Chapter 1 - Chance

THE CONSTRACY OF CHANGE

Keiko, a woman in her sixties, was walking in the underground boarding area in Shinjuku train station, one of the largest train stations in Tokyo, on a spring afternoon. Suddenly she felt very dizzy, so much so that she leaned on a nearby wall to keep from falling. When she looked around, she saw that other people were doing the same thing. It was not dizziness, then, but what was it? There were a few seconds of silence, then a woman nearby shouted, “Jishin da!” (Earthquake!). With that a couple of teenage girls standing nearby screamed.

Earthquakes are a common occurrence in Japan, so after a moment during which there were no signs the building was about to collapse, people contin- ued on their way. Keiko boarded her train, but it did not move; instead there was an announcement: “All train traffic in Tokyo has been suspended.”

No trains meant no subways. Keiko’s apartment was miles away; how was she to get home? As she emerged from the station, another earthquake struck and someone shouted, “Mite!” (“Look!”) and pointed toward the top of a sky- scraper. Keiko looked up at Cocoon Tower, a building with a structure that fits its name, and saw it swaying back and forth, like the inverted pendulum of an old-fashioned clock. She wondered how it could sway so far without the upper part breaking off.

The area outside the station was packed with people, so Keiko knew there was little chance of getting a cab. Perhaps a bus? She got into a long line at a bus stop and waited. When a bus came, those at the front of the line boarded, but the bus went nowhere. Finally someone announced that bus transportationwas suspended. The only way for Keiko to get home was to walk, but it was a long way from Shinjuku station and she had no map. She knew the way to her sister’s home and it was closer, but there was no way to know what she might encounter along the way. Another quake might bring down buildings along her route. Her options were limited, however, so she set off on foot. It took her three hours, but she made it safely. Keiko had been lucky.

Many of the people who lived 150 miles to the northeast, nearer the earth- quake’s epicenter, were not so lucky. The Great East Japan Earthquake of March 11, 2011, at magnitude 9.0, was the strongest earthquake ever known to have hit Japan and one of the five most powerful earthquakes worldwide since 1900. The earthquake moved the northern part of Honshu, Japan’s largest island, about eight feet to the east. A tsunami followed the quake, and a wall of water, in some places well over 100 feet high, crashed into the coast, washing away cars and trucks like leaves in a stream, destroying over 300,000 homes, schools, and facto- ries, and killing over 15,000 people. The tsunami also inflicted severe damage on nuclear power plants, resulting in the release of radioactive material that poi- soned dairy products, crops, and farmland within a 50-mile radius of the plants, and adding the threat of disease throughout much of Japan for weeks afterward.

Earthquakes, tsunamis, and other natural disasters are a part of life, but we tend to regard them as an aberration, a brief disruption in a normally con- stant world. When a great volcano erupts, as Mt. St. Helens in Washington State did in 1980, knocking over thousands of trees and covering the ground for miles around with a blanket of volcanic ash; when a gigantic tsunami hits land, as happened in the Indian Ocean in December 2004, killing over 200,000 people and destroying thousands of homes; when a heat wave kills over 50,000 people, as in Europe in the summer of 2003; when a drought rages across a continent, destroying crops and wiping out herds of sheep and cattle, as in Australia from 2006 until 2011, we think how strange it is that nature should misbehave so. It is, we tell ourselves, a momentary lapse, a kind of geological tantrum; soon our old planet will regain its composure, its constancy. But the truth is, as the Roman philosopher Lucretius said 2,000 years ago, “Change is the only constant.” We live in a world that is ever-changing, like the varicol- ored patterns of a steadily rotating kaleidoscope.

Change is not the exception to the rule, it is the rule. Throughout nature, the struggle to survive is an effort to cope with change: The climate changes, prey animals become harder to see, predators become faster, diseases strike without warning, population increases put added stresses on the availability of food, water, habitable space, and other resources. Some changes, such as the movement of continents, take place over eons; others, such as the advance or retreat of glaciers, normally take hundreds or thousands of years; some, such as the changes in climate we are now seeing due to human use of fossil fuels, take decades; still others, such as the rise and fall of the sun or the abrupt lane change of an aggressive driver, occur on a daily basis. The one constant in our lives is change. Any individual or species must be able to cope with change if it is to survive. But how? By what mechanisms can we and other animals deal with such a fickle world? Charles Darwin offers one answer.

NATURAL SELECTION

Charles Darwin was born in England in 1809. The son of a physician, he went to the University of Edinburgh in 1825 to study medicine. His heart was not in it, however, and when the blood and screams of a patient undergoing sur- gery without anesthesia sent him scurrying to the exit, he decided to pursue a degree in theology at Cambridge University. His heart wasn’t in that field, either, and he spent much of his time pursuing his true love, natural history.

Shortly after graduating, Darwin accepted an offer to join an expedition on the British naval vessel HMS Beagle as the ship’s naturalist. The Beagle was under the leadership of Captain Robert FitzRoy, a 23-year-old aristocrat who wanted a naturalist who would also serve as a suitable dinner companion. The chief purpose of the voyage was to map the shorelines of land areas around the world. It was a great success in this endeavor, but it is now remembered only because it gave Darwin the opportunity to gather hundreds of specimens of plants and animals in his effort to understand “that mystery of mysteries,” the origin of species.

Darwin took a copy of Charles Lyell’s Principles of Geology with him on the Beagle. Lyell, considered by many the father of geology, got many things wrong (he rejected the idea of ice ages, for example), but his view that the earth changes gradually over eons gave Darwin a new perspective on where species came from. If the earth had been around for millions of years and changed slowly, as Lyell argued, there was no reason to think that the vast variety of life forms had appeared overnight in their current form.

Once back in England, Darwin focused attention on animal breeding as a source of insights into the variations in living things. He was himself a breeder of doves, and knew that breeders had long changed the characteristics of cows, horses, pigs, sheep, chickens, dogs, cats, and other domestic animals by selec- tively cross-breeding individuals with desirable characteristics. Such breeding seemed to provide a model for changes in species in the wild, but breeding was the result of deliberate, thoughtful intervention by the breeder. Who was nature’s breeder?

The answer occurred to Darwin when he read a book by a fellow Englishman, Thomas Malthus (1798), called An Essay on the Principle of Population. Malthus was a clergyman who did not accept the then-popular idea that human population growth was the path to utopia. On the contrary, Malthus argued that it was the road to ruin. Resources are limited, and eventu- ally an expanding population spells disaster. There are limits to all resources, and as populations increase, these resources prove inadequate to supply all individuals. “The power of population,” he wrote, “is indefinitely greater than the power in the earth to produce subsistence for man.”

Malthus focused on the effects of human population growth, but Darwin realized that all species of animals and plants produce far more offspring than the environment can possibly support, which inevitably leads to competi- tion for resources. Some survive, but most do not. What determines which individuals and species will win out? Clearly the winners must have fea- tures that give them an advantage. Those of their offspring that share their parents’ advantage will also tend to survive and reproduce. Over generations, these advantages, some of them very subtle, may accumulate and result in very different species. Darwin saw that the mechanism for change wasanalogous to the breeder’s practice of selectively mating animals with desirable characteristics. The difference is that in the wild, nature is the breeder:

Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an indi- vidual of any species, . . . will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving. . . . I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection.

