The Engine Of Evolution

What but the wolfs tooth whittled so fine The fleet limbs of the antelope? What but fear winged the birds, and hunger Jewelled with such eyes the great goshawk's head?

—Robinson Jeffers, "The Bloody Sire"

One of the marvels of evolution is the Asian giant hornet, a predatory wasp especially common in Japan. It's hard to imagine a more frightening insect. The world's largest hornet, it is as long as your thumb, with a two-inch body bedecked with menacing orange and black stripes. It's armed with fearsome jaws to clasp and kill its insect prey, and also a quarter-inch stinger that proves lethal to several dozen Asians a year. And with a 3-inch wingspan, it can fly 25 miles per hour (far faster than you can run), and can cover 60 miles in a single day.

This hornet is not only ferocious, but voracious. Its young larval grubs are fat, insatiable eating machines, who insistently rap their heads against the hive to signal their hunger for meat. To satisfy their relentless demands for food, adult hornets raid the nests of social bees and wasps.

One of the hornet's prime victims is the introduced European honeybee. The raid on a honeybee nest involves a merciless mass slaughter that has few parallels in nature. It starts when a lone hornet scout finds a nest. With its abdomen, the scout marks the nest for doom, placing a drop of pheromone near the entrance of the bee colony. Alerted by this mark, the scout's nestmates descend on the spot, a group of twenty or thirty hornets arrayed against a colony of up to thirty thousand honeybees.

But it's no contest. Wading into the hive with jaws slashing, the hornets decapitate the bees one by one. With each hornet making bee heads roll at a rate of forty per minute, the battle is over in a few hours: every bee is dead, and body parts litter the hive. Then the hornets stock their larder. Over the next week, they systematically ravage the nest, eating honey and carrying the helpless bee grubs back to their own nests, where they are promptly deposited into the gaping mouths of the hornets' own ravenous offspring.

This is nature red in tooth and claw, as the poet Tennyson described.23 The hornets are fearsome hunting machines, and the introduced bees are defenseless. But there are bees that can fight off the giant hornet: honeybees that are native to Japan. And their defense is stunning— another marvel of adaptive behavior. When the hornet scout first arrives at their hive, the honeybees near the entrance rush into the hive, calling nestmates to arms while luring the hornet inside. In the meantime, hundreds of worker bees assemble inside the entrance. Once the hornet is inside, it is mobbed and covered by a tight ball of bees. Vibrating their abdomens, the bees quickly raise the temperature inside the ball to about 47 degrees C. Bees can survive this temperature, but the hornet cannot. In twenty minutes the hornet scout is cooked to death, and—usually—the nest is saved. I can't think of another case (save the Spanish Inquisition) in which animals kill their enemies by roasting them.24

There are several evolutionary lessons in this twisted tale. The most obvious is that the hornet is marvelously adapted to kill—it looks as though it were designed for mass slaughter. Moreover, many traits work together to make the wasp a killing machine. They include body form (large size, stings, deadly jaws, big wings), chemicals (marking pheromones and deadly venom in the sting), and behavior (rapid flight, coordinated attacks on bee nests, and the larval "I am hungry" behavior that prompts the hornet attacks). And then there is the defense of the native honeybees—the coordinated swarming and subsequent roasting of their enemy—certainly an evolved response to repeated attacks by hornets. (Remember, this behavior is genetically encoded in a brain smaller than a pencil point.)

On the other hand, the recently introduced European honeybees are virtually defenseless against the hornet. This is exactly what we would expect, for those bees evolved in an area lacking giant predatory hornets, and therefore natural selection did not build a defense. We can predict, though, that if the hornets are sufficiently strong predators, the European bees will either die out (unless they are reintroduced), or will find their own evolutionary response to the hornets—and not necessarily the same one as the native bees.

Some adaptations entail even more sinister tactics. One of them involves a roundworm that parasitizes a species of Central American ant. When infected, an ant undergoes a radical change in both behavior and appearance. First, its normally black abdomen turns a bright red. The ant then becomes sluggish and raises its abdomen straight up in the air, like a taunting red flag. The thin junction between the abdomen and the thorax becomes flimsy and weakened. And an infected ant no longer produces alarm pheromones when attacked, so it can't alert its nestmates.

