3.4 Natural Selection as a Foundation for Functionalism

The mechanisms underlying behavior are products of natural selection, and, like all products of natural selection, they came about because they promoted survival and reproduction. Just as Tryon, through artificial selection, bred rats to be better at learning a particular maze, natural selection automatically breeds animals to be better at doing what they must to survive and reproduce in their natural environments. This idea provides a foundation for the psychological approach known as functionalism—the attempt to explain behavior in terms of what it accomplishes for the behaving individual.

The functionalist approach to explaining behavior is essentially the same as the functionalist approach to explaining anatomy: Why do giraffes have long necks? Why do humans lack fur? Why do male songbirds sing in the spring? Why do humans have such an irrepressible ability to learn language? The anatomist trying to answer the first two questions, and the behavioral researcher or psychologist trying to answer the latter two, would look for ways by which each trait helped ancestral members of the species to survive and reproduce.

Distal and Proximate Explanations of Behavior

Biologists and psychologists who think in evolutionary terms find it useful to distinguish between two kinds of explanations of behavior—distal and proximate, both of which are needed to properly understand a behavior (Scott-Phillips et al., 2011).

A redwing blackbird at home This male’s singing warns other males of the species to stay away.

Anthony Mercieca/Science Source

• Distal (as in “distant”) explanations are explanations at the evolutionary level. They are statements of the role that the behavior has played in the animal’s survival and reproduction over evolutionary time. Viewed from the vantage point of the gene, they are statements of how the behavior helped the individual’s ancestors’ genes make it into the next generation. Distal causation is sometimes referred to as ultimate causation, in the sense that it is the “ultimate” (in this case evolutionary) cause for a phenomenon.
• Proximate explanations are explanations that deal not with function but with mechanism; they are statements of the immediate conditions, both inside and outside the animal, that bring on the behavior.

Distal and Proximate Explanations Complement Each Other

As an illustration of these two modes of explanation, consider how they might be applied to the question of why male songbirds of many species sing in the spring. A distal explanation goes something like this (Koodsma & Byers, 1991): Over the course of evolution, songbirds have adapted to a mating system that takes place in the spring. The male’s song serves to attract a female for mating and to warn other males to stay away from the singer’s territory in order to avoid a fight. In the evolution of these birds, males whose genes promoted such singing produced more offspring (more copies of their genes) than did males whose genes failed to promote such singing.

A proximate explanation, in contrast, might go as follows (Ball & Hulse, 1998): Through the birds’ visual system, the increased period of daylight in the spring triggers a physiological mechanism that leads to the increased production of the sex hormone testosterone, which in turn acts on certain areas of the brain (which we might call the “song areas”), promoting the drive to sing. Notice the complementary nature of these explanations. The distal explanation states the survival or reproductive value of the behavior, and the proximate explanation states the stimuli and physiological mechanisms through which the behavior occurs.

The Search for Distal Explanations in Human Psychology

All of the complex biological mechanisms underlying human behavior and experience—including the basic mechanisms of perception, learning, memory, thought, motivation, and emotion—are products of evolution by natural selection. They all came about because each small step in their evolution tended to promote the survival and reproduction of our ancestors. Thus, for any basic psychological characteristic that is part of human nature—for any basic drive or emotion, for example—it is legitimate to ask: How did this characteristic improve the survival and reproductive chances of our ancestors? How did it help our ancestors get their genes into the next generation?

The distal explanations of some human traits (especially those that we share with all other mammals) are relatively obvious. We have strong drives to breathe air, drink fluids, and consume foods because our bodies need these things to remain alive. We have strong drives to engage in sex because that is the means by which our genes get from one generation to the next. Individuals who lacked such drives are ancestors to nobody today; their genes died with them.

The distal explanations of some other human traits, however, are not so obvious. It is not obvious, for example, why humans everywhere tend to sleep about 8 hours each night, or why humans everywhere under certain conditions experience the disturbing emotion of guilt. In various places in this book, including in the last sections of this chapter, you will encounter examples of distal explanations that are not intuitively obvious but are supported by research evidence. As you will see, evidence for or against any particular distal explanation can come from detailed analysis of the behavior or trait in question, from cross-species comparisons, and sometimes from studies showing what happens when the behavior or trait is missing.

Limitations on Functionalist Thinking

Before we go deeper into discussions of distal functions, it is useful to know something about the limitations of functionalist thinking. It is not the case that every detail of every trait serves a useful function, and some traits that were once functional may not be so today. Here are four reasons why a particular trait or behavior may not be functional.

Some Traits Are Vestigial

Some traits that evolved because they served the needs of our ancestors are no longer functional today, yet they remain. These remnants of our past are called vestigial characteristics.

As an example, consider the grasp reflex by which newborn infants close their fingers tightly around objects in their hands. This reflex may well be useful today in the development of the infant’s ability to hold and manipulate objects, but that does not explain why prematurely born infants grasp so strongly that they can support their own weight, why they grasp with their toes as well as their hands and why the best stimulus for eliciting this reflex is a clump of hair (Eibl-Eibesfeldt, 1975). These aspects of the reflex make more sense when we observe them in other primates. To survive, infant monkeys and apes cling tightly with hands and feet to their mother’s fur while she swings in trees or goes about her other daily business. In the course of our evolution from ape-like ancestors, we lost our fur, so our infants can no longer cling to us in this way, but the reflex remains.