In writing “its relation to man’s power of selection,” Darwin is drawing attention to the analogy of natural selection to the artificial selection of breed- ers. “There is no obvious reason,” he wrote, “why the principles which have acted so efficiently under domestication should not have acted under nature.”

In Darwin’s day, little was known about how characteristics were transmitted from one generation to the next. As Darwin noted, “The laws governing inheritance are quite unknown. . . .” The Austrian friar and founder of genetics, Gregor Mendel, did not publish his work on inheritance in peas until 1866, and it did not become widely known among scientists until the early 1900s. Nevertheless, it was clear that characteristics were somehow passed from one generation to another, and these characteristics were benefi- cial, detrimental, or of no significance in a particular environment.

It is important to note that natural selection depends on variations among the members of a species. If all members of a species were genetically identical, natural selection would be impossible. As Darwin wrote, “unless profitable variations do occur, natural selection can do nothing.”

Darwin’s critics have often said that even with variation, natural selec- tion cannot possibly account for the sudden appearance of complex organs such as the human eye. You may be surprised to learn that Darwin agreed. But Darwin went on to say that complex organs do not normally appear suddenly. Far from it.

Evidence suggests that the human eye, for example, had its origin mil- lions of years ago with the appearance of a few light-sensitive cells on the skin of some primitive animals. These light-sensitive cells would have proved helpful since anything that cast a shadow on them could, for example, warn of an approaching predator. Through additional variations and natural selec- tion, more light cells appeared, and the light-detecting organ became more and more complicated until it gradually reached the sophisticated light-sensitive organ found in higher animals, including humans (Lamb, 2011; Schwab, 2011).

This may seem to be pure speculation, since we have no videotape of the evolution of the eye, but it is based on evidence suggesting increasing sophis- tication of organs, including the eye, as we go through the animal kingdom from the simplest to the more complex species. Consider, for example, the trematode cercaria, a parasitic flatworm that invades fish brains (Lafferty & Morris, 1996). Cercaria has two spots that, though they may look a bit like eyes, are nothing at all like any mammal’s eye (see Figure 1-1). There is iris, no retina, no vitreous humor, nothing, really, except light-sensitive cells. (Light-sensitive cells form the retina of your more sophisticated eyes.) The cercaria’s eyespots help it to find its way to a higher water level where it is more likely to encounter the killifish on which it depends. Go up the evolu- tionary scale and you see increasingly complex visual equipment. The eye of a reindeer, for example, not only sees more or less what you see, it also responds to ultraviolet light, which helps it to identify both predators and food in snow- covered areas (Hogg et al., 2011).

The human brain is another complex organ that did not appear overnight. The jellyfish has a simple network of neurons, but this primitive nervous system, which serves to coordinate its swimming movements, has no brain at all. The first true brains appear in worms. The brain of the earthworm is about the size of a mustard seed, but it is an advance over the jellyfish. Generally, the further up the evolutionary scale you go, the more complicated the brain and the greater its capacity for dealing with environmental changes.

The devices resulting from natural selection are rarely, if ever, as sim- ple and efficient as they could be, but they generally work (Marcus, 2009; Olshansky, 2009). What natural selection demands is not perfection, but devices that are good enough to aid survival. Natural selection is a crude, inefficient mechanism, so it typically takes countless generations to develop sophisticated tools. You might compare it to carving the sculptures of Mt. Rushmore with a hammer and chisel while blindfolded. Nevertheless, it helps species meet the challenges of a changing environment.

All kinds of changes in the environment can affect the characteristics of a species. Climate change is perhaps the most important of these environmental changes, a point made by Darwin (1859). The west coast of Scotland is famous for its cold winters and its warm wool. Until recently the Soay sheep that produced that wool tended to be large, since smaller sheep often succumbed to the cold before reaching reproductive age. Over the past 25 years, howeverthe sheep of this area have been getting smaller and lighter (Ozgul et al., 2009; Maloney, Fuller, & Mitchell, 2010). This reduction in size parallels a change in climate due to global warming: Winters in Scotland have gotten shorter and milder in recent decades, so smaller sheep now are more likely to survive and reproduce.

Changes in terrain (due to a change in the course of a river or volcanic eruption, for example) can also induce changes in species through natural selection. Evolutionary biologist Erica Rosenblum and her colleagues at the University of Idaho (Rosenblum et al., 2010), studied three species of liz- ards in White Sands, New Mexico. All three species have dark skins in other geographic areas, but those that live in White Sands, an area with dunes com- posed of white gypsum, have developed lighter skins (Figure 1-2). The lizards evolved in areas where their dark skin made them difficult for predators to see, but when they moved into the white dunes, those dark skins made them vulnerable to predators. Those with lighter skins were more likely to survive and reproduce, and this selection process continued over generations until the lizards in the dunes were far lighter than their ancestors.

Pollution provides another example of how changes in the environment affect species characteristics. The classic example is the peppered moth, one of many large moths found in the British Isles. The peppered moth feeds at night and rests during the day on the trunks and limbs of trees. Its survival depends in large part on its ability to escape detection by the birds that find it an appetizing food. At one time, nearly all of these moths were a mottled light gray color, closely resembling the lichen-covered trees on which they rested. A rare black variation of the moth, first observed in 1848, stood out against this background like coal against snow, making it highly vulnerable. But when pollutants from burning coal killed the lichen and darkened the bark of trees, the light-colored moths increasingly fell prey to birds, whereas the dark moths tended to survive and reproduce. In forests near industrial centers, where pollution was common, the black moths increased in number, and the light- colored variety declined. In some areas, 90% of the moths were of the once-rare black variety (Kettlewell, 1959; see Figure 1-3). In recent years, improvement in local air quality has reversed this trend, so that the lighter variety of moth is once again dominant.

Predators are an important part of most animals’ surroundings, and changes in predators play an important role in natural selection. Swanne Gordon of the University of California at Riverside and his colleagues (2009) did an experiment with guppies that demonstrates this. The researchers moved wild guppies in Trinidad from a stream that had no predators to a stream with guppy-eating fish. Eight years later Gordon transferred more guppies from the safer stream and compared the survival rate of their young with the young of guppies that had lived among the predators for many generations. The evolved guppies had a survival rate more than 50% higher than that of the newcomers.

Changes in the environment also affect human characteristics. Human skin color is largely due to the amount of melanin, a substance found in the skin that screens out the sun’s harmful rays. The more melanin, the darker the skin, and the more sunlight is screened out. The people of Scandinavia and northern Europe, where there is relatively little sunlight, are typically fair- skinned, a trait that allows them to absorb the sunlight they need to produce vitamin D. People who live near the equator, where there is an abundance of sunlight, are characteristically dark-skinned, a trait that protects them against the hazards of too much sun. Through natural selection, the human species acquires the coloration that survival in a given environment requires.

