Why Evolution Is True (22 page)

Because certain variations in DNA or protein sequence may be, as Darwin put it, “neither useful nor injurious” (or “neutral” as we now call them), such variants are especially liable to evolve by drift. For example, some mutations in a gene don’t affect the sequence of the protein that it produces, and so don’t change the fitness of its carrier. The same goes for mutations in nonfunctioning pseudogenes—old wrecks of genes still kicking around in the genome. Any mutations in these genes have no effect on the organism, and therefore can evolve only by genetic drift.

Many aspects of molecular evolution, then, such as certain changes in DNA sequence, may reflect drift rather then selection. It’s also possible that many externally visible features of organisms could evolve via drift, especially if they don’t affect reproduction. The diverse shapes of leaves of different tree species—like the differences between oak and maple leaves—were once suggested to be “neutral” traits that evolved by genetic drift. But it’s hard to prove that a trait has absolutely
no
selective advantage. Even a tiny advantage, so small as to be unmeasurable or unobservable by biologists in real time, can lead to important evolutionary change over eons.

The relative importance of genetic drift versus selection in evolution remains a topic of hot debate among biologists. Every time we see an obvious adaptation, like the camel’s hump, we clearly see evidence for selection. But features whose evolution we don’t understand may reflect only our ignorance rather than genetic drift. Nevertheless, we know that genetic drift
must
occur, because in any population of finite size there are always sampling effects during reproduction. And drift has probably played a substantial role in the evolution of small populations, although we can’t point to more than a few examples.

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 act 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 ten thousand 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 carry 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 coats, the length and pointiness of their ears, the size and shape of their skeletons, the quirks of their behaviors and temperaments, 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 ten thousand 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 two-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 the last twelve thousand 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:

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.

WE CAN GO A STEP FURTHER. Instead of breeders picking out favored variants, we can let this happen “naturally” in the laboratory, by exposing a captive population to new environmental challenges. This is easiest to do in microbes like bacteria, which can divide as often as once every twenty minutes, allowing us to observe evolutionary change over thousands of generations in real time. And this is
genuine
evolutionary change, demonstrating all three requirements of evolution via selection: variation, heritability, and the differential survival and reproduction of variants. Although the environmental challenge is created by humans, these sorts of experiments are more natural than artificial selection because humans don’t choose which individuals get to reproduce.

Let’s start with simple adaptations. Microbes can adapt to virtually anything that scientists throw at them in the lab: high or low temperature, antibiotics, toxins, starvation, new nutrients, and their natural enemies, viruses. Probably the longest-running study of this type has been carried out by Richard Lenski at Michigan State University. In 1988, Lenski put genetically identical strains of the common gut bacterium
E. coli
under conditions in which their food, the sugar glucose, was depleted each day and then renewed the next. This experiment was thus a test of the microbe’s ability to adapt to a feast-and-famine environment. Over the next eighteen years (40,000 bacterial generations), the bacteria continued to accumulate new mutations adapting them to this new environment. Under the varying-food conditions, they now grow 70 percent faster than the original unselected strain. The bacteria continue to evolve, and Lenski and his colleagues have identified at least nine genes whose mutations result in adaptation.

But “laboratory” adaptations can also be more complex, involving the evolution of whole new biochemical systems. Perhaps the ultimate challenge is simply to take away a gene that a microbe needs to survive in a particular environment, and see how it responds. Can it evolve a way around this problem? The answer is usually yes. In a dramatic experiment, Barry Hall and his colleagues at the University of Rochester began a study by deleting a gene from
E. coli.
This gene produces an enzyme that allows the bacteria to break down the sugar lactose into subunits that can be used as food. The geneless bacteria were then put in an environment containing lactose as the only food source. Initially, of course, they lacked the enzyme and couldn’t grow. But after only a short time, the function of the missing gene was taken over by another enzyme that, while previously unable to break down lactose, could now do so weakly because of a new mutation. Eventually, yet another adaptive mutation occurred: one that increased the
amount
of the new enzyme so that even more lactose could be used. Finally, a third mutation at a different gene allowed the bacteria to take up lactose from the environment more easily. All together, this experiment showed the evolution of a complex biochemical pathway that enabled bacteria to grow on a previously unusable food. Beyond demonstrating evolution, this experiment has two important lessons. First, natural selection can promote the evolution of complex, interconnected biochemical systems in which all the parts are codependent, despite the claims of creationists that this is impossible. Second, as we’ve seen repeatedly, selection does not create new traits out of thin air: it produces “new” adaptations by modifying preexisting features.

We can even see the origin of new, ecologically diverse bacterial species, all within a single laboratory flask. Paul Rainey and his colleagues at Oxford University placed a strain of the bacteria
Pseudomonas fluorescens
in a small vessel containing nutrient broth, and simply watched it. (It’s surprising but true that such a vessel actually contains diverse environments. Oxygen concentration, for example, is highest on the top and lowest on the bottom.) Within ten days—no more than a few hundred generations—the ancestral free-floating “smooth” bacterium had evolved into two additional forms occupying different parts of the beaker. One, called “wrinkly spreader,” formed a mat on top of the broth. The other, called “fuzzy spreader,” formed a carpet on the bottom. The smooth ancestral type persisted in the liquid environment in the middle. Each of the two new forms was genetically different from the ancestor, having evolved through mutation and natural selection to reproduce best in their respective environments. Here, then, is not only evolution but speciation occurring in the lab: the ancestral form produced, and coexisted with, two ecologically different descendants, and in bacteria such forms are considered distinct species. Over a very short time, natural selection on
Pseudomonas
yielded a small-scale “adaptive radiation,” the equivalent of how animals or plants form species when they encounter new environments on an oceanic island.