What is the Evolutionary Approach?

As stated in Section 1-2, a theoretical approach is a set of assumptions and concepts with which one constructs explanations of phenomena. In other words, it is the general viewpoint or perspective taken when developing the descriptions and explanations that make up a theory. Four of the most important theoretical approaches in psychology during the last 100 years have been the psychodynamic, behavioristic, cognitive, and biological approaches. Each approach focuses on a different set of causes. The psychodynamic approach focuses on motives and conflicts among them; the behavioristic approach focuses on environmental events and learning; the cognitive approach focuses on mental processes and cognitions; and the biological approach focuses on biological factors, their development in individuals, and their evolution in species. These primary theoretical approaches, which include assumptions and concepts that tend to be general in scope, each consists of a number of secondary approaches, which tend to be much more specific in terms of fundamental assumptions and concepts. Within the psychodynamic approach, for instance, there are many secondary approaches. Three examples are:

• The psychoanalytic approach of Sigmund Freud, which assumes that early childhood experiences involving expressions of the sexual motive result in unconscious conflicts that the individual must resolve, often in at least somewhat maladaptive ways. The resolutions of these conflicts determine the individual's adult personality;
• The individual psychology approach of Alfred Adler, which assumes that, during early childhood, the individual develops feelings of inferiority that cause the development of strivings for superiority (feelings of significance and attaining life success). If the individual's strivings for superiority become unrealistic, an inferiority complex may develop.
• The interpersonal approach of Harry Stack Sullivan, which assumes that personality is determined by the individual's interactions and relationships with others, which are constantly changing, especially as the individual develops through the lifespan. The individual's social roles, expectations, and responsibilities during childhood differ from those during adolescence, as well as from the different phases of adulthood. As these factors change, so does personality.

Within the behavioristic approach, there also are several secondary approaches. Two examples are:

• The radical-behavioristic approach of B. F. Skinner, which assumes that mental events (cognitions and emotions), bodily movements, and physiological responses are all behaviors and, hence, that they are all determined by the same learning principles. Even though mental events are not verifiable by others, they are observable by the individual and, therefore, can be subjected to scientific study.
• The social-learning approach of Albert Bandura, which assumes that the individual learns behavior by observing and imitating (or not) the behaviors, attitudes, and emotional responses of others, who are referred to as "models." Cognitive processes (attending, encoding, storing, etc.) underlie this learning.

Among the secondary approaches within the biological approach is the evolutionary approach, which assumes that cognitions, emotions, and behaviors are determined by biological structures and biological processes that have evolved over many generations of a species or a sequence of species. In short, the evolutionary approach in psychology assumes that animal species have evolved particular ways of responding — cognitively, emotionally, and behaviorally — to environmental events. These evolved ways of responding, in general, lead to greater survival and reproductive success of individual members of a species.

To take one example, the human spinal cord develops in such a way that it can rapidly process sensory information related to the temperature of objects being touched by a finger; and, when the objects are very hot, the spinal cord immediately activates a reflexive reponse that rapidly jerks the finger away from them. Because this response occurs automatically (without deliberation), we can't explain it as the result of conscious choice. In fact, the hand typically is jerked away before the information reaches the cerebral cortex, where electrical activity in the parietal lobes causes the conscious perception of pain. The existence of this spinal-cord reflex, according to the evolutionary approach, may be explained as the product of a long evolutionary process based on the increased survival and reproductive success of ancient vertebrates who were, because of the existence of this reflex, able to prevent severe bodily damage caused by extreme temperatures.

In biology, evolution refers to changes over generations in the frequencies of variants[∂] of a biological, psychological, or behavioral characteristic[∂]. For example, the development and widespread use (and misuse) of antibiotic medications since about 1940 have caused the rapid evolution of antibiotic resistance in many species of bacteria. Antibiotic resistance is due to the evolution of physiological characteristics in these species that render the antibiotics ineffective. Dogs are another good example of relatively rapid evolutionary changes. Dogs make up one biological group within the family Canidae, which comprises 35 extant[∂] species of carnivorous mammals that includes foxes, wolves, jackals, dingoes, and coyotes.