Many changes due to natural selection are not as obvious as those consid- ered thus far. New diseases can result in widespread deaths, as happened with the bubonic plague that killed a third or more of the people living in Europe in the 14th century. The disease is caused by a bacterium, but the cause of the epidemic was garbage: People routinely threw garbage into the streets, rats fed on the garbage, and the fleas the rats carried infected people with the plague bacteria. Although the mortality rate of bubonic disease was high, some peo- ple were genetically resistant or immune to the disease. These people passed these advantages on to their young. This change no doubt proved helpful in subsequent epidemics of the same disease, which were generally less devastat- ing. Not only that, but today this same genetic variation protects some people from the HIV virus that causes AIDS.

Today there is great concern among health professionals about global epi- demics. The bacteria that causes bubonic plague is alive and well, as are any number of other microorganisms that pose a threat of pandemics. Increases in international travel and trade bring vectors (carriers of disease-causing organisms) into ports around the world (Smith et al., 2012). The failure of governments to monitor legal and illegal imports adequately makes a pan- demic likely, and when it occurs those individuals who have genetic features that convey resistance or immunity will survive and pass on their advan- tage to their offspring. The descendants of survivors may look no different from those who died, but they will be slightly different, thanks to natural selection.

These examples illustrate how changes in the environment induce changes in species through natural selection. They also suggest that if there are no significant changes in a species’ surroundings, there are likely to be no significant changes in its characteristics. We can see this, for example, in the American alligator, a creature that has scarcely changed in 200 million years. The alligator is ideally suited to its habitat, the ponds and wetlands of the American Southeast, particularly the states of Louisiana and Florida, where 2 million of them survive today. The alligator’s territory has shrunk drastically over the millennia, but otherwise changed little. Once a species is well adapted to its environment, the rule seems to be: no change in the environment, no change in the species.

When we think of the changes produced by natural selection, we usually think of physical characteristics, such as size, shape, and color. In fact, the same pressures that select a physical feature, such as wings, can also select a behavior, such as wing flapping. For example, partridge tend to spend the bulk of their time on the ground, flying into trees only when in danger. In many instances they actually run up trees while flapping their wings. Biologist Kenneth Dial (reported in Wong, 2002) found that wing flapping while running up trees assists the partridge in getting out of reach of predators. Even par- tridge chicks with immature wings have an easier time reaching the branches as a result of flapping their stubby wings. Chicks that flap their wings when attempting to climb trees are more likely to escape predators than those that don’t, so this behavioral tendency is as likely to be selected as are the wings.

Three kinds of behavior are largely the products of natural selection. What are these kinds of behavior? You’re about to find out.

REFLEXES

A reflex is a relationship between a specific event and a simple response to that event. A reflex is not, as is often thought, a particular kind of behavior. Rather, it is a relationship between certain kinds of events, usually events in the immediate surroundings, and relatively simple forms of behavior (see Figure 1-4). An example is the tendency to blink when a speck of dirt hits your eye. The reflex is not the eyeblink itself, but the relation between the speck of dirt and the movement of your eyelid.

Reflexes are either present at birth or appear at predictable stages in devel- opment. Most reflexes are found in virtually all members of a species and are part of the adaptive equipment of the animal. All animals, from protozoa to college professors, have reflexes.

Many reflexes serve to protect the individual from injury. The amoeba is an irregularly shaped, one-celled animal that travels by extending a part of its perimeter forward and then pulling the rest along after. When the amoeba encounters a noxious substance, it immediately withdraws from it; this reflex minimizes the harmful effects of the substance. Larger animals do much the same thing when they withdraw a limb from a painful object. The professor who picks up a very hot skillet will immediately release it and withdraw the injured hand. Other protective reflexes in humans include the the pupillary reflex, in which the iris contracts or relaxes in response to changes in light; the sneeze, by which irritants such as dust and pollen are expelled from the nose and lungs; and the vomit reflex, which removes toxic substances from the stomach in an efficient, if indelicate, manner. Other reflexes are important in food consumption. When an amoeba en- counters some edible object, such as a dead bacterium, it immediately re- sponds to the object by engulfing it and making a meal of it. Humans have a number of such consummatory reflexes: Touch a baby’s face, and she will turn toward what touched her; this rooting reflex is useful in finding the moth- er’s nipple. When the nipple touches the baby’s lips, this evokes the sucking reflex, which brings milk into the baby’s mouth. Food in the mouth elicits the salivary reflex, the flow of saliva that begins the process of digestion. The presence of saliva and food in the mouth triggers swallowing. Swallowing trig- gers peristalsis, the rhythmic motion of the lining of the esophagus that car- ries food to the stomach. Thus, the simple act of eating is, in large measure, a chain of reflexes.

We tend not to notice useful reflexes until they fail to function properly. Reflex failure is seen in people who have consumed excessive amounts of alcohol or other drugs that depress the central nervous system. Death from al- coholic intoxication can occur, for example, when the alcohol interferes with the respiratory reflex (inhaling and exhaling) or when the intoxicated person vomits inadequately and chokes to death on the vomit—a fact that ought to make people think twice about binge drinking. Fortunately, most of us are blissfully unaware that we even have reflexes until one of them malfunctions.

Not all reflexes are useful. Some people have life-threatening allergic reac- tions to certain foods, such as peanuts. In some people, epileptic seizures can be triggered by flickering lights, a loud noise, a certain kind of music, or a par- ticular odor. Reflexes are subject to natural selection. Those that contribute to survival will appear more frequently in succeeding generations; those that hinder survival will literally tend to “die out.”

Reflexes are highly stereotypic; that is, they are remarkably consistent in form, frequency, strength, and time of appearance during development. This is not to say, however, that they do not vary at all. The rooting reflex mentioned above may first appear in one infant at the age of seven days but may not show up in a second infant for another week. There is also some variation in the form a reflex takes. A tap below the knee may produce a barely detectable knee jerk in one person, whereas in another person of the same age and simi- lar health, the same light blow may result in a kick that looks like an attempt to make a field goal. Reflexes also change across the life span. Motor reflexes tend to slow with age, which is probably one reason people are more likely to fall in their seventies than in their thirties.

MODEL ACTION PATTERNS

Another kind of naturally selected behavior is the modal action pattern (MAP), a series of related acts found in all or nearly all members of a species (Tinbergen, 1951). They used to be called instincts, but this term fell out of favor, partly because it came to refer to any more or less automatic act (as in, “Martha instinctively slammed on the brakes”). Other terms seen in the literature include fixed action patterns and species-specific behavior.

Modal action patterns resemble reflexes in that they have a strong genetic basis; display relatively little variability from individual to individual or from day to day in the same individual; and often are reliably elicited by a particular kind of event, called a releaser. MAPs differ from reflexes in that they involve the entire organism rather than a few muscles or glands; are more complex, often consisting of long series of reflex-like acts; and are more variable than reflexes, though still rather stereotypic.

