Why We Get Sick (4 page)

Another apparent exception to the nice-guys-finish-last principle in evolution is the result of reciprocal exchanges of favors between individuals who need not be relatives. If Elsa is an expert maker of shoes and Fritz is a skillful hunter of animals that supply excellent
leather, trading resources will benefit them both. It pays me to be nice to you, and vice versa. Ever since Robert Trivers’s classic 1971 paper on reciprocity theory, biologists have routinely interpreted cooperative relations among organisms in nature as resulting from either reciprocal exchanges or kin selection. The biology of social life has grown thanks to the efforts of pioneers such as E. O. Wilson, author of
Sociobiology
, and Richard Alexander, author of
Darwinism and Human Affairs
. Early controversies and misunderstandings have been largely supplanted by growing work in this new field of science.

T
here is a widespread misconception that evolution proceeds according to some plan or direction, but it has neither, and the role of chance ensures that its future course will be unpredictable. Random variations in individual organisms create tiny differences in their Darwinian fitness. Some individuals have more offspring than others, and the characteristics that increased their fitness thereby become more prevalent in future generations. Once upon a time (at least) a mutation occurred in a human population in tropical Africa that changed the hemoglobin molecule in a way that provided resistance to malaria. This enormous advantage caused the new gene to spread, with the unfortunate consequence that sickle-cell anemia came to exist, as will be discussed in later chapters.

Chance can influence the outcome at each stage: first in the creation of a genetic mutation; second in whether the bearer lives long enough to show its effects; third in chance events that influence the individual’s actual reproductive success; fourth in whether a gene, even if favored in one generation, is, by happenstance, eliminated in the next; and finally in the many unpredictable environmental changes that will undoubtedly occur in the history of any group of organisms. As Harvard biologist Stephen Jay Gould has so vividly expressed it, if one could rewind the tape of biological history and start the process over again, the outcome would surely be different. Not only might there not be humans, there might not even be anything like mammals.

We will often emphasize the elegance of traits shaped by natural selection, but the common idea that nature creates perfection needs to be analyzed carefully. The extent to which evolution achieves perfection depends on exactly what you mean. If you mean “Does natural selection always take the best path for the long-term welfare of a species?” the answer is no. That would require adaptation by group selection, and this is, as noted above, unlikely. If you mean “Does natural selection create every adaptation that would be valuable?” the answer again is no. For instance, some kinds of South American monkeys can grasp branches with their tails. This trick would surely also be useful to some African species, but, simply because of bad luck, none have it. Some combination of circumstances started some ancestral South American monkeys using their tails in ways that ultimately led to an ability to grab onto branches, while no such development took place in Africa. Mere usefulness of a trait does not necessarily mean that it will evolve.

There is a sense, however, in which natural selection does regularly come close to perfection, and that is in optimizing some quantitative features. If a trait serves a specific function, selection among minor modifications over many generations tends to make its quantitative aspects closely approach the functional ideal. For instance, a bird’s wings must be long enough to give good lift but short enough to allow the bird to maintain control. Measurements on birds found killed after a major storm showed more than expected numbers of unusually long or unusually short wings. The survivors showed a bias toward intermediate (more nearly optimal) wing lengths.

In human physiology, there are hundreds of similar examples in which traits have been shaped to nearly optimal values: the sizes and shapes of bones, blood pressure, glucose level, pulse rate, age at onset of puberty, stomach acidity—the list could go on and on. The observed values may never be exactly perfect, but they usually come close. When we think that natural selection has erred, it is more likely that we have missed some important consideration. For instance, stomach acid aggravates ulcers, yet people who take antacids can still digest their food. So is there too much acid? Probably not, given the importance of stomach acid in digestion and in killing bacteria, including those that cause tuberculosis. To identify the imperfections of the body, one must first understand its perfections and the compromises on which many of them are based.

Like any engineer, evolution must constantly compromise. An auto designer could increase the thickness of the fuel tank in order to decrease the risk of fire, but at some point increased cost and decreased mileage and acceleration require a compromise. Thus, fuel tanks do rupture in some collisions, and this compromise costs some lives each year. While natural selection cannot achieve perfection in every character simultaneously, its compromises are not random but are accurately shaped to give the greatest net benefit.

An apocryphal story tells of Henry Ford looking at a junkyard filled with Model Ts. “Is there anything that never goes wrong with any of these cars?” he asked. Yes, he was told, the steering column never fails. “Well then,” he said, turning to his chief engineer, “redesign it. If it never breaks, we must be spending too much on it.” Natural selection similarly avoids overdesign. If something works well enough that its deficiencies do not constitute a selective force, there is no way natural selection can improve it. Thus, while every part of the body has some reserve capacity to deal with occasionally encountered extreme circumstances, every part is also vulnerable when its reserve capacity is exceeded. There is nothing in the body that never goes wrong.

Moderate increments of a resource often have enormous value, while higher amounts may have less benefit. If you are making a stew, two onions may be better than one, but ten onions would be much more expensive yet offer little, if any, extra benefit. Such cost-benefit analyses are routine procedures in economics, but they are useful in biology and medicine as well. Consider the use of an antibiotic for pneumonia. A tiny dose will probably have no detectable benefit, a moderate dose will cost more but offer much greater benefits, while a high dose will have still higher costs with no additional benefits and perhaps significant danger.

