Animal And Plant Breeding

The theory of natural selection predicts what types of adaptations we'd expect to find and—more important—not find in nature. And these predictions have been fulfilled. But many people would like more: they'd like to see natural selection in action, and witness evolutionary change in their lifetime. It's not hard to accept the idea that natural selection could cause, say, the evolution of whales from land animals over millions of years, but somehow the idea of selection becomes more compelling when we see the process before our eyes.

This demand to see selection and evolution in real time, while understandable, is curious. After all, we easily accept that the Grand Canyon resulted from millions of years of slow, imperceptible carving by the Colorado River, even though we can't see the canyon getting deeper over our lifetime. But for some people this ability to extrapolate time for geological forces doesn't apply to evolution. How, then, can we determine whether selection has been an important cause of evolution? Obviously, we can't replay the evolution of whales to see the reproductive advantage of each small step that took them back to the water. But if we can see selection causing small changes over just a few generations, then perhaps it becomes easier to accept that, over millions of years, similar types of selection could cause the big adaptive changes documented in fossils.

Evidence for selection comes from many areas. The most obvious is artificial selection—animal and plant breeding—which, as Darwin realized, is a good parallel to natural selection. We know that breeders have worked wonders in transforming wild plants and animals into completely different forms that are good to eat, or that satisfy our aesthetic needs. And we know that this has been done by selecting variation present in their wild ancestors. We also know that breeding has wrought huge changes in a remarkably short period of time, for animal and plant breeding has been practiced for only a few thousand years.

Take the domestic dog (Canis lupus familiaris), a single species that comes in all shapes, sizes, colors, and temperaments. Every single one, purebred or mutt, descends from a single ancestral species—most likely the Eurasian gray wolf—that humans began to select about 10,000 years ago. The American Kennel Club recognizes 150 different breeds, and you've seen many of them: the tiny, nervous Chihuahua, perhaps bred as a food animal by the Toltec of Mexico; the robust Saint Bernard, thick of fur and able to fetch kegs of brandy to snow-stranded travelers; the greyhound, bred for racing with long legs and a streamlined shape; the elongated, short-legged dachshund, ideal for catching badgers in their holes; retrievers, bred to fetch game from the water; and the fluffy Pomeranian, bred as a comforting lap dog. Breeders have virtually sculpted these dogs to their liking, changing the shade and thickness of their coat, the length and pointiness of their ears, the size and shape of their skeleton, the quirks of their behavior and temperament, and nearly everything else.

Think of the diversity you'd see if all these dogs were lined up together! If somehow the recognized breeds existed only as fossils, paleontologists would consider them not one species but many—certainly more than the thirty-six species of wild dogs that live in nature today.29 In fact, the variation among domestic dogs far exceeds that among wild dog species. Take just one trait: weight. Domestic dogs range from the 2-pound Chihuahua to the 180-pound English mastiff, while the weight of wild dog species varies from 2 pounds to only about 60 pounds. And there is certainly no wild dog having the shape of a dachshund or the face of a pug.

The success of dog breeding validates two of the three requirements for evolution by selection. First, there was ample variation in color, size, shape, and behavior in the ancestral lineage of dogs to make possible the creation of all breeds. Second, some of that variation was produced by genetic mutations that could be inherited—for if it were not, breeders could make no progress. What is most astonishing about dog breeding is how fast it got results. All those breeds have been selected in less than 10,000 years, only 0.1 percent of the time that it took wild dog species to diversify from their common ancestor in nature. If artificial selection can produce such canine diversity so quickly, it becomes easier to accept that the lesser diversity of wild dogs arose by natural selection acting over a period a thousand times longer.

There's really only one difference between artificial and natural selection. In artificial selection it is the breeder rather than nature who sorts out which variants are "good" and "bad." In other words, the criterion of reproductive success is human desire rather than adaptation to a natural environment. Sometimes these criteria coincide. Look, for example, at the greyhound, which was selected for speed, and wound up shaped very much like a cheetah. This is an example of convergent evolution: similar selective pressures give similar outcomes.

The dog can stand for the success of other breeding programs. As Darwin noted in The Origin, "Breeders habitually speak of an animal's organization as something quite plastic, which they can model almost as they please." Cows, sheep, pigs, flowers, vegetables, and so on—all came from humans choosing variants present in wild ancestors, or variants that arose by mutation during domestication. Through selection, the svelte wild turkey has become our docile, meaty, and virtually tasteless Thanksgiving monster, with breasts so large that male domestic turkeys can no longer mount females, who must instead be artificially inseminated. Darwin himself bred pigeons, and described the huge variety of behaviors and appearance of different breeds, all selected from the ancestral rock dove. You wouldn't recognize the ancestor of our ear of corn, which was an inconspicuous grass. The ancestral tomato weighed only a few grams, but has now been bred into a 2-pound behemoth (also tasteless) with a long shelf life. The wild cabbage has given rise to five different vegetables: broccoli, domestic cabbage, kohlrabi, Brussels sprouts, and cauliflower, each selected to modify a different part of the plant (broccoli, for example, is simply a tight, enlarged cluster of flowers). And the domestication of all wild crop plants occurred within last 12,000 years.

It's no surprise, then, that Darwin began The Origin not with a discussion of natural selection or evolution in the wild, but with a chapter called "Variation under Domestication"—on animal and plant breeding. He knew that if people could accept artificial selection—and they had to, because its success was so obvious—then making the leap to natural selection was not so hard. As he argued:

Under domestication, it may be truly said that the whole organization becomes in some degree plastic____Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations?

Since domestication of wild species took place only in the relatively short period since humans became civilized, Darwin knew that it wouldn't be much of a stretch to accept that natural selection could create much greater diversity over a much longer time.

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