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Chapter 16

BIOLOGY 1M03 Chapter 16: Chapter 16 - Evolution and Human Behaviour

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Ben Evans

1 Chapter 16: Evolution and Human Behaviour Why Evolution Is Relevant to Human Behaviour  The application of evolutionary principles to understanding human behaviour is controversial  All phenotypic traits, including behavioural traits, reflect the interactions between genes and the environment o The nature-nurture debate is based on a false dichotomy o It assumes that there is a clear distinction between the effects of genes and the effects of the environment o However, gene are not like blueprints that specify phenotype o Every trait results from the interaction of a genetic program with the environment o The expression of any genotype always depends on the environment o The expression of behavioural traits is usually more sensitive to environmental conditions than is the expression of morphological and physiological traits o Traits that develop uniformly in a wide range of environments, such as finger numbers, are said to be canalized o Traits that vary in response to environmental cues, such as subsistence strategies, are said to be plastic o Every trait however, results from the unfolding of a developmental program in a particular environment  Natural selection can shape developmental processes so that organisms develop different adaptive behaviours in different environments o There is a misunderstanding that natural selection cannot create adaptations unless behavioural differences between individuals are caused by genetic differences o If this were true, adaptive explanations of human behaviour must be invalid because there is no doubt that most of the variation in behavioural traits, such as foraging strategies, is not due to genetic differences, but is instead the product of learning and culture o Belief is false because natural selection shapes learning mechanism so that organisms adjust their behaviour to local conditions in an adaptive way o Ex. Male soapberry bugs guard their mates when females are scarce, but not when females are abundant ▪ Individual males vary their behaviour adaptively in response to local sex ratio ▪ In order for this flexibility in behaviour to evolve, there had to be small genetic differences in male propensity to guard a mated female and small genetic differences in how mate guarding is influenced by local sex ratio ▪ If such variation exists, then natural selection can mold responses of males so that they are locally adaptive 2 ▪ In any given population, most of the observed behavioural variation is due to fact that individual males respond adaptively to environmental cues o The crucial point is that evolutionary approaches do not imply that differences in behaviours among humans are product of genetic differences between individuals Understanding How We Think  Evolutionary analyses provide important insights about how our brains are designed o Natural selection hasn’t just made our brains big, it has shaped our cognitive abilities in very specific ways and molded the way we think  Even the most flexible strategies are based on special-purpose psychological mechanisms o A considerable body of empirical evidence indicates that animals are predisposed to learn some things and not others o Ex. Rats quickly learn to avoid novel foods that make them sick ▪ Based solely on taste of a food that has made them sick, not the food’s size, shape, colour or other attributes ▪ Makes sense because rats live in a very wide range of environments, where they frequently encounter new foods and usually forage at night when it is dark o There are certain limits to the flexibility of this learning mechanism ▪ Certain items that rats will never sample and in this way their diet is rigidly controlled by genes ▪ Learning process not affected equally by all environmental contingencies ▪ Ex. Rats are affected more by the association of novel tastes with gastric distress than they are with other possible associations  Our brains may be designed to solve the kinds of problems that our ancestors faced when they lived in small foraging bands o Researchers argue that complex adaptations like the brain evolve slowly, so our brains are designed for life in foraging societies o Use the term environment of evolutionary adaptedness (EAA) to refer to the social, technological and ecological conditions under which human mental abilities evolved o People living in foraging groups face certain kinds of problems that affect their fitness ▪ Ex. Food sharing is an essential part of life in modern foraging groups ▪ Although vegetable foods are typically distributed only to family members, meat is nearly always shared more widely ▪ Food sharing is a form of reciprocal altruism o Researchers have hypothesized that human cognition should be finely tuned to detect cheaters and data suggests that people are very attentive to imbalances in social exchange and violations of social contracts 3  Evolved psychological mechanisms cause human societies to share many universal characteristic o Two examples of mechanisms found in all human societies ▪ Inbreeding avoidance, may have inherited from primate ancestors ▪ Capacity for language, derived  Inbreeding Avoidance o The offspring of genetically related parents have lower fitness than the offspring of unrelated parents do ▪ The offspring of inbred matings are much more likely to be homozygous for deleterious alleles than are offspring of outbred matings ▪ Therefore, inbred offspring are less robust and have higher mortality than offspring of outbred matings ▪ Mating with close relative is deleterious because it greatly increases the chance that both partners will carry a deleterious allele at the same locus ▪ Inbreeding leads to substantial reduction in fitness, suggesting that natural selection should favour behavioural adaptations that reduce chance of inbreeding o Matings between close relatives very rare among non-human primates ▪ In all species of non-human primates, members of one or both sexes leave natal groups near time of puberty ▪ Likely that dispersal is an adaptation to prevent inbreeding ▪ Natural selection as provided at least some primates with another form of protection against inbreeding, a strong inhibition against mating with close kin ▪ Experimental studies have shown that primates generally have a strong aversion to mating with kin ▪ Ex. In chimpanzees, females tend to avoid mating with males older than them and males seem to be generally uninterested in females much younger than themselves • These mechanisms may protect females from mating with fathers and vice versa o Humans rarely mate with close relatives o Adults are not sexually attracted to people with whom they grew up ▪ The fact that inbreeding avoidance is common among primates suggests that our human ancestors probably also had psychological mechanisms preventing them from mating with close kin ▪ Evidence towards psychological mechanisms • Taiwanese minor marriage o In minor marriages, children were betrothed and prospective bride was adopted into family of future husband during infancy o Partners in minor marriages found each other sexually unexciting • Kibbutz age-mates 4 o In these communities, children were raised in communal nurseries and lived intimately with a small group of unrelated age-mates from infancy to adulthood o Ideology did not discourage sexual experimentation or marriage by children in such groups, but neither occurred • Third-party attitudes toward incest o People find it disgusting  Human Language o The cognitive capacities that give rise to language are universal o Humans can perceive speech sounds at a much more rapid rate than other sounds ▪ Phonemes are basic units of speech perception, the smallest bits of sound that we recognize as meaningful elements of language ▪ The ability to communicate rapidly was most likely favoured by natural selection o We perceive speech as a sequence of discrete words, even though this is an auditory illusion ▪ There are no periods of silence in ordinary speech o Language is based on grammatical properties that allow us to interpret sequences of words ▪ In order to create meaning, our brain interprets information using rules that specify meaning ▪ These rules are called grammar ▪ These are the unconscious rules that allow almost every person to speak and understand spoken language effortlessly o The psychological mechanisms that structure language are different from other cognitive abilities ▪ Many researchers believe that language is a manifestation of a powerful
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