Just as there are costs as well as benefits involved in every engineering or medical decision, there are costs associated with every beneficial genetic change preserved in evolution. Natural selection isn’t weak or capricious; it just selects for genes that give an overall fitness advantage, even if those same genes increase vulnerability to some disease. Is there any way, for instance, for anxiety to be a functionally desirable trait? Consider what would happen to those rabbits we discussed if they had no anxiety in a year when foxes were especially abundant. Even some genes that cause aging are not necessarily
maladaptive. They may give benefits during the early years of life, when selection is the strongest, benefits that are more important to fitness than the later costs of aging and inevitable death. To understand disease better, we need to understand the hidden benefits of apparent mistakes in design.

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his chapter started with a quotation from Aristotle for a serious reason. We can think of him as the originator of the general procedure for functional analysis that has been particularly fruitful in many kinds of biological research and that we expect to be similarly rewarding in medicine. There is, of course, a big difference between Aristotle’s outlook and that of modern biologists. He had almost no grasp of the physical and chemical principles that underlie the workings of any organism. He didn’t think experiments were necessary. He had no notion of the principle of natural selection and certainly did not realize that organisms were designed entirely to maximize their success in reproduction. Whether applied to the human hand or brain or immune system, Aristotle’s powerful question, “What is it for?” now has a very specific scientific meaning: “How has this trait contributed to reproductive success?” His conviction that the body as a whole exists for the sake of some complex action is correct. Only in the past few decades has it become clear that that complex action is reproduction.

Many people have the notion that questions about the function of a trait are not scientific, that they are “teleological” or “speculative” and therefore not appropriate objects of scientific inquiry. This idea is incorrect, as many examples in this book will demonstrate. Questions about the adaptive function of a biological trait are just as amenable to scientific inquiry as are questions about anatomy and physiology. It makes sense to ask about the adaptive significance of biological traits such as eyes, ears, and the cough reflex because they are products of historical processes that have gradually modified them in ways that improve their capacity to serve special functions.

Yet when we ask these “why” questions, we must guard against too readily believing fanciful stories. Why do we have prominent noses? It must be to hold up eyeglasses. Why do babies cry for no
apparent reason? It must be to exercise their lungs. Why do we nearly all die by age 100? It must be to make room for new individuals. Almost anything can be the subject of such speculation, but if this is as far as it goes it is not science. The problem is not in the questions but in a lack of adequate investigation and critical thinking about suggested answers.

The above absurd examples demonstrate how easily some expianations can be tested and proven false. Noses could not have evolved to hold up glasses, since we had noses long before we had eyeglasses. Crying cannot be to develop the lungs, since lung health in adulthood does not require crying in infancy. Aging cannot have evolved to make room for new individuals, because natural selection cannot favor such benefits to the group and the details of aging simply do not conform to the expectations for such a function.

Other functional hypotheses are so easily supported that they are of little interest. Anyone thoroughly familiar with the heart’s structure and operation can see that it pumps blood. One can also see that coughing expels foreign material from the respiratory tract and that shivering increases body heat. You don’t need to be an evolutionary scientist to figure out that teeth allow us to chew food. The interesting hypotheses are those that are plausible and important but not so obviously right or wrong. Such functional hypotheses can lead to new discoveries, including many of medical importance.

S
tudies of the functional reasons for human attributes are based on a method of investigation recently named the
adaptationist program
. By suggesting the functional significance of some known aspect of human biology, you may logically be able to predict some other, unknown aspects. An appropriate investigation can then confirm that these characteristics are either there or not. If they are there, they may be of medical significance. If they’re not, we can eliminate our hypothesis and go back to the drawing board.

We will give three examples here of interesting discoveries made by considering questions on how various features might contribute to fitness. They relate to beavers and birds but not to medical questions, for
which we will give many examples in the chapters to come. To various degrees these examples show that intuitive ideas about fitness, even the intuitions of professional biologists, may not always be adequate. Serious, often mathematical, theorizing is needed to provide the logical answers that can then be tested by investigating real organisms.

Beavers harvest trees in or near their ponds for their food and shelter. They use their teeth to chop through the trunks near the ground, drag the trees to the water if they are not already in it, and tow them to their lodges. How do beavers decide which trees to chop down? They do so
adaptively
, was the hypothesis considered by Michigan biologist Gary Belovsky. This implies an economically rational decision based on a tree’s likely value to a beaver, the difficulty expected in chopping it down and moving it, and how far it is from home. Belovsky’s calculations showed that an efficient beaver ought to be increasingly discriminating as the distance from the pond increases. Small trees may be rejected for not being worth the time to transport them, large ones for not being worth the labor of felling and transporting them, especially dragging them or pieces of them through the woods to where they can be floated in the pond. Belovsky predicted that the range of sizes of trees harvested by beavers would steadily decrease as the distance from the pond increased. At some point, only trees of an ideal size would be harvested; beyond that, none at all. Observation of stumps of beaver-felled trees near their ponds confirmed the prediction. The next time you see a beaver pond, admire not only the beaver’s legendary industry but also its cleverness at setting priorities.

Now imagine a woodland songbird about to lay a clutch of eggs that she and her mate will incubate. Her reproductive success for this breeding season will depend entirely on those eggs. How many should she lay? Remember, she is not trying to assure the survival of the species, she is trying to maximize her own lifetime reproductive success. Laying too few eggs would obviously be foolish, but laying too many can also decrease her total lifetime reproduction if there is not enough food and some of the chicks die, or if she exhausts her energy reserves in caring for her brood and thus jeopardizes her chances of living until the next breeding season. These considerations apply equally to every individual in the woodland, but different birds reach different decisions on how many eggs to lay. If the average for a species is four eggs per pair, some pairs may have five and some only three. Do we conclude that all are trying for four but some can’t
count? Or do we perhaps conclude that egg numbers are not subject to optimization by natural selection?