The ancestors of modern dogs separated from wolves in North America about 100,000 years ago, although there seems to have been some interbreeding between the two groups after that time (Vila, et. al, 1997). Our species, Homo sapiens, arose about 200,000 years ago in Africa (see the Genetic Migrations web page). Groups of Homo sapiens left Africa and reached Europe about 40,000 to 50,000 years ago, quickly spreading from there into Asia and eventually into Australia. At some point, these migrating humans came into contact with dogs, which had migrated from North America across the "land bridge" from Alaska to Siberia, which emerged at the beginning of the last ice age about 70,000 years ago and submerged after the end of that ice age about 11,000 years (see this article).

Thus, it seems likely that humans and dogs have interacted for tens of thousands of years. When groups of humans settled into an agricultural lifestyle about 10,000 to 12,000 years ago, the close human-dog interaction led to large and rapid changes in the physical appearance of dogs, as well as in their psychological and behavioral characteristics (Coren, 1994). In general, compared to wolves, dogs are much more puppy-like in their behavior, especially in terms of their playfulness, submissiveness, and obedience; have shorter snouts; are smaller in body size; are more variable in color; are more variable in the appearance and texture of their coats; have floppier ears; and are less fearful of novel situations and people. These changes are due to the choices made by humans regarding which dogs were allowed to breed and which were not. This will be explained later when the principles of natural selection and artificial selection are described.

Study Questions

1. Why would it not be sufficient to describe Sigmund Freud as a psychodynamic theorist or B. F. Skinner as a behavioristic theorist?
2. How are the various primary theoretical approaches similar to one another?
3. How do the primary theoretical approaches differ from one another?
4. How are the secondary theoretical approaches within a primary theoretical approach similar to one another?
5. How do the secondary theoretical approaches within a primary theoretical approach differ from one another?
6. What is the evolutionary approach in psychology?
7. How would you define "evolution" in your own words?
8. What is an example of the rapid evolution of a characteristic not mentioned in the textbook?
9. When did the species, Homo sapiens, first emerge in the hominid lineage?
10. When did the first group of Homo sapiens migrate from Africa to Europe?

What is Evolution By Natural Selection?

As already stated, the concept of evolution refers to changes over generations in the frequencies of variants of a biological, psychological, or behavioral characteristic in a population of organisms. A characteristic is a feature of an individual, such as eye color, that can be distinguished from other features, such as hair color. Characteristics often have variants that involve observable differences among individuals with respect to that characteristic. For example, the characteristic of eye color has many variants, such as various shades of brown, green, gray, and blue eye colors; and the characteristic of hair color has many variants, such as various shades of black, brown, red, and blonde hair colors. We will refer to such variants as expressions of the characteristic. Evolution, therefore, is a change over generations in the frequencies of expressions of a characteristic within a population of organisms; or, stated in a different but equivalent way, evolution is a change over generations in the average expression of a characteristic within a population of organisms. For example, a population consisting of 99% brown-eyed individuals and 1% blue-eyed individuals may evolve over generations into a population consisting of 1% brown-eyed individuals and 99% blue-eyed individuals. The average expression of eye color in this population evolved from brown to blue.

What causes evolution to occur in populations? For two decades beginning in 1836, Charles Darwin developed a credible[∂] naturalistic theory able to explain evolutionary changes — a theory that he began to develop when trying to interpret observations he had made during his five-year voyage on the H.M.S. Beagle (Darwin, 1839), as well as in research that he and others performed during the 23 years after Darwin returned from that voyage. This was the theory of evolution by natural selection. He published a detailed description of the theory in the first edition of the book, On the Origin of Species. (The sixth edition generally is considered to represent Darwin's mature views on evolution and its causes.) No one before Darwin had so masterfully marshaled such an enormous amount of supporting evidence for the evolution of organisms. In addition, no one before Darwin had outlined such a compelling explanation of evolution. Natural selection may be defined as the increased reproductive success of individuals with particular expressions of physical, mental, and/or behavioral characteristics. To put the matter most simply, Darwin (1859) argued that natural selection occurs when a subset of individuals in a population produce a greater number of offspring, on average, than others in that population because they express a physical, mental, or behavioral variant that allows them to adapt better to their environments.