All of these changes are caused by the genes of the parasitic worm as an ingenious ploy to reproduce themselves. The worm alters the appearance and behavior of the ant, which advertises itself to birds as a scrumptious berry, and in so doing brings on its own death. The berrylike red abdomen of the ant is raised up for all birds to see, and easily plucked because of the ant's sluggishness and the weakened junction between the abdomen and the rest of the body. And birds gobble up these abdomens, which are full of worm eggs. The birds then pass the eggs in their droppings, which ants scavenge and take back to their nests to feed the larvae. The worm eggs hatch within the ant larva and grow. When the ant larva becomes a pupa, the worms migrate to the ant's abdomen and mate, producing more eggs. And so the cycle begins again.

It is staggering adaptations like this—the many ways that parasites control their carriers, just to pass on the parasites' genes—that gets an evolutionist's juices flowing.25 Natural selection, acting on a simple worm, has caused it to commandeer its host and change the host's appearance, behavior, and structure, turning it into a tempting mock fruit.26

The list of adaptations like this is endless. There are adaptations in which animals look like plants, camouflaging themselves among the vegetation to hide from enemies. Some katydids, for example, look almost exactly like leaves, complete with leaf-like patterns and even "rotten spots" resembling the holes in leaves. The mimicry is so precise that you'd have trouble spotting the insects in a small cage full of vegetation, much less in the wild.

And we have the converse: plants that look like animals. Some species of orchids have flowers that superficially resemble bees and wasps, complete with fake eyespots and petals shaped like wings. The resemblance is good enough to fool many shortsighted male insects, who alight on the flower and try to mate with it. While this is happening, the pollen sacs of the orchid attach to the insect's head. When the frustrated insect departs without consummating his passion, he unwittingly carries the pollen to the next orchid, fertilizing it during the next fruitless "pseudocopulation." Natural selection has molded the orchid into a bogus insect because genes that attract pollinators in this way are more likely to be passed on to the next generation. Some orchids further seduce their pollinators by producing chemicals that smell like the sex pheromones of bees.

Finding food, like finding a mate, can involve complex adaptations. The pileated woodpecker, a crested bird that is the largest woodpecker in North America, makes its living by hammering holes into trees and plucking insects like ants and beetles from the wood. Besides its superb ability to detect prey beneath the bark (probably by hearing or feeling their movements—we're not sure), the woodpecker has a whole group of traits that help it hunt and hammer. Perhaps the most remarkable is its ridiculously long tongue.27 The base of the tongue attaches to the jawbone, and then the tongue runs up through one nostril, completely over and around the back of the head, and finally reenters the beak from below. Most of the time the tongue is retracted, but it can be extended deep into a tree to probe for ants and beetles. It is pointed and covered with sticky saliva to help extract those tasty insects from holes. Pileated woodpeckers also use their bills to excavate large nest cavities and to drum on trees, attracting mates and defending their territories.

The woodpecker is a biological jackhammer. This poses a problem: how can a delicate creature drill through hard wood without hurting itself? (Think of the force it takes to drive a nail into a plank.) The punishment that a pileated woodpecker's skull takes is astounding—the bird can strike up to fifteen blows per second when it's "drumming" for communication, each blow generating a force equivalent to banging your head into a wall at 16 miles per hour. This is a speed that can crumple your car. There is a real danger of the woodpecker injuring its brain, or even having its eyes pop out of its skull under the extreme force.

To prevent brain damage, the woodpecker's skull is specially shaped and reinforced with extra bone. The beak rests on a cushion of cartilage, and the muscles around the beak contract an instant before each impact to divert the force of the blow away from the brain and into the reinforced base of the skull. During each strike, the bird's eyelids close to keep its eyes from popping out. There is also a fan of delicate feathers covering the nostrils so that the bird doesn't inhale sawdust or wood chips when hammering. It uses a set of very stiff tail feathers to prop itself against the tree, and has an X-shaped, four-toed foot (two forward, two back) to securely grip the trunk.

