Chapter 4. Genetics and Evolution
Chapter 4 Summary, Key Terms, and Self-Test
The biological perspective emphasizes bodily events and changes associated with behaviour. Biological psychology emerged from scientific and philosophical traditions of the 18th and 19th centuries, including the ideas of William James (1890), who argued that psychological science should be grounded in biology.
Biological psychology applies the principles of biology to the study of mental processes and behaviour. Psychologists in this framework study human behaviours that are both different and alike.
Genetic and evolutionary approaches inform the nature versus nurture debate and seek to determine the origins of behavioural traits as being either biological or due to environment. Both frameworks contribute to the question, ‘why do we behave the way we do?”
The genetic influence on behavior is a relatively recent discovery. Behavioural genetics is an interdisciplinary field concerned with how genes and the environment influence individual behaviour and traits including brain function.
Human genetic code is unique and sets us apart from other species, but the focus of this field is on the genetic bases of individual difference in how we think and act.
Our code involves genes, located on chromosomes, which are the rod-shaped structures found in the centre or ‘nucleus’ of every cell of the body. This genetic material is composed of thread-like strands of deoxyribonucleic acid or ‘DNA’. DNA refers to the chromosomal molecule that transfers genetic characteristics by way of coded instructions.
Cells that develop from the union of sperm and ova (egg) will produce a karyotype of 46 chromosomes arranged in 23 pairs that an individual will inherit at conception. 22 of the pairs are autosomal chromosomes and one pair consists of two sex chromosomes.
DNA contains the four nucleotides adenine, cytosine, guanine, and thymine. Each coded gene is a unique combination of the four nucleotides, abbreviated using the first letter of each (A, C, G, T).
Gene sequences are responsible for guiding the process that creates the structure of proteins. Genes provide the instructions to make the proteins that we need to carry out life functions. Proteins make up our physical structures and regulate development and physiological process throughout the lifespan.
Some genes contribute directly to the development of individual traits while others are inherited in the same way for all humans. Offspring will inherit a combination of dominant and recessive alleles, the variations of a given gene, from their parents. The variation is sometimes observable as in the case of the interaction and variation of the genes involved in eye color.
The sum of this inherited information is called genotype. Genotype represent the unique set of genes that comprise an individual’s unique code. Heredity also results in phenotype, which refers to the inherited physical traits and behavioural characteristics that show genetic variation in attributes like eye colour, height, and body weight, and personality and intelligence as well. Error in copying the original DNA sequence during division of sperm and egg cells are referred to as mutations that result from environmental hazards, toxins, and radiation.
Heredity and environment are constantly interacting to influence our psychological and physical traits. Genes affect the kind of experiences we have but experience also affects our genes.
Epigenetics is the study of heritable changes in gene expression that does not involve changes to the underlying DNA sequence. Epigenetic research seeks to understand the influence of genes on our behaviour and mental processes, and how the environment affects our genes, and influences their expression through biological mechanisms that switch them on and off. Epigenetics research also helps us to understand that the influence of nature and nurture on behaviour is more than an either-or equation.
While our genetic code is found in the nucleus of most of the cells in our body, cells also contain noncoding DNA that contributes indirectly to a trait by switching certain genes on or off over the course of a lifetime. Genes also contain information about environmental factors that influence whether a gene is ‘expressed’ or stays inactive. Whether a gene is expressed or not depends on heredity and environment.
Behavioural genomics is a complementary interdisciplinary field in the study of DNA, inherited traits, and the ways in which specific genes are related to behaviour. This perspective involves a shift in focus away from the influence of specific individual genes to genome, which refers to an organism’s complete set of genes in each cell with the exceptions of sperm and egg cells.
Genetics research involves looking at markers across complete sets of DNA or genome, in the cells of many people to identify genetic variations associated with a disease or disorder.
Behavioural genomics research questions the role of genomes, genetics, heritability, and environmental factors as main contributors to behaviour. Research in this framework addresses the differences between genotypes of groups of people in order to determine and better understand the relevance of genome to health conditions in the population.
Researchers examine the influence of genomes on behaviour to understand the heritability of traits, and which genomes cause certain conditions. The technology behind the Human Genome Project has opened doors for research on complex diseases and behaviours affecting human populations including mental illness
Researchers are also interested in the interaction of multiple genes and numerous environmental factors that influence human behaviour. The Human Genome Project, for example, involves classifying the sequences of billions of nucleotides (ACGT) making up the genes. Between 20,000 to 25,000 genes have been identified in scientific research so far.