Let’s consider, for example, a fictional species of fruit fly that has just arrived on a windy and tiny island hundreds of miles from other land. (Perhaps individuals from this species were brought to the island accidentally by human visitors who had fruit stored in their boat). Let's say that, in this founding population (the initial group of organisms that arrive at a location not already occupied by members of their species), there exists a variety of expressions of the characteristic of wing size, as shown in the following graph.

Figure 1. The Percentage of Individuals in a Founding Population of
Fruit Flies That Have Wings of Various Lengths.

As can be seen in the graph, some individuals have large wings, which are advantageous for flying speed and for the ability to stay airborne, whereas others have small wings, which result in slower flying speeds and greater difficulties with staying airborne. On this small and windy island, however, larger-winged flies are more likely to get blown out to sea where they die, whereas the smaller-winged flies are less likely to get blown out to sea. Thus, smaller-winged flies are more likely to survive long enough to reproduce than is the case with larger-winged flies.

This fictional example illustrates well the simple idea behind natural selection: individuals differ in their reproductive success because they have variants of characteristics that are associated with the ability to adapt to local environmental conditions. Because individuals with particular variants adapt better relative to individuals with other variants, the former survive longer, on average, and, hence, have more opportunities to reproduce. In other words, the local environmental conditions consist of factors that impose biological, psychological, and behavioral demands on organisms. These factors “naturally select” those organisms best able to deal with the environmental demands: they survive longer and reproduce more than others in their local population. For example, animals living in environments where food is scarce tend to survive longer when they are able to efficiently use the food they eat and are able to store fat reserves. Camels are an excellent example of this: the humps on their backs store enough fat to allow them to live up to two weeks without food, which is an advantageous trait for desert-dwelling animals that generally do not have frequent access to food (see the article here).

Given the obvious fact that natural selection occurs, how does it produce evolutionary changes in populations of organisms? There are three requirements that must be met in order for evolution in the average expression of a characteristic to occur through natural selection:

1. There must be individual differences in the expression of the characteristic.
2. These individual differences must be associated with genetic differences.
3. The increased reproductive success of individuals with particular expressions of the characteristic must remain stable over generations.

The first requirement has already been discussed (for example, see Figure 1). The second requirement involves the existence of genetic variants that affect the development of characteristics. A gene is the basic unit of biological heredity. Genes consist of sequences of chemical units (sections of DNA molecules) that are contained in chromosomes carried by the sperm of males and the ova (eggs) of females (see articles here and here, and the web pages following each). In human reproductive cells (sperm and ova), there are 23 chromosomes, which together contain from 20,000 to 25,000 genes (Coghlan, 2004). This means that, on average, each human chromosome contains about 1000 genes.

Genes influence the production of proteins and their use in developing and maintaining the body. For example, there are at least three genes that influence the development of eye color (for example, see article here), which, for the sake of simplicity, we'll refer to as Gene A, Gene B, and Gene C. As can be seen in the following table, babies receive one copy of each gene from their biological fathers (labelled as 1) and one copy of each gene from their mothers (labelled as 2):

Gene A has two variants: a brown variant and a nonbrown variant. If at least one brown variant is inherited from either parent, then, regardless of what is inherited at Gene B and Gene C, the person will develop brown eyes:

 If, on the other hand, the nonbrown variant is inherited from each parent, then eye color is determined by what is inherited at Gene B and Gene C.

Gene B has two variants: a brown variant and a blue variant. If at least one brown variant of Gene B is inherited from either parent, the person will develop brown eyes, regardless of what is inherited at Gene C:

If, on the other hand, the blue variant is inherited from each parent, the person will develop blue eyes depending on what is inherited at Gene C (which we will ignore for the moment):

Gene C has two variants: a green variant and a blue variant. If the blue variant of Gene B is inherited from each parent, then, if at least one green variant of Gene C is inherited from either parent, the person will develop green eyes:

If, on the other hand, the blue variant of Gene C is inherited from each parent, the person will develop blue eyes:

Thus, eye color is determined by interactions among variants of at least three genes: two on Chromosome 15 (Gene A and Gene B), and one on Chromosome 19 (Gene C). Of course, there must be other factors (including genes) that affect eye color: there are many shades of eye color (such as gray) that are not explained by this theory; and eye colors often change color from infancy to childhood.