Because of their complexity and their utility, many modal action patterns appear to be thoughtful acts. In fact, they are probably no more thoughtful than is the behavior of a person who responds to a rap on the knee by jerking her leg. An illustration of the unthinking nature of modal action patterns is provided by the tropical army ant. Entire colonies of these ants charge across the forests in what appears to be a highly organized, intelligently directed campaign. However, the ants are merely following a chemical trail laid down by the ants ahead of them. T. C. Schneirla (1944) demonstrated that on a flat surface, such as a road, where no obstacles direct the course of the march, the lead ants tend to move toward the ants beside them. The column then turns in on itself, and the ants soon march round and round in a circle. This is not very intelligent behavior. Various people (e.g., Carr, 1967; Dawkins, 1995; Skinner, 1966, 1975, 1984) have suggested that some modal action patterns are selected by grad- ual changes in the environment. Consider, for example, salmon that migrate upstream to breed. This act often requires the fish to ascend steep cliffs and swim against rushing currents. At one time, returning to the breeding area might have constituted a relatively easy swim up a gently rising stream. As geological changes gradually increased the steepness of the slope, those fish with the ability to make the trip bred successfully and reproduced their kind, whereas those not up to the challenge failed to reproduce. It seems likely that other modal action patterns (such as migration and mating rituals) are molded by the environment in much the same way.

MAPs evolve through natural selection because they contribute to the survival of the species. They do this chiefly by helping the individual find food, deal with threats to their safety, or pass their genes on to the next gen- eration. The pine bark beetle burrows into pine trees to find a meal. Some spiders spin webs with which they capture prey, while others lie in wait and pounce on an unsuspecting meal as it passes by. The buck moth caterpillar climbs deciduous trees, particularly oaks, to feed on the leaves. Pigs root for worms, larvae, and truffles beneath the ground. Woodpeckers peck on trees to get at the insects that feed there, while the yellow-billed cuckoo and other birds feed on the caterpillars that feed on leaves.

Some apparently pointless aggressive behavior is actually part of food gathering. The cowbird lays its eggs in the nest of another, smaller species, such as a wren. The cowbird chick, being bigger than its nest mates, gets most of the daily menu and eventually pushes the smaller chicks from the nest. The mother wren ends up providing meals to a single chick that is twice her own size.

Many MAPs serve to protect the individual from environmental threats, such as predators. The rattlesnake shakes its rattle when approached by an animal, such as a Boy Scout, that may harm it. When confronted by a threatening dog, the house cat arches its back, hisses, growls, and flicks its tail. These acts make the cat appear larger and more formidable than it really is and may therefore serve to put off an attacker. The opossum responds quite differently to predators: It plays dead. Some of the opossum’s predators eat only animals they themselves have killed; others will cover a dead animal they find and return to eat it later, so a “dead” opossum has a chance of surviving. The beaver avoids predators by building a dam and then a lodge with an un- derwater entrance. It then packs mud on top of the lodge, which adds to the difficulty of getting at the beaver by tearing at the roof of the lodge.

Another kind of threat is posed by seasonal changes. Bears eat voraciously from spring to fall, thereby adding layers of fat. Then they seek shelter in a cave or in a cavity under a rock or fallen trees. There they spend most of the winter in a state of hibernation: They do not eat, drink, or eliminate wastes. A recent study of black bears in Alaska found that their metabolic rate fell by 75%, thus drastically reducing their consumption of calories (TØien et al., 2011). This enables them to live off of the calories they stored as fat during the warmer months. Many birds and some mammals deal with cold by migrating to a warmer area in the fall. The Canada goose flies from its nesting grounds in the north to the much more temperate climate in the mid-Atlantic region.

Some of the most interesting modal action patterns have to do with re- production. The male peacock attracts a female by spreading its flamboyantly decorated tail and causing it to tremble. (The tail, that is, not the female.) The male bighorn sheep wins a partner by bashing its head against that of its rival. The genital area of certain female primates becomes swollen and red when she is capable of being impregnated, and she displays these features to a potential partner, nonverbally saying, “I’m available!”

Modal action patterns also govern the care and rearing of the young. After mating, the female of a certain species of wasp builds a nest, places a para- lyzed spider in it, lays an egg on top of the spider, closes the nest, and goes on its way, leaving the young wasp to fend for itself after it eats its first meal. The newborn of many higher species of animals require more nurturing, for which task their parents are genetically equipped. Most birds feed their young at least until they leave the nest. When the adult robin arrives at the nest, the chicks chirp loudly and then open their beaks wide; the parent responds to the gaping mouths by regurgitating a mix of worms and insects it has eaten. As the chicks get older the parent shoves undigested bits of prey between the chicks’ gaping jaws.

Are there modal action patterns in human beings? It is hard to say. Darwin (1874) wrote of the instincts of self-preservation, lust, and vengeance, among others, in humans. Several decades ago, textbooks listed dozens of human in- stincts, including the sex instinct, the social instinct, the maternal instinct, and the territorial instinct (see, e.g., McDougall, 1908). But the list of human instincts has grown shorter over the years. Today, many researchers maintain that there are no true modal action patterns in people, that the “instincts” previously attributed to them lack the monotonous character of web spin- ning in spiders and nest building in birds. For instance, for thousands of years, people around the planet obtained food mainly by hunting and gather- ing, with men typically doing the hunting and women doing the gathering. But the hunting and gathering took different forms in different regions and at different times, and the gender roles were not absolute. And today people are more likely to obtain their food by sitting at a desk than by shooting deer or digging up roots. Similarly, among humans the method of finding a sexual partner varies tremendously from culture to culture, from individual to individual, and even within the same individual from time to time. Humans have invented mar- riage, dating services, singles bars, personals columns, Internet chat rooms, massage parlors, and all sorts of rules and customs for defining how, when, where, and with whom sexual acts may be performed. The complexity and variability of mating rituals among humans is a far cry from the stereotypic mating behavior of most other animals.

The same sort of case can be made against the so-called maternal instinct. True, many women do desire to have children and to protect and nurture them, as do many men. But there is tremendous variation in how mothers perform these tasks. In some societies, young children are fondled and held constantly, and their slightest need is met immediately; in others, children are left largely to their own resources. Moreover, women in Western societies in- creasingly delay or forgo altogether the traditional maternal role. True modal action patterns are not so readily discarded.

There are, then, few if any modal action patterns in humans. The role of genetics in behavior is, however, seen in both nonhumans and humans in the form of general behavior traits.

GENERAL BEHAVIOR TRAITS

Over the past few decades, a great deal of research has focused on the role of genes in determining what I will refer to here as general behavior traits. By this I mean the tendency to engage in a certain kind of behavior. Examples include the tendency to be shy (or outgoing), aggressive (or mild-mannered), adventurous (or cautious), anxious (or relaxed), and obsessive-compulsive (or impulsive).

