Читать книгу Social Psychology - Daniel W. Barrett - Страница 85

The tiniest physiological aspects of the brain that are most relevant to our discussion of the biological basis of social behavior are chromosomes, genes, and DNA. A typical human cell body contains 46 chromosomes, consisting of 23 chromosomal pairs. The only exceptions to the 23-pair rule are sex cells, which contain just one-half of a pair (the missing half is supplied by the other parent at fertilization). Chromosomes are composed of both DNA molecules and proteins. DNA or deoxyribonucleic acid consists of two strands of genes arranged in the familiar ascending staircase structure of a double helix. Chromosomes can also be considered strands of genes, because each gene is a segment of DNA. Chromosomes provide the blueprint for thousands of proteins, whereas a gene directs the synthesis of a particular protein. Finally, variants of genes are called alleles. People often refer to genes as units of heredity; although that is not incorrect, it is more accurate to say that alleles carry the information essential for the expression of traits. Psychologists who study the effects these traits have on social behavior focus primarily on alleles.

As we’ve noted, gene variants or alleles are the basic unit of heritability. That is to say that evolutionary processes are thought to work at the level of genes, not individuals, groups, or species (Keller, Howrigan, & Simonson, 2011). Although I may at times refer to the natural selection of individuals, it is simply a manner of speaking. I do so because genes cannot be passed down and cannot survive except by residing in human vehicles. Evolutionary theorists have largely agreed that genes are selected for, not individuals. In recent years, several experts have argued that natural selection does in fact work at the level of groups, although this issue remains quite controversial (Boyd & Richerson, 2007; Wilson, 2007).

Exactly how does gene-level natural selection occur? As mentioned in Chapter 1, genes that are adaptive are selected for, which is to say that individuals carrying those genes have an evolutionary advantage over those who do not. As a result of that advantage, those adaptive genes may spread throughout a population over time via sexual reproduction. This of course begs the question of how the “new” adaptive genes appear in the first place. There are two ways that gene variation can occur. One is through recombination: The fertilization of the egg by the sperm results in the combination of one-half of the female’s chromosomes with one-half of the male’s. Thus, reproduction can produce novel combinations of alleles and, by extension, traits and individuals.

The second source of variation is mutation, which is the result of random errors in the duplication of genes within a given individual. Mutations typically produce recessive alleles, are “invisible,” and are generally not adaptive. These mutations will only “appear” if the carrier reproduces with another carrier of the same gene (such as a close relative). However, mutations occasionally create both dominant and adaptive genes and, if the carrier successfully passes them down to the offspring, then they may eventually become present in the entire population. Dominant genes (such as for brown eyes) are expressed if the parts of a pair of genes are different, but the expression of recessive genes (such as for blue eyes) only occurs when both halves of a pair are identical. Psychological tendencies and personality traits, such as extraversion or conscientiousness, are partially inherited in much the same way.

Although we tend to think about the evolution of genes in terms of natural selection, recent research suggests that advances in culture have also affected the evolution of genes. In other words, genes and culture have coevolved and together have produced the human mind as we know it today (Chiao, 2011; Richerson, Boyd, & Henrich, 2010). Cultural variation in diet and disease exposure have affected specific alleles, such that these alleles differ in prevalence across cultures. For instance, differences in the desirability and frequency of culture-related traits like individualism-collectivism may be correlated with genetic variation (Fincher, Thornhill, Murray, & Schaller, 2008). Later we discuss the role that disease may have played in the historic development of collectivistic cultures (see Chapter 6). The takeaway message here is that not only can genes affect social behavior, but also that social behavior can impact genes.

Alleles: Gene variants that carry the information essential for the expression of traits