Regardless of its limitations, this theory shows that gene variants, and interactions among them, are causes of the development of the physical characteristics of the body. In fact, you see evidence supprting this claim all around you with respect to the physical resemblances among biological relatives. Furthermore, when a male and female from the same species mate, their offspring obviously look like other members of that species. On the other hand, when a male and female from different species mate, their offspring generally express physical characteristics intermediate between the two species. For example, matings between male donkeys and females horses produce mules; and matings between male horses and female donkeys produce hinnies. Mules and hinnies have physical and behavioral characteristics that are intermediate between those of horses and donkeys (see the article here).

It is of interest that mules and hinnies are almost always unable to reproduce. This is because horses have 64 chromosomes (32 pairs) and donkeys have 62 chromosomes (31 pairs). In addition to having different numbers of chromosomes, the two species have chromosomes that differ somewhat in terms of their structures. Their offspring (mules and hinnies) have 63 chromosomes — a number that represents various combinations of the horse and donkey chromosomes in different individuals. This creates difficulties when each chromosome attempts to pair with its corresponding chromosome in the biological process that produces sperm and ova. Nevertheless, there have been a few reports of successful matings.

Study Questions

11. What are some physical characteristics not mentioned in the textbook?
12. What are some variants of the characteristics you listed in the previous question?
13. How would you define "natural selection" in your own words?
14. What is natural selection such an important concept in evolutionary biology?
15. How is natural selection related to the notion of adaptation?
16. What are the three main requirements of evolution by natural selection?
17. What is a example, not mentioned in the textbook, of a characteristic that probably has evolved through natural selection of variants of this characteristic?
18. How is the genetic situation associated with the development of eye color a good example of multifactorial causation?
19. Given the theory of eye color described above, are you able to explain how two blue-eyed parents can have a brown-eyed child? Why or why not?
20. When individuals from two different species mate, what typically is true regarding the physical and behavioral characteristics of the offspring?

Natural Selection and Genes
When particular expressions of a characteristic are naturally selected and those expressions are associated with particular gene variants, then those gene variants will tend to be inherited by the resulting offspring. For example, let's say that there exists a gene with two variants, T and t, that are associated with changes in the average height of plants from a particular (fictional) species, Pirasus arizonensis: from a baseline height of five inches, T increases the average height of Pirasus arizonensis by 1/2 inch, whereas t decreases the average height by 1/2 inch. If Pirasus arizonensis seeds are transported by birds into an environment in which the fully grown plant is surrounded by plants from another (fictional) species, Torensi mojavensis, that are six inches tall, on average, the taller species will limit the smaller species access to sunlight, which will lead to its reduced survival and reproductive success. Any plant from Pirasus arizonensis that grows taller than five inches will tend to survive longer and reproduce more than plants at or below the baseline height. Let's say that the following 300 Pirasus arizonensis plants have grown in this new environment:

• 100 TT plants, which will have an average height of 6 inches;
• 100 Tt plants, which will have an average height of five inches;
• 100 tt plants, which will have an average height of four inches.

If 80 of the tt plants die before producing seeds, 50 of the Tt plants die before producing seeds, and 10 TT plants die before producing seeds, then 72% of the seeds in the next generation will contain T and only 28% will include t (don't worry about how this was calculated: you can ask me if you are curious). This means that, if 300 plants grow in the next generation, there numbers will be as follows:

• 156 TT plants;
• 120 Tt plants;
• 24 tt plants.

As you can see, the number of plants with at least one copy the T variant, 276 plants, is much larger than the number of plants with at least one t variant, 144 plants (again, you need not worry about how these numbers were obtained).

The third requirement of evolution by natural selection — the increased reproductive success of individuals with particular expressions of a characteristic must remain stable over generations — means that the "selective pressure"[∂] on Pirasus arizonensis plants with respect to their heights must not change. That is, the six-inch tall (on average) Torensi mojavensis plants must continue to limit the amount of sunlight obtained by smaller Pirasus arizonensis plants. If this occurs, then over generations, we will expect the following change in the frequencies of the T and t gene variants in this species:

Figure 2. Changes in the Frequencies of T and t Over Five Generations.

As you can see, the frequency of T becomes almost 100% within five generations, which means that the population in this new environment is made up almost entirely of plants that are six inches tall, on average. Thus, over a very short period of time, natural selection can lead to a large change in the average expression of a characteristic in a population when individual differences in that characteristic are strongly associated with genetic differences.

