science I have written about sidelines history. Ideas such as D’Arcy Thompson’s “diagram of forces,” niches, network models, and energy equivalence assume that there has been enough time, and evolutionary flexibility, for evolution to find a solution to a problem—for nature to find a balance—that can be explained without reference to an organism’s past or the constraints on it. The successes of such an approach show that sometimes this assumption is justified. Robert MacArthur explicitly sought nonhistorical explanations: “The ecologist and the physicist tend to be machinery oriented,” he wrote, “whereas the paleontologist and most biogeographers tend to be history oriented.”

But a historical viewpoint is as critical in understanding life today as it ever was. As an example, imagine the following thought experiment about how some animal groups become more diverse than others. You release a small animal species and a large one—a rat and a deer, say—onto an island with no other animals, and monitor the evolutionary results for a few million years, until many more species have evolved. Which of the original species will give rise to the most descendents? You might expect that the small animal would split into more new species because, as we have seen, there are more small niches and small animals are more diverse. The small animal would evolve more rapidly. Yet DNA evidence of some animals’ evolutionary history gives another picture. Andy Purvis and his colleagues have found that small, diverse groups such as rodents do not seem to have split into new species, and filled niches, any more quickly than large mammals such as primates have. But small mammals did emerge relatively unscathed from whatever killed the dinosaurs off 65 million years ago, and so the reason there might be more of them today is that they made up a greater proportion of the ancestral stock from which today’s mammals evolved. They won the race not by being faster but by having a head start. However much we might be able to explain about ecology using concepts that ignore an organism’s history, the historical perspective is still crucial to understand how today’s nature came to be.

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