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Chapter 2

Chapter 2: Evolution and Genetics

Chapter Review

GENETICS AND DNA

  • The nucleus of each biological cell contains chromosomes, which each contain a single molecule of DNA. Within this molecule, specific sections called genes govern the cell’s functioning by providing detailed instructions for making proteins. Humans have 23 pairs of chromosomes and roughly 25,000 protein-coding genes; these are collectively called the person’s genome.
  • In each cell, some genes are expressed at any point in time and others are not. Gene expression is controlled by the biochemical environment inside the cell, which in turn is influenced by many factors, including the organism’s overall environment, its experience, and its behavior. An organism’s genome therefore specifies only its genotype; the overt traits and behaviors of the organism define its phenotype, which is the product of the genotype and experience in constant interaction.
  • Most characteristics are influenced by the action of many genes. Each gene, though, is paired with another gene; the pairs are located at corresponding positions on the pairs of chromosomes. The genes may be the same allele or not; and if they are different, one gene may be dominant and the other recessive, or the genes may be codominant, or one may be incompletely dominant.

EVOLUTION BY NATURAL SELECTION

  • Charles Darwin hypothesized that all modern organisms are descended from a small set of shared ancestors and have emerged over time through the process of evolution. An enormous amount of evidence has confirmed these proposals. The key mechanism is natural selection: If individuals with certain traits are more likely to survive and reproduce, then their genes will be better represented in the next generation; and if the genes gave rise to the advantageous traits, then those traits will be more common in the next generation.
  • It is important to avoid the naturalistic fallacy, however—the idea that evolution somehow improves organisms, or that anything natural is good.
  • Darwin’s broad proposal boils down to three principles: there must be variation among the individuals within a population; certain of the variants must survive and reproduce at higher rates than others; the traits associated with this advantage must be passed from parents to offspring. Both the variation itself and the transmission of traits, across generations, depends on the organism’s genome.
  • The emphasis on the survival of genes helps explain a number of behaviors, including behaviors in which organisms endanger their own survival to protect their offspring or relatives. These behaviors are best understood in terms of the survival of genes, rather than the survival of individuals.
  • The evidence for modern evolutionary theory comes from many sources, including the fossil record as well as the examination of the resemblance between the genomes of various organisms.We can also document the unfolding of evolution in some modern organisms. The shared ancestry for many organisms is evident in the extraordinary unity of life we can document in many ways.

THE GENETICS AND EVOLUTION OF BEHAVIOR

  • Evolution by natural selection has shaped behaviors just as it shapes an organism’s physical traits. In many cases, this natural selection has favored flexibility in an organism’s behavior, and so organisms have evolved mechanisms through which they can alter their responses and learn new skills.
  • The behavior of smiling seems to be species general for humans, but is not species specific. The behavior is also evident in individuals blind since birth, making it clear that this behavior does not depend on a history of learning.
  • Examination of humans and other primates indicates that there are at least two types of smiles: One type is expressive of an individual’s inner state, and is produced even if no other people are around; the other type is more social and functions as a greeting or a means of defusing tense situations. All of these points provide powerful indications that smiles have ancient roots, and were selected by evolution as a means of communication, allowing others to read our inner states and intentions.
  • An individual’s level of intelligence is influenced by genetic factors; we can see this in the fact that individuals who resemble each other genetically (identical twins) tend to resemble each other in their intelligence, even if the twins were reared separately. Intelligence is also influenced by environmental factors, and researchers use the heritability ratio as a summary of the effect of genetic differences within a given population and environment.
  • The value of the heritability ratio depends on the group being examined. In lower-SES groups, the heritability may be zero; the heritability also increases with a person’s age, plausibly because each person chooses environments that amplify the individual’s genetic potential.
  • Human intelligence was favored by natural selection because our ancient ancestors had a reproductive advantage if they could communicate, solve problems, and draw conclusions. However, it’s unclear, from an evolutionary perspective, why humans vary in their intelligence—reminding us that not all inherited characteristics are the direct result of natural selection.
  • Most mammals are polygynous, and this pattern is easily understood in evolutionary terms. To maximize their reproductive success, most male mammals should mate with as many females as possible; female mammals, in contrast, maximize their reproductive success by mating just a few times during their lives, but taking steps to ensure the well-being of each of their progeny.
  • This logic helps explain why human males express a desire for multiple partners, and why it’s usually the female, in most species, that makes the choice of whether to mate with a particular male or not. By the same logic, natural selection explains why it’s usually males who take the larger role in courtship, seeking to attract the female.
  • Human males seem to care more about their partner’s appearance than females do; women seem to put greater weight on their partner’s social status than males do. Natural selection likely favored these tendencies, based on the idea that a partner’s appearance indicates health and fertility; a partner’s status indicates the likelihood that he will provide resources needed to raise the young.
  • An evolutionary perspective also leads to the expectation that men will be more distressed by sexual infidelity in their partners than by emotional infidelity; women should show the reverse pattern.
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