Let's return to the example of a founding population of fruit flies on a tiny and isolated island buffeted by strong winds. Differences in the size of fruit-fly wings are strongly linked to differences in genes (Robertson & Reeve, 1952), to a degree similar to that described above for height differences in the fictional account of "Pirasus arizonensis." Thus, if there the environmental conditions naturally select flies with smaller wings, gene variants causing the development of smaller wings will increase in fequency in the population. If the selective pressure remains stable over generations, the population will evolve a smaller average wing size, as shown in Figure 3. The black bars represent the founding population and the white bars represent the population after perhaps 10 to 15 generations of natural selection for smaller wing sizes.

Figure 3. Evolution of a Population of Fruit Flies Undergoing
Natural Selection for Smaller Wing Size.

Although these two examples are fictional ones, there are many examples of natural selection and artificial selection (selection in which humans breed over generations organisms that express particular variants). For introductions to and histories of the concepts of evolution and natural selection, see Colby (1996), Endler (1986), and Zimmer (2001), and these course notes on evolution.

Is Evolution Goal-Directed?

Given enough time, natural selection can change the average expressions of the physical, mental, and behavioral characteristics of a lineage of species to such a degree that the modern species looks and acts very different, on average, from the ancestral species that gave rise to it. For example, the lineage that led to modern whale species occurred because of the natural selection of variants that transformed ancestral species over millions of years from what were probably unintelligent “bear-like” creatures that walked on land to highly intelligent “fish-like” creatures that swim in the oceans (Sutera, 2001). In a similar way, the highly complex mental and behavioral characteristics typical of modern humans are the result of millions of years of natural selection of variants that transformed less complex species of primates and hominids into a species with very complex mental and behavioral adaptations[∂] (see Human Evolution). An abridged version of the geneology of modern Homo sapiens is shown in Figure 4.

Figure 4. A General Sketch of the Evolution of Primates and Hominids.

While it probably is true that there has been a general evolutionary trend among mammals towards increasing physical, mental, and behavioral complexity, such as that observed in the lineages of modern humans and cetaceans (whales, porpoises, and dolphins), it is important to keep in mind that there is no reason to believe that natural selection, in some mysterious way, has had as "its goal" the eventual evolution of Homo sapiens or any other present-day species:

evolution by natural selection is not forward looking or intentional. A giraffe does not notice juicy leaves stirring high in a tree and “evolve” a longer neck. Rather, those giraffes that happen to have slightly longer necks than other giraffes have a slight advantage in getting to those leaves. Hence, they survive better and are more likely to live to pass on genes for slightly longer necks to offspring. (Buss, et al., 1998)

In the case of modern humans, if past environments had developed in even slightly different ways than they did, our species probably would not be here today (see, for example, the interview of Stephen Jay Gould by Schleier, 1998).

Evolution by natural selection should not be thought of as leading to organisms that are superior (“higher”) in some way when compared to ancestral species. To various extents, every species (whether past or present) is adapted to the environment that it inhabits. Modern humans might be at a significant disadvantage if we somehow could be transported back in time five million years and inhabit the environments of our primate ancestors. We might even become extinct in these environments. Over time, as the environment changes, each species tends to change (or to become extinct). As the species evolves, it may lose characteristics that we tend to think of as “higher” ones. For example, certain cave-dwelling species of animal have evolved into creatures without eyes even though their ancestors had and their modern "cousins" have well-developed eyes (see, for example, Ternes, 2004). Humans might think of this as some sort of "de-evolution": evolution from a “higher” to a “lower” form. The human tendency to place values on physical, mental, and behavioral variants causes us to incorrectly think of evolution as tending to lead from “lower” to “higher” species. [EVOLUTIONARY SCALE]

Why Should One "Believe In" Evolution?

Many Americans state that they don't "believe in" evolution: recent polls show that more than 50% of Americans believe that humans were created by the Judeo-Christian God as described in the two different creation stories presented in the first chapter of the Book of Genesis (see Origin of Human Life for the survey data; see Linder, 2004, for a discussion of the two creation stories). But, if the term, "believe in," means to "have faith"[∂] that a claim is true, then the theory of evolution is not something that one can "believe in" or not. Instead, one must evaluate whether the empirical evidence available either supports or fails to support the claim that extant species (including Homo sapiens) have evolved from ancestral species, almost all of which are extinct; and that all extant and extinct species evolved from one or a few groups of organisms that emerged billions of years ago when Earth was very young.