Some behavior traits were once classified as modal action patterns, but they differ from the latter in important ways. As noted previously, modal action patterns are elicited by fairly specific kinds of environmental events, called releasers. The gaping mouth of a fledgling induces the parent bird to provide food; a closed beak does not have this effect. Behavior traits, on the other hand, occur in a wide variety of situations. For instance, under certain circumstances, unpleasant experiences will reliably produce aggressive behav- ior in many animals, including people (Berkowitz, 1983; Ulrich & Azrin, 1962). But unpleasant experience covers a lot of territory. It can include, among other things, an electric shock, a pinprick, a spray of cold water, a threatening stare, an insult, an air temperature above 90 degrees, and so on. All can increase the likelihood of aggressive behavior. Modal action patterns are not released by so many different kinds of events.

Another difference between modal action patterns and behavior traits con- cerns the plasticity of the behavior. Compare the modal action pattern of the web-spinning spider with the aggressiveness of a shocked rat. Each web-spinning spider spins a web with a specific pattern, and it goes about the task with a re- markable sameness, like someone living a recurrent dream. Moreover, the web spinning of one spider is remarkably like that of other members of the same species (Savory, 1974). But the rat that attacks its neighbor goes about it in a far less stereotypic manner, and there may be considerable difference between the attack of one rat and that of another of the same species.

Behavior traits are more variable than modal action patterns, but there is little doubt that heredity plays an important role in them. Selective breeding has, for example, produced strains of animals differing in fearfulness (Hall, 1951; Marks, 1986; see Figure 1-5), excitability (Viggiano et al., 2002), aggres- siveness (Dierick & Greenspan, 2006), activity level (Garland et al., 2011), drug abuse (Matson & Grahame, 2011), and risk-taking (Jonas et al., 2010), among others.

Today, researchers can use genetic engineering to demonstrate the role of genes in behavior traits. Gleb Shumyatsky, a geneticist at Rutgers University, and his colleagues (Shumyatsky et al., 2005) removed a particular gene from mice and then bred a line of mice without the gene. Although these mice looked and generally behaved normally, the missing gene had a profound ef- fect on one kind of behavior. When mice are placed on an unfamiliar white surface, they normally display caution. The engineered mice, however, were much bolder: They spent twice as much time exploring the area as the ordinary mice did. A similar study identified a genetic component in social dominance (Garfield et al., 2011).

Genes also play a significant role in the behavior traits of people. People have long recognized this with the expression blood will tell. Selective breeding and genetic engineering are not available as research tools where humans are concerned, but twin studies and studies of specific genes serve researchers well. Researchers have shown that, as in other animals, genes play an important role in fearfulness (Hettema et al., 2003), excitability (Pellicciari et al., 2009), aggressiveness (Rhee & Waldman, 2002), activity level (Perusse et al., 1989), drug abuse (Li & Burmeister, 2009; Nielsen et al., 2008), and risk-taking (Kuhnen & Chiao, 2009). If this list of traits sounds familiar, it’s because they are characteristics mentioned earlier that have been bred in animals. All sorts of other human characteristics have proven to be influenced by genes (e.g., Knafo et al., 2008; Kreek et al., 2005).

It is easy to see how some behavior traits can contribute to survival, thereby being naturally selected. The rabbit that flees a fox may escape and go on to create more rabbits, whereas the rabbit that stands its ground becomes fox food. But if rabbits must compete with other rabbits for food, the more aggressive ones are likely to fare better. So the desirable level of aggressive tendencies in rabbits will depend on the kind of threat they face. When there are lots of foxes about, the running rabbits will prevail; when there are few foxes but lots of rabbits competing for food, the more aggressive rabbits will dominate.

The same thing is undoubtedly true of people. For example, some peo- ple tend to be bold and adventurous, while others are cautious and reserved. Hunting gazelle with a spear requires walking into the grassy area where the gazelle feed, but these are often the same areas where lions prowl. Those peo- ple who lead the way are most likely to secure food for themselves and their group, but they are probably also the most likely to fall victim to lions. The more cautious people risk going hungry, but hungry people are more likely to reproduce than dead people. Even today we need people who “boldly go where no one has gone before,” but we also need people who stay behind and balance their checkbooks.

Thanks to genetic variation and natural selection, then, adaptive forms of behavior (reflexes, modal action patterns, and general behavior traits) spread throughout a species and help it survive. As the environment changes, new adaptive forms of behavior appear and old ones that are no longer adaptive fade. Natural selection is a marvelous mechanism for producing both physical and behavioral characteristics suited to a changing environment, but it has its limits.

LIMITS OF NATURAL SELECTION

The chief problem with natural selection as a way of coping with change is that it is slow. It is slow because it occurs over generations.

Consider Gordon’s study of guppies put into a stream where they were exposed to new predators. The guppies adapted, but it took between 13 and 26 generations. Guppies are short-lived, so those generations occurred over only eight years. Other species take longer to reach sexual maturity and have longer gestational periods, which makes adaptation through natural selection even slower. Humans, for example, reach sexual maturity in their teens and gestation is nine months, so it takes a minimum of about 13 years to produce a new generation. For us to get the kind of adaptive changes seen in Gordon’s guppies would likely take more than 200 years, and that is assuming that women would routinely have children in their teens.

Natural selection is therefore of limited value in coping with abrupt changes. Hawaii, “the extinction capital of the world,” provides an apt ex- ample. The Hawaiian islands were home to a great variety of wildlife when Captain Cook stumbled onto them in 1778. Unfortunately, Cook and other early visitors brought with them not only trinkets but rats. Other invasive species followed, including cats, dogs, and snakes. Some of the indigenous ani- mals on the islands had little or no defenses against these new predators. In the 200 years since Cook’s arrival, close to 200 species of animals that once lived on the islands have become extinct.

Another example is provided by the passenger pigeon. These North American birds, which resembled their cousin, the mourning dove, once num- bered in the millions. Sometimes their flocks were so large that they blocked the sun and turned day into twilight. But natural selection was no help to the passenger pigeon against the shotgun and unregulated hunting, and the last passenger pigeon died in 1914. A rapid change that threatens humans is infectious diseases. As men- tioned earlier, bubonic plague killed millions of Europeans in the 14th cen- tury, and the flu pandemic of 1918 killed 27 million worldwide. Pandemics of similar magnitude are not only possible, but probable (Garrett, 1995). The Ebola virus, for instance, has no effective treatment, almost inevitably ends in a horrible death, and is highly contagious. The disease originated in and is currently limited to Africa, but this is the age of the jet plane and tourism, imports and exports, not to mention international terrorism, so it is quite pos- sible that the Ebola virus will threaten people on every continent during this century. Natural selection is unlikely to work fast enough to prevent massive casualties from such a highly infectious and deadly disease.