Behavioural genetics researchers use the methods of twin and adoption studies to calculate heritability, which is a measure of variability of behavioural traits among individuals that can be accounted for by genetic factors. Variability in IQ scores, for example, can be denoted by a heritability coefficient, which is a statistic expressed as a number between zero and one that represents the degree to which genetic differences between individuals contribute to individual differences in a behaviour or trait found in a population. This method is used to compare people of different levels of relatedness and measure any similarities or resemblance for a specific trait of interest.
Behavioural genomics research also involves twin studies to identify specific genes than can be linked to behavioural phenotypes.
Linkage mapping is also used to indicate the order of genes on a chromosome in behavioural genetics research.
The Candidate Gene approach involves assessing the impact of genes on inherited disorders by comparing the genome of people who express a trait or behaviour with those who do not.
Whole-genome association like the Human Genome and HapMap studies are undertaken with the goal of better detection, treatment, and prevention of physical and mental disease.
While behavioural genetics and genomics perspectives focus on the roles of genes, heredity, and environment in explaining individual differences in behaviour, researchers in evolutionary psychology concentrate on the evolutionary mechanisms that might explain the commonalities that aid in our survival and reproductive success, including human cognition, development, emotion, and social practices.
The focus of the evolutionary perspective is on the genetic dispositions that shape our similarities relating to survival and reproductive success. It is an approach that interprets and explains modern human behaviour in terms of forces acting upon our distant ancestors.
Evolutionary psychology is a field of psychology that emphasizes the evolutionary mechanisms at work in the commonalities of human behaviour including cognition, emotion, development, and social practice.
An evolutionary approach aims to interpret and explain modern human behaviour in terms of how our brains and behaviours have been shaped by the physical and social environment encountered by our ancestors, and the forces that acted upon them.
Genes hold messages about the past and our shared evolutionary heritage. While researchers in the perspectives of behavioural genetics and genomics search for the influence of genetics on human differences, evolutionary psychologists look for the genetic bases of our similarities.
The theories of Charles Darwin have a profound influence on evolutionary psychology. Natural selection and sexual selection are theories developed through his observations of the fitness of a species’ characteristics to its environment.
Natural selection refers to the ability for a species to adapt to its environment, find food and water, and mate in order to stay alive long enough to reproduce and pass on genetic traits favorable to that setting. When this pattern continues, more offspring are born with adaptive traits that are ‘selected’ by reproductive success and spread throughout the species. In this theory, Darwin proposed that nature determines the fate of genes in terms of which genes survive and reproduce. Species without adaptive traits are more likely to die before being able to reproduce and if this pattern continues, the species will eventually disappear.
Gene selection theory is a modern theory of evolution by selection, that suggests differential gene replication is the defining process of evolutionary change. Evolution through natural selection requires a trait to be heritable, and individuals within the breeding population must have a reproductive advantage for having the trait. If there is an environmental pressure – a change in climate or shortage of food for example – a certain trait may be better suited to survival and this will produce an increase in that trait within a population.
Darwin’s theories can be applied to human behavior as well. All modern species are versions of the species that were the ‘fittest’ for their specific time and environment. Humans have several innate physical adaptations or traits that we are either born with or develop quickly thanks to the evolution of our species that enhance the chance of our survival. We have the physical ability to respond to temperature including sweat glands and shivering mechanisms. We crave foods rich in vitamins, fats, and sugars. We have a fear response to snakes, spiders, darkness, heights, and strangers.
Humans also have a ‘thinking’ capacity to solve inevitable problems with creative solutions regarding our survival and reproduction. The human brain contains specialized adaptations for performing certain cognitive and behavioural functions including perception.
What makes Homo sapiens stand out in evolutionary history is our roomy prefrontal cortex. This extra space enabled the development of more brain cells, which led to higher cognitive functions including logic and intelligence. The ability to calculate and ‘think’ our way out of challenging situations helped our ancestors overcome environmental problems.
Evolutionary useful behaviors have had a beneficial function in the cognitive development of our species. The brain, for example, has a set of cognitive adaptations for solving problems related to survival and reproductive fitness. A guiding assumption among many evolutionary psychologists is that the human mind developed as a collection of mental modules to help handle specific survival problems.