Evaluating the empirical evidence requires a critical examination of the empirical evidence, which means that reason and logic, not faith and intuition, are central to the evaluation process. Those who express faith in the truth of a claim reject (and sometimes revile) the critical examination of empirical evidence relevant to the evaluation of the claim. Thus, one can "believe in" the claims made in the creation stories described in the Book of Genesis by having faith that they were written by a supernatural creator of the universe. But one cannot "believe in" the claims made about evolution by scientific researchers (at least, one cannot do this and still claim to be critically examining the claims). The acceptance or rejection of scientific claims should never be a matter of faith.

Since the publication of Darwin’s book in 1859, there has been so much empirical evidence supporting the claim that all species, including Homo sapiens, have evolved from other species that biologists accept the claim as a "fact" (something that is true beyond a reasonable doubt). And evolution is considered to be the central organizing principle of biology. One of the greatest geneticists and evolutionary biologists of the twentieth century, Theodosius Dobzhansky, stated that, "nothing in biology makes sense except in the light of evolution," and then provided his reasons for making this claim (Dobzhansky, 1973). To biologists, the claim that evolution has occurred (and is occurring) is as obviously true as is the claim that the Earth revolves around the sun about once every 365 days and makes a complete rotation about once every 24 hours.

Study Questions

21. Why do humans have two copies of every gene?
22. How does natural selection of particular expressions of a characteristic (such as greater height in men) cause changes in the gene frequencies of a population over generations ?
23. Would it be correct to say that, since evolution by natural selection has produced a species, Homo sapiens, with a high degree of intelligence, humans will continue to evolve towards even greater heights of intellect? Why or why not?
24. How would you define "adaptation" in your own words?
25. What is an example of a probable human adaptation not mentioned in the textbook?
26. About how long ago did the first hominids evolve?
27. About how long ago did the first species of "Homo" (a Latin term meaning "man") evolve?
28. To which primate are we most closely related to: orangutans, chimpanzees, or gorillas?
29. Why is it incorrect to state that some extant species are "higher" and other extant species are "lower" on the "evolutionary scale"?
30. Read the article by Dobzhansky (1973). Why did he claim that "nothing in biology makes sense except in the light of evolution"?

Why Are Humans Good General Learners?

An adaptation is a variant of a physical, mental, or behavioral characteristic that has been naturally selected in a species over many generations. In general, an adaptation is expressed in all normal members of either the species or a biological subgroup of the species (such as one of the sexes). Adaptations help individual members of a species deal with environmental demands in such a way that they tend to survive longer and reproduce more than those without the adaptations. Many human mental functions, such as the ability to easily learn associations between events, may be thought of as adaptations. But there is a difficulty with this claim: how can nonphysical mental processes be naturally selected if natural selection works only on the physical structures that make up our bodies? In order to answer this question, we have to remember that mental activity is caused by neural activity in the central nervous system.

The evolutionary approach in psychology assumes that human mental abilities and behaviors are adaptations that helped our ancestors to survive longer and reproduce more. How can the human mind evolve in the way that the evolutionary approach assumes? The answer involves the fact that mental and behavioral activity are caused by neural activity. The following argument spells out the details of the answer:

1. The bodies of humans have evolved by natural selection over thousands of generations.
2. The brain is a part of the human body and, therefore, also has evolved by natural selection over thousands of generations.
3. Chemical and electrical activity in brain structures cause cognitions, emotions, and behaviors.
4. Natural selection of cognitions, emotions, and behaviors includes natural selection of the brain structures and brain activity that produces them.
5. Therefore, natural selection of cognitions, emotions, and behaviors over thousands of generations causes the evolution of the brain.

Evidence for the evolution of the human brain is plentiful. For example, fossil evidence demonstrates that the capacity of the human skull has increased dramatically over the last one million years or so, such that our modern melon-sized brains have evolved from the smaller orange-sized brains of our hominid ancestors (see article here). It is virtually certain that the greatly increased size of the modern human brain has led to enormous changes in how we process sensory information and use this processed information to respond to environmental demands. In other words, modern humans think, feel, and behave in much more complex ways relative to our hominid forebears.