A related problem with natural selection is that adaptations that served a species for thousands or even millions of years can become useless almost overnight. Lee Cronk (1992) provides several examples of this phenomenon in a delightful article called Old Dogs, Old Tricks. “Behavioral and physical ad- aptations that seem to make no sense in an organism’s current environment,” Cronk writes, “can be traced to the legacy of an earlier, different environment in which those traits were favored” (p. 13). He cites the example of the rabbit that dodges back and forth when pursued by foxes, bobcats, and coyotes. This practice is still helpful in eluding these predators, but it is less effective when the rabbit finds itself on a highway “pursued” by a truck. Similarly, Cronk notes, armadillos befuddled approaching predators for thousands of years by springing into the air. Once again, however, this behavior is not adaptive on modern highways. As Cronk puts it, “Leap two feet high in front of a Buick, and you’re buzzard bait” (p. 13) Human beings also have become hostages to their genetic history. B. F. Skinner (1984) notes that humans evolved in a world in which salt and sugar were not readily available. Those individuals who had a natural preference for these foods were more likely to get the sodium and the calories needed for survival. We have, as a consequence, evolved into a species with strong prefer- ences for both salty and sweet foods. But our world has changed. In industrial societies, salt and sugar are abundant, and many of us consume far too much of them, endangering our health in the process. Now, heart disease, stroke, and diabetes are major killers and are considered diseases of civilization. They could be called diseases of natural selection.

Sometimes abrupt changes in genes, called mutations, appear, and al- though most of them do not help in the struggle for survival, occasionally they prove useful. When the mutation provides a significant advantage, it may “sweep” through the population and could conceivably ensure the survival of the species. However, having desirable mutations sweep through the popula- tion is definitely not something we can count on (Hernandez et al., 2011).

Hybridization, the cross-breeding of closely related species, can also aid species adaptation. Mating between different species of animals sometimes oc- curs in the wild when the species share the same territory. A grizzly bear is known to have mated with a polar bear, for example, producing offspring with characteristics from each parent (Carroll, 2010). Your ancestors (and hence, you) may have profited from such hybridization: From 1% to 4% of the genes of people of European and Asian descent are those of Homo neanderthalensis, a species distinct from Homo sapiens (Carroll, 2010; Finlayson, 2010, Wong, 2000). Matings across species should increase the variabilty of genes in the next generation and thereby lead to useful adaptations, but hybrid animals are often sterile, so whether hybridization could speed up the adaptive process is uncertain.

There is also evidence that some genes “jump” from one area of a chro- mosome to another, changing the influence the genes would otherwise have (Gage & Muotri, 2012). Like mutations and hybridization, this might increase the variability of features, including behavioral characteristics, but even if useful features appear, it takes multiple generations for natural selection to take advantage of them.

It seems clear that natural selection is not up to the challenge of rapid change. What is needed is the evolution of a characteristic that allows organisms to change, not over many generations, but within the lifetime of the individual. Fortunately, such a mechanism has evolved. I like to think of it as evolved modifiability, but most people call it learning.

LEARNING: EVOLVED MODIFIABILITY

Although learning has been defined in countless ways, among learning research- ers it is often defined as a change in behavior due to experience. As you will soon see, this means a change in behavior due to a change in the environment. The definition is simple enough, but it deserves careful examination.

LEARNING MEANS CHANGE

Consider the word change. Why should learning be said to be a change in be- havior? Why not say, for example, that learning is the acquisition of behavior? The word change is preferred over acquisition because learning does not always involve acquiring something, but it does always involve some sort of change. Joan would like to quit smoking; Bill wants to stop biting his nails; and Mary and Harry would like to quarrel less often. All of these reductions in behavior, if they occur, are examples of learning, but nothing has been acquired—at least, not in the ordinary sense of that word. Learning means a change in some aspect of behavior such as its frequency, intensity, speed, or

form (see Chapter 2).
Some authorities (e.g., Kimble, 1961) insist that only durable changes

qualify as learning, but since there is no consensus about what durable means (a few milliseconds? a second? a minute? a week? a year?), adding durability to the definition does not seem to help. Besides, why should durability be a re- quirement? If you see your doctor because you had a severe pain in your chest that lasted six seconds, is your doctor going to say, “Six seconds? Oh, then it didn’t happen”? She might say it’s unimportant, but she isn’t going to say it didn’t happen. If an astronomer sees a star explode and disappear in three sec- onds, are other astronomers going to say, “Forget it. If it didn’t last for at least a minute it didn’t happen.” The key issue in learning is whether a change in behavior occurred, not how long it lasted. The fact that you no longer remem- ber all of those Spanish verbs you learned in high school doesn’t mean that you didn’t learn them.

WHAT CHANGE IS BEHAVIOR

What changes when learning occurs is behavior. Behavior may be defined as anything a person or other animal does that can be measured (Moore, 2011; Reber, 1995; Skinner, 1938). Actually, anything an animal does might qualify as behavior, whether measurable or not, but for scientific analysis we are nec- essarily limited to those things we can measure (Baum, 2011).

The concept of behavior seems simple enough, but it can get fuzzy when you examine it closely (e.g., see Angier, 2009). Is a heartbeat an example of behavior? What about the firing of a single neuron? How about the secretion of adrenaline into the bloodstream? Probably most people think of these as physiology, not behavior. But these are all things that people (and many other animals) do that can be measured, so they qualify as behavior. You have no doubt heard that dogs can learn to salivate at the sound of a bell. Salivation is the secretion of saliva by a gland, a topic for physiologists, but the dog pro- duces the saliva and the amount of saliva can be measured, so it qualifies as behavior.

People think, so does thinking qualify as behavior? Most people would probably say No, arguing that behavior involves physical movement, whereas thoughts are mental. The word mental is usually defined as “of the mind,” and the implication is that thoughts exist in a different dimension from the physical world (Descartes, 1637, 1641). That takes thoughts out of the reach of science. However, thinking is something people (and presumably some ani- mals) do, so if it can be measured, it qualifies as behavior.

The chief difference between thinking and other forms of behavior is that one is private and the other is public. There is considerable evidence that much of what we call thinking is simply covert behavior. That is, what we do “in our head” is often merely a more subtle form of public behavior. For exam- ple, we can speak to others, or we can “think out loud” (mumble), or we can engage in “inner speech” (Huang et al., 2001; Schlinger, 2009; Watson, 1920). Schizophrenics often hear voices, for example, but research suggests that the voices they hear are actually their own—they are talking to themselves si- lently or softly (Lindsley, 1963; McGuigan, 1966; Slade, 1974; Stephane, Barton, & Boutros, 2001). Similarly, deaf people who use sign language in communicating with others seem to think with their fingers—signing subtly as they wrestle with a problem (Max, 1937; Watson, 1920).

Neurological evidence supports the idea that our covert speech is es- sentially a diminutive form of speech. In one experiment researchers found that a kind of magnetic stimulation of an area of the brain involved in lan- guage interfered with both overt and covert speech (Aziz-Zadeh, Cattaneo, & Rizzolatti, 2005). If thinking were fundamentally different from overt speech, it seems unlikely that stimulation that interferes with one would interfere with the other. Similarly, when the neuroanatomist Jill Bolte Taylor (2008) had a severe stroke, she lost the ability to speak both to others and to herself. We unwittingly acknowledge the shared nature of overt and covert speech when someone asks us what we are mumbling and we reply, “Oh, I was just thinking out loud.”

