Monique and Philip Lehner Professor for the Study of Latin America, Professor of Organismic and Evolutionary Biology, Harvard University; Curator in Herpetology, Museum of Comparative Zoology
Natural Selection

It’s easy to think of natural selection as omnipotent. As Darwin said, “natural selection is daily and hourly scrutinizing…every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good.” And the end result? Through time, a population becomes better and better adapted. Given enough time, wouldn’t we expect natural selection to construct the ideal organism, optimally designed to meet the demands of its environment?

If natural selection worked like an engineer—starting with a blank slate and an unlimited range of materials, designing a blueprint in advance to produce the best possible structure—then the result might indeed be perfection. But that’s not a good analogy for how natural selection works. As Nobel laureate Francois Jacob suggested in 1977, the better metaphor is a tinkerer who “gives his materials unexpected functions to produce a new object. From an old bicycle wheel, he makes a roulette; from a broken chair the cabinet of a radio.” In just this way “evolution does not produce novelties from scratch. It works on what already exists, either transforming a system to give it new functions or combining several systems to produce a more elaborate one.”

What a tinkerer can produce is a function of the materials at hand, and the consequence is that two species facing the same environmental challenge may adapt in different ways. Consider the penguin and the dolphin. Both are speedy marine organisms descended from ancestors that lived on land. Although they live similar lifestyles, chasing down swift prey, they do so in different ways. Most fast-swimming marine predators propel themselves by powerful strokes of their tails, and the dolphin is no exception. But not the penguin—it literally flies through the water, its aquatic celerity propelled by its wings.

Why haven’t penguins evolved a powerful tail for swimming like so many other denizens of the sea? The answer is simple. Birds don’t have tails (they do have tail feathers, but no underlying bones). Natural selection, the tinkerer, had nothing to work with, no tail to modify for force production. What the penguin’s ancestor did have, however, were wings, already well-suited for moving through air. It didn’t take much tinkering to adapt them for locomotion in a different medium.

Sometimes, the tinkerer’s options are limited and the outcome far from perfect. Take, for example, the panda’s “thumb,” made famous by Stephen Jay Gould. As opposable digits go, the modified wrist bone is subpar, limited in flexibility and grasping capabilities. But it gets the job done, helping the panda grasp the bamboo stalks on which it feeds.

Or consider another example. The long and flexible neck of the swan is constructed of twenty-five vertebrae. Elasmosaurus, a giant marine reptile from the Age of Dinosaurs with a contortionist’s neck as long as its body, took this approach to the extreme, with seventy-two vertebrae. Pity, then, the poor giraffe, with only seven long, blocky vertebrae in its seven-foot bridge. Wouldn’t more bones have made the giraffe more graceful, better able to maneuver through branches to reach its leafy fare? Probably so, but the tinkerer didn’t have the materials. For some reason, mammals are almost universally constrained to have seven cervical vertebrae, no more, no less. Why this is, nobody knows for sure—some have suggested a link between neck vertebra number and childhood cancer. Whatever the reason, natural selection didn’t have the necessary materials—it couldn’t increase the number of vertebrae in the giraffe’s neck. So, it did the next best thing, increasing the size of the individual vertebrae to nearly a foot in length.

There are several messages to be taken from the realization that natural selection functions more like a tinkerer than an engineer. We shouldn’t expect optimality from natural selection—it just gets the job done, taking the easiest, most accessible route. And as a corollary: we humans are not perfection personified, just natural selection’s way of turning a quadrupedal ape into a big-brained biped. Had we not come along, some other species, from some other ancestral stock, might eventually have evolved hyper-intelligence. But from different starting blocks, that species probably would not have looked much like us.