Natural selection of the human brain has produced mental and behavioral adaptations such as our ability to easily learn associations between events. In fact, Seligman (1971) argued that we are "biologically prepared" to learn particular associations easier than others. Biological preparedness may be defined as the innate (inborn) tendency to easily learn certain associations between events, probably because of the effects of natural selection on the evolution of the central nervous system. For example, humans who quickly learned to fear snakes after being attacked by one would survive longer than those who learned the association between snakes and danger more slowly.

Language Learning
Not only do we learn some classically conditioned associations very easily, we also learn languages very easily, especially during early childhood. Without much explicit instruction, we learn to speak our native language very well by about five years of age. By late adolescence, almost all of us have become extremely proficient at producing and comprehending spoken language. Pinker (1994) reported that the average high-school graduate knows about 60,000 words:

Word learning generally begins around the age of twelve months. Therefore, high school graduates, who have been at it for about seventeen years, must have been learning an average of ten new words a day continuously since their first birthdays, or about a new word every ninety waking minutes. Using similar techniques, we can estimate that an average six-year-old commands about 13,000 words.... A bit of arithmetic shows that preliterate [nonreading] children, who are limited to [listening to the speech that they hear around them]..., must be lexical [word] vacuum cleaners, inhaling a new word every two waking hours, day in, day out.... Think about having to memorize a new batting average or treaty date or phone number every ninety minutes of your waking life since you took your first steps. (pp 150-151)

How does this amazing development of language occur? Perhaps it is simply because we hear language all around us right from birth and, therefore, we just “pick it up.” But this is not a satisfactory answer: we experience all sorts of things around us right from birth and much of this is not learned unless we pay close attention and practice it again and again. For example, you listen to your instructors each and every day, but most of this information is very difficult to learn and remember unless you pay very close attention to it and think about it very carefully later when you are studying. The same thing is true when learning to play a musical instrument or a sport.

Compared to the learning of such material, language learning during the first few years of life is almost ridiculously easy. Young children hear language around them and quickly learn to use it in novel ways without exerting much effort at all. Could we explain language learning with either classical conditioning or operant conditioning? Speaking might be seen as a voluntary activity, so perhaps language is learned through operant conditioning — perhaps we learn a new word by first hearing it and then by being reinforced for using it correctly and punished for using it incorrectly. If this were true then, over time, the incorrect usages would slowly weaken and disappear, whereas the correct usages would slowly strengthen and become firmly established. This may seem to be a plausible answer. But, if you spend some time observing young children, you soon will see that this explanation probably is wrong: children typically begin to use a new word in correct ways without explicit reinforcement. For example, how often have you heard a parent praising a preschooler for using, say, the word “light” (or any other word) correctly? The learning of language is a much more subtle process than can be explained by operant conditioning.

Even though language learning may be the highest form of human learning, we also are very good at learning many other things about our environments. Why are we such good general learners? Is it because, somehow, we have become more than mere animals? Have we gone beyond instinct into a new plane of existence where only learning exists? Are we infinitely malleable creatures as John Watson (1930) suggested?

Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in, and I’ll guarantee to take any one at random and train him to become any type of specialist I might select—doctor, lawyer, artist, merchant-chief, and, yes, even begger-man and thief, regardless of his talents, penchants, tendencies, activities, vocations, and race of his ancestors. (p. 65)

None of this seems very likely. We are not able to learn just anything we want to learn, and we definitely learn some things much more easily than others. This is true even with language learning: learning to read and write a language is much more difficult than learning to speak a language. And many animals easily learn certain things that most humans would find very difficult or impossible to learn. For example, rats quickly learn to navigate very complex mazes that many humans probably could learn only with great difficulty, if they could learn them at all. Nevertheless, there is no doubt that we have a much better general learning ability than any other animal. The most likely reason that we are such good general learners is that we have evolved an ability to easily associate many kinds of events in our environments—an ability that probably has much to do with the enormous size and complexity of our cerebral cortex. But some other mammals, such as chimpanzees, whales, and dolphins, also have very good general learning abilities. And most animal species are able to learn very well at least some things about their environments.