Everywhere we look in nature, we see animals that seem beautifully designed to fit their environment, whether that environment be the physical circumstances of life, like temperature and humidity, or the other organisms—competitors, predators, and prey—that every species must deal with. It is no surprise that early naturalists believed that animals were the product of celestial design, created by God to do their jobs.

Darwin dispelled this notion in The Origin. In a single chapter, he completely replaced centuries of certainty about divine design with the notion of a mindless, mechanistic process—natural selection—that could accomplish the same result. It is hard to overestimate the effect that this insight had not only on biology, but on people's worldview. Many have not yet recovered from the shock, and the idea of natural selection still arouses fierce and irrational opposition.

But natural selection posed a number of problems for biology as well. What is the evidence that it operates in nature? Can it really explain adaptations, including complex ones? Darwin relied largely on analogy to make his case: the well-known success of breeders in transforming animals and plants into organisms suitable for food, pets, and decoration. But at the time, he had little direct evidence for selection acting in natural populations. And because, as he proposed, selection was extremely slow, altering populations over thousands or millions of years, it would be hard to observe it acting during a single human lifetime.

Fortunately, thanks to the labors of field and laboratory biologists, we now have this evidence—lots of it. Natural selection, we find, is everywhere, scrutinizing individuals, culling the unfit and promoting the genes of the fitter. It can create intricate adaptations, sometimes in surprisingly little time.

Natural selection is the most misunderstood part of Darwinism. To see how it works, let's look at a simple adaptation: coat color in wild mice. Normal-colored or "oldfield" mice (Peromyscus polionotus), have brown coats and burrow in dark soils. But on the pale sand dunes of Florida's Gulf Coast lives a light-colored race of the same species called "beach mice": these are nearly all white with only a faint brown stripe down the back. This pale color is an adaptation to camouflage the mice from predators, like hawks, owls, and herons, that hunt among the white dunes. How do we know this is an adaptation? A simple (albeit slightly gruesome) experiment by Donald Kaufman at Kansas State University showed that mice survive better when their fur matches the color of the soil in which they live. Kaufman built large outdoor enclosures, some with light soil and others with dark soil. In each cage he put equal numbers of mice with dark and light coat colors. He then released a very hungry owl into each cage, returning later to see which mice survived. As expected, mice whose coats contrasted most conspicuously with the soil were picked off more readily, showing that camouflaged mice really do survive better. This experiment also explains a general correlation that we see in nature: darker soils harbor darker mice.

Since white color is unique among beach mice, they presumably evolved from brown mainland mice, possibly as recently as 6,000 years ago when the barrier islands and their white dunes were first isolated from the mainland. This is where selection comes in. Oldfield mice vary in coat color, and among those that invaded the light beach sand, individuals with a lighter coat would have a higher chance of surviving than darker mice, which are easily spotted by predators. We also know that there is a genetic difference between light and dark mice: beach mice carry the "light" forms of several pigmentation genes that together give them their light-colored coats. Darker oldfield mice have the "dark" alternative form of the same genes. Over time, due to the differential predation, lighter mice would have left more copies of their light genes (they have a higher chance of surviving to reproduce) and, as this process continued for generation after generation, the population of beach mice would have evolved from dark to light.

What happened here? Natural selection, acting on coat color, has simply changed the genetic composition of a population, increasing the proportion of genetic variants (the light-color genes) that enhance survival and reproduction. And while I said that natural selection acts, this is not really accurate. Selection is not a mechanism imposed on a population from outside. Rather, it is a process, a description of how genes that produce better adaptations become more frequent over time. When biologists say that selection is acting "on" a trait, they're merely using shorthand saying that the trait is undergoing the process. In the same sense, species don't try to adapt to their environment. There is no will involved, no conscious striving. Adaptation to the environment is inevitable if a species has the right kind of genetic variation.

Three things are involved in creating an adaptation by natural selection. First, the starting population has to be variable: mice within a population have to show some difference in their coat colors. Otherwise this trait cannot evolve. In the case of mice, we know this is true because mice within mainland populations show some variability in coat color.