What about unconscious thoughts—are they behavior? No, but they are not thoughts, either. The phrase unconscious thoughts is an oxymoron. True, your brain is routinely engaged in activities of which you are unaware, and some of these activities may affect your behavior (your covert and overt speech, for example). But the same thing can be said of your salivary glands, stomach, liver, intestines, and bone marrow; are the unconscious activities of these organs thoughts?

Yet thoughts are not identical to overt behavior. In particular, they often have different effects. If I’m in a stuffy room I can open a window and let in fresh air; thinking about opening a window will not have that effect. (Neither will talking about it.) Similarly, if you call someone an idiot, this behavior is likely to have consequences that will not follow if you merely think the person is an idiot. But these differences in effect do not justify assigning thoughts to a fundamentally different and mysterious realm. The effects of publicly performed behavior also can have different consequences. Shouting names at someone, for example, will have different effects than whispering the same words in their ear, and those whispered thoughts will have different effects than inaudible speech—that is, thoughts.

Like thoughts, feelings are often assumed to be excluded from behavior, but they need not be. Emotions are, after all, part of our response to events around us and sometimes in us—toothaches do not feel good, for example. Like thoughts, feelings pose special problems since much of what we call feelings are not normally publicly observable. However, feelings tend to “spill out” from the body in the form of readily observed behavior. When you feel happy you often have a smile on your face and a spring in your step; when you are sad, you are apt to “wear a frown” and may shed tears; when you have a toothache you may moan and hold your jaw. We can also “look in on” a person’s feelings by recording physiological activity that is reliably cor- related with expressed feelings. A person who reports feeling angry is likely to have an increase in heart rate and blood pressure; a person who is afraid is likely to have an increase in electrical conductivity of his skin; a person who is sexually aroused is likely . . . well, you get the idea.

Some people might argue that learning should be defined as a change in the nervous system that makes a change in behavior possible. In this view, behavior change is merely an indicator of learning. Great progress has been made in recent years in our understanding of the way learning experiences change the brain (e.g., Cohen et al., 2012; Holy, 2012; Kandel, 1970, 2007). No learning researcher denies that learning involves a change in the nervous system, but there are at least two problems with equating learning with neu- rological changes.

One problem is that we are just beginning to understand the biological mechanisms involved in learning. No one can point to changes in a rat’s brain and say, “This animal can run a maze better today than it did yesterday.” Nor can anyone point to features of a person’s brain and say, “This person can play the piano.” At present, the only reliable measure of learning is a change in behavior.

A second problem with defining learning as neurological change is that it denies the importance of behavior. It is, of course, very important to under- stand how experience changes the nervous system. But even if we were able to say, solely on the basis of physiological measures, “This rat can run a maze better today than it did yesterday” or “This person can play the piano,” the changes in behavior would be paramount. When we go to a concert, we go to hear the pianist play, not to watch her neurons fire.

Thus, insofar as learning is concerned, behavior is literally “where it’s at.” The changes in behavior are not the result of learning; they are learning. They are the result of experience.

WHAT CHANGES BEHAVIOR IS EXPERIENCE

Our definition says that learning is due to experience. Experience means changes in the environment, so our definition of learning is, in effect, a change in behavior due to changes in the environment. These changes in the environ- ment are events that affect, or are capable of affecting, behavior. Such events are called stimuli.

Stimuli are physical changes in an organism’s environment. They are the changes in air pressure we call sound, the light waves we call sights, the tac- tile pressures we call touch. The delicate fragrance of a rose derives from just so many molecules of “rose matter” arising from the flower. Even the gentle caress and softly whispered words of a lover are, in scientific terms, merely physical events. Stimuli often have significance beyond their physical proper- ties (the fragrance of a rose may remind us of a friend), but it is their physical properties that define them.

Often in learning research the stimuli are very simple events such as a lightbulb coming on or going off or a buzzer sounding, but this does not mean that the experiences that change behavior are typically simple. A given stimu- lus is always part of a complex array of stimuli. A light that is illuminated in a small room may have different effects, for example, than the same light in a large room. Researchers typically define experience in terms of simple events in order to simplify the problem being studied.

In most studies of learning, the stimuli investigated occur outside the per- son or animal. However, physical events occur inside the body as well, and they, too, can affect behavior. We tend to define these internal events in terms of sensations—the pain of a toothache, the nausea of an upset stomach—but these sensations, like those from stimuli outside the body, have a physical basis. The toothache may be due to an inflamed root, the nausea caused by spoiled meat in the stomach.

This point raises an interesting philosophical question: Is it the physical event that affects behavior, or the sensation that is produced by the physi- cal event? Some unfortunate people are incapable of experiencing pain, so if they pick up a hot skillet they do not immediately release it and their hand gets burned. Similarly, a deaf person does not respond to the sound of a telephone or the warning sound of a car horn. This issue, like most philosophical issues, is complicated and difficult to resolve. For practical reasons, however, scientists generally focus on readily observable physical features.

We saw earlier that changes in environment are largely responsible for the changes we see in natural selection. The rule of thumb is, no change in the environment, no change in the species. A parallel rule of thumb applies to learning: no change in the environment, no change in behavior.

Not all changes in behavior, even those resulting from changes in the en- vironment, qualify as learning, however. A physician may give an emotionally distraught man a tranquilizer, but we do not then say that the patient learned to be calm. A woman who is usually very agreeable may, following a head injury, become very argumentative. If this change in behavior is due to brain damage, we do not say that she learned to be quarrelsome. Changes in behav- ior that are due to drugs, injury, aging, or disease do not qualify as learning.

There is a great deal more that might be said about the meaning of learn- ing, but perhaps you now have a better understanding of why it is defined here as a change in behavior due to experience. To illustrate this definition, let’s take a brief look at what is arguably the simplest example of learning, habituation.

HABITUATION: AN EXAMPLE OF LEARNING

Habituation is a reduction in the intensity or probability of a reflex response as a result of repeatedly evoking the response. Many versions of habituation have been demonstrated by researchers. Seth Sharpless and Herbert Jasper (1956), for example, noted the effects of loud noises on cats by recording their brain waves on an electroencephalograph (EEG). The EEG showed marked arousal at first, but the reaction declined steadily with each repetition of a given sound until the noise had hardly any effect. Wagner Bridger (1961) studied habitua- tion in infants and found that when babies first heard a noise, they responded with an increase in heart rate. With repetition of the noise at regular intervals, however, the change in heart rate became less and less pronounced until, in some cases, the noise had no measurable effect. Habituation has even been demonstrated in the human fetus. A stimulus to the mother’s abdomen dur- ing the last three months of pregnancy will produce movement in the fetus. If the stimulus is applied repeatedly in a regular way, the fetal response becomes steadily weaker (Leader, 1995). When the course of habituation is plotted on a graph, it usually reveals a fairly smooth, decelerating curve (see Figure 1-6).