The concept of vestigial traits becomes especially relevant to psychologists when applied to our inherited drives, or motives. Because of culture, our habitats and lifestyles have changed dramatically in just a few centuries, a speck on the evolutionary time scale. Essentially all of our evolution as a species occurred in conditions that were quite different from those present today, and some of our inherited tendencies may be harmful, rather than helpful, in the habitat that we occupy today. An example is our great appetite for sugar. In the world of our ancestors, sugar was a rare and valuable commodity. It existed primarily in fruits and provided energy needed for vigorous physical activity. But today sugar is readily available in most areas of the world, and life (for many of us) is less physically strenuous. Yet our preference for sugar persists as strong as ever, despite such negative consequences as tooth decay and obesity.

Some Traits Are Side Effects of Natural Selection for Other Traits

Useless changes can come about in evolution as by-products of natural selection for other useful changes. A simple example in humans is the navel, or belly button (Buss et al., 1998). To the best of anyone’s knowledge, the navel serves no function related to survival or reproduction. It is simply a remnant left from the umbilical cord. The umbilical cord, of course, does serve a survival and reproductive function: It conveys nutrients from the mother to the developing fetus. As such, we refer to the navel as an adaptation. It is a universal and reliably developing inherited feature that arose as a result of natural selection and helped to solve some problem of survival. But navels are simply the necessary by-products of umbilical cords and have no function themselves. An anatomist from Mars who observed belly buttons on adult earthlings, but who never observed a fetus or the birth process, would be at a loss to explain why such a structure would have evolved.

It seems quite possible that some human psychological capacities, even some that are so general that we would consider them to be part of human nature, came about as side effects of the evolution of other capacities. It is reasonable to ask, for example, whether the universal human proclivities for art and music are direct effects of natural selection or side effects. Perhaps these proclivities served to attract mates during much of our evolutionary history (as they seem to today), and were therefore selected for directly, much as song was selected for in birds. It is also possible, however, that they emerged simply as by-products of selection for other proclivities, such as those for planning, constructing tools, and communicating through language. A third possibility, combining the first two, is that proclivities for art and music may have initially emerged as by-products and then been selected for because of their usefulness for attracting mates or other helpers. At present, we do not have evidence to support strongly any of these theories over the others.

Some Traits Result Simply from Chance

Some inheritable characteristics that result from just one or two mutations are inconsequential for survival and reproduction. Different races of people have somewhat differently shaped noses. Maybe that variation is caused by natural selection. Perhaps one shape worked best in one climate and another worked best in another climate, so natural selection molded the noses differently. But we can’t automatically assume that. The different shapes might be a result of mutations that didn’t matter and therefore were never weeded out by natural selection. Maybe the small group of people who migrated to a specific part of the world, and who were the ancestors of a particular racial group, just happened to carry along genes for a nose shape that was different from the average for the group they left. Such variation, due to chance alone without selection, is referred to as genetic drift.

Many years ago, researchers discovered that the incidence of schizophrenia (a serious mental disorder, discussed in Chapter 16) is three times greater among people living in northern Sweden, above the Arctic Circle, than among people in most other parts of the world (Huxley et al., 1964). There are at least three possible explanations of this observation: (a) Environmental conditions, such as the harsh climate or the isolation it produces, might tend to bring on schizophrenia in people who are prone to it. (b) Natural selection might have increased the frequency of schizophrenia-promoting genes among these people, perhaps because such genes help protect people from the harmful effects of physical stressors such as cold climate. (This was the hypothesis suggested by Huxley and his colleagues.) (c) The Arctic population may have been founded by a small group of Swedish migrants who, just by chance, had a higher proportion of schizophrenia-promoting genes than the population at large. This last possibility (also mentioned by Huxley and his colleagues) would be an example of genetic drift. To this day, scientists do not know which of these theories is correct.

Evolved Mechanisms Cannot Deal Effectively with Every Situation

Our basic drives, emotions, and other behavioral tendencies came about in evolution because, on balance, they promoted survival and reproduction more often than they interfered with survival and reproduction. That does not mean, however, that every instance of activation of such a drive, emotion, or tendency serves survival or reproductive ends. The emotion of guilt serves the distal function of helping us to preserve our relationships with people whose help we need for survival and reproduction. When we hurt someone we depend on, we feel guilty, which motivates us to make amends and patch up the relationship. That does not mean, however, that every manifestation of guilt in every person serves that function. Sometimes guilt can be crippling; sometimes our capacity for guilt is exploited by others for their ends at the expense of ours. The best that natural selection could do was to develop a guilt mechanism that is triggered by certain general conditions. It could not build a mechanism capable of distinguishing every possible condition from every other one and triggering guilt only when it is useful, never when it is harmful. The same is true for all of our other evolved emotions and drives (and that is why we have psychotherapists).