Second, some proportion of that variation has to come from changes in the forms of genes, that is, the variation has to have some genetic basis (called heritability). If there were no genetic difference between light and dark mice, the light ones would still survive better on the dunes, but the coat-color difference would not be passed on to the next generation, and there would be no evolutionary change. We know that the genetic requirement is also satisfied in these mice. In fact, we know exactly which two genes have the largest effect on the dark/light color difference. One of them is called Agouti, the same gene whose mutations produce black color in domestic cats. The other is called Mcir, and one of its mutant forms in humans, especially common in Irish populations, produces freckles and red hair.28

Where does this genetic variation come from? Mutations—accidental changes in the sequence of DNA that usually occur as errors when the molecule is copied during cell division. Genetic variation generated by mutation is widespread: mutant forms of genes, for example, explain variation in human eye color, blood type, and much of our—and other species'—variation in height, weight, biochemistry, and innumerable other traits.

On the basis of many laboratory experiments, scientists have concluded that mutations occur randomly. The term "random" here has a specific meaning that is often misunderstood, even by biologists. What this means is that mutations occur regardless of whether they would be useful to the individual. Mutations are simply errors in DNA replication. Most of them are harmful or neutral, but a few can turn out to be useful. The useful ones are the raw material for evolution. But there is no known biological way to jack up the probability that a mutation will meet the current adaptive needs of the organism. Although it's better for mice living on sand dunes to have lighter coats, their chance of getting such a useful mutation is no higher than for mice living on dark soil. Rather than calling mutations "random," then, it seems more accurate to call them "indifferent": the chance of a mutation arising is indifferent to whether it would be helpful or hurtful to the individual.

The third and last aspect of natural selection is that the genetic variation must affect an individual's probability of leaving offspring. In the case of mice, Kaufman's predation experiments showed that the most camouflaged mice would leave more copies of their genes. The white color of beach mice, then, meets all the criteria for having evolved as an adaptive trait.

Evolution by selection, then, is a combination of randomness and lawfulness. There is first a "random" (or "indifferent") process—the occurrence of mutations that generate an array of genetic variants, both good and bad (in the mouse example, a variety of new coat colors); and then a "lawful" process—natural selection—that orders this variation, keeping the good and winnowing the bad (on the dunes, light-color genes increase at the expense of dark-color ones).

This brings up what is surely the most widespread misunderstanding about Darwinism: the idea that, in evolution, "everything happens by chance" (also stated as "everything happens by accident"). This common claim is flatly wrong. No evolutionist—and certainly not Darwin—ever argued that natural selection is based on chance. Quite the opposite. Could a completely random process alone make the hammering woodpecker, the tricky bee orchid, or the camouflaged katydids and beach mice? Of course not. If suddenly evolution was forced to depend on random mutations alone, species would quickly degenerate and go extinct. Chance alone cannot explain the marvelous fit between individuals and their environment.

And it doesn't. True, the raw materials for evolution—the variations between individuals—are indeed produced by chance mutations. These mutations occur willy-nilly, regardless of whether they are good or bad for the individual. But it is the filtering of that variation by natural selection that produces adaptations, and natural selection is manifestly not random. It is a powerful molding force, accumulating genes that have a greater chance of being passed on than others, and in so doing making individuals ever better able to cope with their environment. It is, then, the unique combination of mutation and selection—chance and lawfulness—that tells us how organisms become adapted. Richard Dawkins provided the most concise definition of natural selection: it is "the non-random survival of random variants."

The theory of natural selection has a big job—the biggest in biology. Its task is to explain how every adaptation evolved, step by step, from traits that preceded it. This includes not just body form and color, but the molecular features that underlie everything. Selection must explain the evolution of complex physiological traits: the clotting of blood, the metabolic systems that transform food into energy, the marvelous immune system that can recognize and destroy thousands of foreign proteins. And what about the details of genetics itself? Why do pairs of chromosomes separate when eggs and sperm are formed? Why do we have sex at all, instead of budding off clones, as some species do? Selection has to explain behaviors, both cooperative and antagonistic. Why do lions hunt cooperatively in a pack, and yet when intruding males displace resident males from a social group, why do the intruders kill all the unweaned cubs?