Although habituation is a relatively simple phenomenon, it is not as sim- ple as this discussion implies. Variations in the stimulus used to elicit the response affect the rate of habituation. For example, a sudden loud noise will typically elicit the startle reflex—the “jump” you experience when, for ex- ample, the wind causes a door to slam shut. But the rate at which habituation occurs depends on the loudness of the sound, variations in the quality of the sound, the number of times the sound occurs, the time interval between re- peated exposures to the sound, and other variables. (For more on this, see Thompson, 2000, 2009.) You can probably see how habituation would be helpful in survival. The world is a dangerous place, full of predators, venomous reptiles, stinging insects and plants, lightning, forest fires, hurricanes, not to mention drunk drivers and jealous boyfriends. The world also presents us with opportunities: An unsus- pecting prey animal may come our way, for example, providing an easy meal. We and other animals need to be alert to events around us that might signal trouble or opportunity. However, events that occur repeatedly and do not sig- nal trouble or opportunity can distract us from more important things, such as eating or sleeping. Habituation allows us to go on with whatever we are doing without interruption. If you live next to a train line, for example, at first the noise from passing trains prevents you from falling asleep, but with repeated exposure to the sounds you sleep like a rock, thanks to habituation.

Habituation isn’t what most people think of when they think of learning, but it illustrates the essential nature of learning: a change in behavior due to experience. It also illustrates the survival value of learning. We have seen that changes in the environment produce both changes in the species (through nat- ural selection) and changes in the individual (through learning). Which of those influences, nature and experience, is more important? That is our next topic.

NATURE VS NURTURE

One of the longest-running arguments in the study of behavior—indeed, in all of science—concerns the relative importance of nature and nurture (basically, heredity and learning), in determining behavior. Do we, as individuals, behave a certain way because we were “born that way,” or do we behave that way because our environment “taught” us to behave that way? The debate is evi- denced in aphorisms people use every day, often without thinking about their larger significance. Is it true, for instance, that “blood will tell,” or is it more accurate to say that “as the twig is bent, so the tree is inclined”? Are leaders born, or are they made? Can a person “turn over a new leaf,” or is the leopard stuck with its spots?

Of course, no one denies that learning is important, at least in higher animals, and no one completely ignores the role of heredity. The most influ- ential behavior scientist of the 20th century, B. F. Skinner, is often called an “extreme environmentalist,” and has even been accused of denying that biol- ogy plays any role in behavior. Yet his earliest research interest was in biology, and throughout his career he wrote repeatedly of the role of biology in behav- ior (Morris, Lazo, & Smith, 2004; Skinner, 1969, 1975). In one passage, for example, Skinner (1953) writes that “behavior requires a behaving organism which is the product of a genetic process. Gross differences in the behavior of different species show that the genetic constitution . . . is important” (p. 26). Similarly, biologists such as E. O. Wilson (1978) who emphasize hereditary factors in behavior are often labeled “biological determinists,” yet they ac- knowledge that experience plays an important role as well. Nevertheless, for centuries people have lined up on one side or the other of the nature–nurture debate, arguing that one or the other, heredity or learning, is the true determi- nant of behavior.

The trouble with the nature–nurture debate is that it creates an artifi- cial division between the contributions of heredity and learning. The debate wrongly implies that the answer must be one or the other (Kuo, 1967; Midgley, 1987). In fact, nature and nurture are inextricably interwoven in a kind of Gordian knot; the two strands cannot be separated. As William Verplanck (1955) put it long ago, “learned behavior is innate, and vice versa” (see also Moore, 2001; Ridley, 2003; Schneider, 2003; see Figure 1-7).

Let’s take, as an example, the question of aggression. Douglas Mock (2006), a zoologist at the University of Oklahoma, compared the aggression of great blue heron and great egret chicks toward their siblings. The egrets were much more likely to kill their siblings than were the herons. At first glance, it appears that egrets are innately more aggressive, but Mock performed an experiment to see if that is the case. He had egrets raise herons and herons raise egrets. If the difference in siblicide were solely determined by genetics, this switch should have made no difference. What Mock found, however, was that while egrets showed the same amount of aggression to their siblings, the herons showed more. The difference was due to parental behavior: The herons bring their young large fish they can share; the egrets bring small fish the birds can swallow—if they get them before their nest mate does. The difference in environment influences the aggressiveness of the chicks.

Cats provided another example. It seems obvious that cats have a natural attraction to rats—as food, not as playmates. Zing Yang Kuo (1930) of China reared kittens under different conditions. Some grew up with their mothers and had the opportunity to see them kill rats. Others grew up away from their mothers and never saw rats killed. When the kittens matured, Kuo put them together with rats. He found that 86% of the cats that had been reared with their mothers killed rats, but only 45% of the others did. Thus, even some- thing as basic as killing “natural” prey is strongly influenced by experience.

Aggression in humans also seems to be the product of a tangled mix of na- ture and nurture. Wilson (1978) reports that among the !Kung San, an aboriginal people of Africa, violence against their fellows was almost unknown. But Wilson points out that several decades earlier, when the population density among these people was greater and when there was less governmental con- trol over their behavior, their per capita murder rate rivaled that of America’s most dangerous cities. Wilson (1978) adds that the Semai of Malaya also dem- onstrated the capacity for both gentleness and violence. Murder is unknown among these people; they do not even have a word in their language for the concept. Yet when the British colonial government trained Semai men to fight against communist guerrillas in the 1950s, the Semai became fierce warriors. One anthropologist wrote that “they seem to have been swept up in a sort of insanity which they call ‘blood drunkenness’” (Dentan, quoted in Wilson, 1978, p. 100). Wilson concludes from such evidence that “the more violent forms of human aggression are not the manifestations of inborn drives . . . [but are] based on the interaction of genetic potential and learning” (Wilson, 1978, p. 105). Even the ability to learn is itself the product of both heredity and experi- ence. Numerous studies suggest that genes play a role in determining learning ability, but many other studies have shown that learning experiences are also important. For instance, Betty Hart and Todd Risley (1995; for a summary see Chance, 1997) did a longitudinal study of the verbal environment provided by parents with different educational backgrounds. They found huge differences in the amount and kinds of verbal exchanges between parents and their young children. Those children whose parents talked to them a lot (providing lengthy explanations and lots of positive feedback, for example) later scored higher on intelligence tests than those children whose parents were more reticent. The differences remained when race and socioeconomic level were held constant.

It is clear, then, that while we are biological organisms, we are also en- vironmental organisms. Someone has said that asking, “Which is more important in determining behavior, heredity or environment?” is like asking, “Which is more important in determining the area of a rectangle, width or length?” All behavior reflects a blending of nature and nurture so complex and intricate that it is impossible to say where one begins and the other ends. Heredity and learning are merely different aspects of the same process, the ef- fort to cope with life’s one constant—change.