Error management theory (EMT) is a theory that deals with the evolution of how we think and evaluate certain situations to minimize costly mistakes that can cause death or damage, and impact on our survival and the ability to reproduce. We psychologically adapt to make choices that minimize a ‘cost’ in judgement, or error. Visual descent illusion, auditory looming bias, and sexual overperception bias are all examples of psychological adaptations in this regard.
All modern species are versions of the species that were the ‘fittest’ for their specific time and environment. Humans have several innate traits that we are either born with or develop quickly thanks to the evolution of our species. We pass physical and behavioural traits from one generation to the next through sexual reproduction. The ability to anticipate and solve problems helped our ancestors overcome food shortages and other environmental problems. Individuals with these advantageous traits are more likely to survive and reproduce offspring with similar genes.
Humans also have adaptations for reproduction as proposed in Darwin’s sexual selection theory. Sexual selection theory attempts to explain the fate of genes by suggesting that certain traits evolve to help some individuals increase their chances of mating and passing on their genes. Members of the same sex will compete for access to the other sex in a process called intrasexual selection.
Intersexual selection refers to the influence of physical factors signalling reproductive health and fitness, including a preference for youthful and beautiful females for males, and a preference for tall and muscular males by females.
Cultural factors are also important influences for female preferences including love, kindness, and social security.
Evolutionary research on attraction also highlights the importance of facial symmetry as an indicator of reproductive health in mate selection and reproduction.
Members of the same sex will compete for access to the other sex in a process called intrasexual selection.
Sexual strategies theory is a comprehensive evolutionary theory of human mating that defines the menu of mating strategies humans pursue, the adaptive problems women and men face when pursuing these strategies, and the evolved solutions to these mating problems.
Sexual overperception bias is a mating theory that suggests that males often misread sexual interest from women to prevent the costs of missing out on an opportunity for reproduction.
Evolutionary research on attraction highlights the importance of facial symmetry to mate selection and reproduction. Humans are genetically programmed to have, and to recognize symmetrical features. There is no sex difference in mate selection behaviour in this context.
An evolutionary psychologist would suggest that an individual with a symmetrical face is more likely to be fertile and in possession of good genes. Symmetry signals health to a potential mate, however, sometimes disease or environmental factors slightly alter facial symmetry.
Sociobiology contends that evolution has given us a genetic tendency to act in ways that maximize our chances of passing on our genes onto the next generations. Psychological traits are thought to be ‘selected’ to aid individuals in propagating their genes. Male and female problem-solving skills sometimes differ because of different survival issues. Physical characteristics often play a role in initial attraction and influence our desire to approach. For example, humans show a preference for body symmetry in mate selection. Sex role socialization appears to be another factor in mate selection in American samples.
One notable field is the study of sex differences in cognitive abilities examined in the framework of hunter-gatherer theory. Hunter-gatherer theory illustrates sex differences and suggests that our labour was divided based on sex as a means of survival. This theory suggests that males hunted, and females gathered because of physical and behavioural skills that are reproductively fit to this environment. Also, that the competencies selected for during the process of human evolution are still present today, including cognitive abilities and tasks with varying results.
The hunter-gather theory of human spatial sex differences starts from the premise that most of our time on earth has been spent living on the lands of the African savannah. In this environment, our labour was divided based on sex. Because of their size and strength, males took on the role of hunter and tracker of animals, and females remained closer to the home to care for children and gather plants and medicines rich in the vitamins needed for health.
Sex roles required individuals to use certain behavioural skills or ‘competencies’ needed to thrive and survive in their environment). Males required stamina and good wayfaring, orienteering, and spatial skills to be able to travel great distances without getting lost, as well as good coordination for throwing weapons used in the hunt. Females had to have good memory for the location and variety of edible plants. The theory further suggests that males with good spatial skills and females with good location memory would have been more successful or ‘fit’ to this environment, than males and females without these skills.
Biopsychosocial theory takes a complex approach to understanding human behaviour. Aspects of biology (genes), psychological components (thoughts, personality, mood), and social conditions (family support, stress, culture) are all considered in analyses of why we do what we do from this perspective. Research on specialized adaptation also relates to how the brain processes attraction. Cross-cultural studies have found “universals” in feelings of attraction.
Research in the evolutionary perspective also applies the principles of biology to the study of human behaviour. Evolutionary psychologists start from the position that cognitive structures are designed by natural selection to serve survival and reproduction. From this perspective, hearing, smell, vision, pain, and motor control are examined as functions of the nervous system that have been involved in survival and reproduction for thousands of generations and years.
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