And selection has to mold these features in a particular way. First, it has to create them—most often gradually—step by step from precursors. As we have seen, each newly evolved trait begins as a modification of an earlier feature. The legs of tetrapods, for example, are simply modified fins. And each step of the process, each elaboration of an adaptation, must confer a reproductive benefit on individuals possessing it. If this doesn't happen, selection won't work. What were the advantages of each step in the transition from a swimming fin to a walking leg? Or from an unfeathered dinosaur to one having both feathers and wings? There is no "going downhill" in the evolution of an adaptation, for selection by its very nature cannot create a step that doesn't benefit its possessor. In the world of adaptation, we never see the sign that's the bane of freeway drivers: "a temporary inconvenience—a permanent improvement."

If an "adaptive" trait evolved by natural selection instead of having been created, we can make some predictions. First, in principle we should be able to imagine a plausible step-by-step scenario for the evolution of that trait, with each step raising the fitness (that is, the average number of offspring) of its possessor. For some traits this is easy, like the gradual alteration of the skeleton that turned land animals into whales. For others it is harder, especially for the biochemical pathways that leave no trace in the fossil record. We may never have enough information to reconstruct the evolution of many traits, or even, in extinct species, to understand precisely how those traits functioned (what were the bony plates on the back of the Stegosaurus really for?). It is telling, however, that biologists haven't found a single adaptation whose evolution absolutely requires an intermediate step that reduces the fitness of individuals.

Here's another requirement: an adaptation must evolve by increasing the reproductive output of its possessor. For it is reproduction, not survival, that determines which genes make it to the next generation and cause evolution. Of course, passing on a gene requires that you first survive to the age at which you can have offspring. On the other hand, a gene that knocks you off after reproductive age incurs no evolutionary disadvantage. It will remain in the gene pool. It follows that a gene will actually be favored if it helps you reproduce in your youth but kills you in your old age. The accumulation of such genes by natural selection, in fact, is widely thought to explain why we deteriorate in so many ways ("senesce") as we reach old age. The very genes that help you sow your wild oats when young may give you wrinkles and an enlarged prostate gland later in life.

Given how natural selection works, it shouldn't produce adaptations that help an individual survive without also promoting reproduction. One example would be a gene that helps human females survive after menopause. Neither do we expect to see adaptations in one species that benefit only members of another species.

We can test this last prediction by looking at traits of one species that are useful to members of a second species. If those features arose by selection, we predict that they'll also useful for the first species. Take tropical acacia trees, which have swollen, hollow thorns that act as homes for colonies of fierce, stinging ants. The trees also secrete nectar and produce protein-rich bodies on their leaves that provide the ants with food. It looks as if the tree is housing and feeding the ants at its own expense. Does this violate our prediction? Not at all. In fact, harboring ants gives a tree huge benefits. First, herbivorous insects and mammals that stop by for a leafy treat are repelled by a furious ant horde—as I discovered to my chagrin when brushing up against an acacia in Costa Rica. The ants also cut down seedlings around the base of the tree— seedlings, which, when larger, could compete with the tree for nutrients and light. It is easy to see how acacias that were able to enlist ants to defend them from both predators and competitors would produce more seeds than acacias lacking this ability. In every case, when one species does something to help another, it always helps itself. This is a direct prediction of evolution, and one that does not follow from the notion of special creation or intelligent design.

And adaptations always increase the fitness of the individual, not necessarily of the group or the species. The idea that natural selection acts "for the good of the species," though common, is misguided. In fact, evolution can produce features that, while helping an individual, harm the species as a whole. When a group of male lions displaces the resident males of a pride, this is often followed by a gruesome slaughter of the unweaned cubs. This behavior is bad for the species since it reduces the total number of lions, increasing their likelihood of extinction. But it's good for the invading lions, as they can quickly fertilize the females (who come back into estrus when they're not nursing) and replace the slaughtered cubs with their own offspring. It is easy—though unsettling—to see how a gene causing infanticide would spread at the expense of "nicer" genes, which would have the invading males simply babysit the unrelated cubs. As evolution predicts, we never see adaptations that benefit the species at the expense of the individual—something that we might have expected if organisms were designed by a beneficent creator.

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