Evolutionary psychologists believe that species tend to evolve a general learning ability when their environments are unpredictable. This claim was argued by Steven Pinker (1994):

Many social scientists believe that learning is some pinnacle of evolution that humans have scaled from the lowlands of instinct, so that our ability to learn can be explained by our exalted braininess. But biology says otherwise. Learning is found in organisms as simple as bacteria.... Learning is an option, like camouflage or horns, that nature gives organisms as needed — when some aspect of the organisms’ environmental niche is so unpredictable that anticipation of its contingencies [in this context, a contingency refers to an association between events] cannot be wired in [in an instinctive way].... [On the other hand,] when an environment is stable, there is a selective pressure for learned abilities to become increasingly innate [inborn or instinctive]. (p. 242)

According to the evolutionary approach, modern humans are such good learners probably because the environments of our ancestors were often unpredictable — so unpredictable that they had to be able to learn and remember many important aspects of it if they were to survive. For example, in the arid environment inhabited by our ancestors during a particular time period a few hundred thousand years ago, they had to learn where sources of water were and where food was plentiful if they were to live long enough to reproduce. This ability to easily learn general information about the environment probably evolved in our ancestors over at least hundreds of thousands of years. We now often put this ability to uses of which our ancient ancestors never dreamed (such as being able to learn and remember the information in this course). Nonetheless, our general ability to easily associate many different events was first forged in those rough and dangerous environments of our hominid forebears. This, at least, is what is suggested by an evolutionary approach to learning.

Evolution and Mental Continuity
Darwin’s theory of evolution by natural selection had a major impact on the emerging science of psychology by affecting how psychologists conceived of the human mind and behavior. In his later writings, Darwin argued that, over millions of years, gradual changes occurred in the mental processes of  our nonhuman ancestors. These changes accumulated gradually, he suggested, until the complex minds of modern-day humans emerged. Since these same nonhuman ancestors gave rise to other modern-day primates, such as chimpanzees, our minds should share similarities with the minds of these animals. In other words, Darwin argued that there exists a mental continuity between humans and other animals: the mental abilities of humans are similar to those of other related species because we all have descended from a common ancestral species. Any differences in mental abilities that are found among closely related species should generally tend to be differences in the amount or degree of these abilities. This suggests that, if we are trying to find evidence that a mental ability or behavior has evolved in humans, we should measure the ability or behavior in our primate relatives to see if they share this characteristic to some extent. For example, chimpanzees and gorillas have been taught to use the symbols of nonspoken languages, such as sign language, although their ability in this regard is much less than that of humans (see Kosseff, undated; Nature: A Conversation With Koko; Nisbet, 1998; the Primate Language Research Center web site).

The acceptance of the principle of mental continuity led many nineteenth-century behavioral scientists to study animals closely related to humans in the belief that this would tell us much about the nature of the human mind and human behaviors. The science of psychology itself emerged from this research activity:

The influence of Darwinism upon psychology during the last quarter of the nineteenth century probably did as much as any single factor to shape the science as it exists today. Psychology was certain to become consistently more biological; mental processes tended more and more to be stated in terms of the functions served in the task of adjusting to the world.... As a natural consequence, interest in animal psychology rapidly increased. Many books appeared which concerned themselves with the nature of instincts and with the phylogenetic [that is, evolutionary] study of intelligence. Studies of animal behavior appeared in psychological journals. The great gulf established by Descartes between human and animal psychology had been bridged. The life of the organism was seen as a whole. Human psychology was to be seen in relation to all the phenomena of life. (Murphy, 1949, p. 116)

Darwin’s work brought the study of human nature fully within the realm of scientific inquiry. After the publication of Darwin’s book, many came to believe that they could use the methods of science to resolve once and for all the various philosophical controversies that had brewed for centuries.

Study Questions

31. How can mental abilities and functions, which are not physical things, evolve through natural selection?
32. How would you define "biological preparedness" in your own words?
33. How is evolution by natural selection thought to be related to biological preparedness?
34. According to evolutionary psychologists, why do humans learn associations so easily and so generally?
35. How would you define the concept of "mental continuity" in your own words?
36. How does mental continuity help us to understand the evolutionary origins of human mental abilities and behaviors?