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Chapter 2

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University of Toronto St. George

Chapter 2: Evolution, Genetics, and Experience  Thinking about the biology of behaviour: from dichotomies of interactions  Zeitgeist: the general intellectual climate of our climate  Cartesian dualism: the idea that the human brain and the mind are separate entities  Is it inherited or is it learned?  Nature vs. Nurture  Most psychologists are committed to the nurture part of n v. n  John B. Watson, the father of behaviourism, states that there is no real evidence of the behavioural traits  This started experimental psychology taking root in North America  But in Europe, this was around when ethology emerged: the study of animal behaviour in nature  European ethology focused on instinctive behaviours, unlike North American ethology.  This emphasised the role of nature in behavioural development  Nature vs. Nurture (physiological -or -psychological) criticisms:  Many demonstrations show that even the most complex psychological changes can be produced by damage to, or stimulation of, the brain  The man who fell out of bed:  Oliver Sacks is a patient suffering from asomatognosia (a deficiency in the awareness of parts of one’s own body)  Usually consists of the left side of the body due to damage to the right parietal lobe  Some nonhuman species, particularly primate species, have abilities that were thought to be confined to the human mind.  The case of chimps in mirrors:  G. G. Gallup’s research on self-awareness in chimpanzees shows that chimps show self-awareness.  Neural development  Neurons become active long before they are fully developed  The subsequent course of their development (if they survive, number of connections, etc.) depends greatly on their activity, much of which is triggered by external experience  Experience continuously modifies genetic expression  Behaviour is a product of three factors:  1. Genetic endowment (a product of evolution)  2. Experience  3. Perception of the current situation  Part 1: Human Evolution  Darwin’s theory of evolution:  Not the first to propose that we evolve (undergo gradual orderly change), but first with lots of supporting evidence. 3 main kinds of arguments:  1. Documented evolution of fossil records through progressively more recent geological layers  2. Described striking structural similarities among living species, suggesting that we evolved from a common ancestor  3. Pointed out the changes in animal and plants over selective breeding  Most importantly, he observed and tracked the evolution of finches on the Galapagos Islands  Natural selection- the heritable traits that are associated with high rates of survival and reproduction are the most likely to be passed on to future generations  Fitness- the ability of an organism to survive and contribute its genes to the next generation  Scientific theory- an explanation that provides the best current account of some phenomenon based on the available evidence  Social Dominance  The males of many species establish a stable hierarchy of social dominance through combative encounters with other males  This is important so that the dominant males are the ones that pass down their genes to contribute to a stronger offspring than the lower-ranking males would produce  Courtship Display  Thought to promote the evolution of new species  Species: a group of organisms that is reproductively isolated from other organisms  A new species can be formed by geographic isolation  But, a species can also form via a behavioural barrier, such as differing courtship displays  Course of Human Evolution  Evolution of Vertebrates  Complex multi-cellular water-dwelling organisms appeared 600 million years (my) ago (mya)  About 150my later, chordates evolved  Chordate- animals with dorsal nerve cells (large nerves that run along the centre of the back, or dorsum)  25my later, chordates with spinal bones to protect their dorsal nerve cords evolved, called vertebrates  First, there were only primitive bony fish, then 7 classes of vertebrates:  3 classes of fish  Amphibians  Reptiles  Birds  Mammals  Amphibians (frogs, toads, salamanders)  410mya fish ventured out of water  400mya fins and gills evolved into legs and lungs to make the first amphibians  Amphibians must stay in water during larval form; only adults can survive on land  Reptiles (lizards, snakes, turtles)  300mya the first reptiles evolved from amphibians  First vertebrates to lay shell-covered eggs and be covered in dry scales  Can live its whole life out of water, but spend the first stage of life in a watery environment  Scales help reduce water loss to allow terrestrial life  Mammals  180mya, in the height of the dinosaurs, mammals evolved  The females feed their young with mammary glands and eventually stopped laying eggs to nurture the young in a water environment within the body  There are 20 orders of mammals, we belong to the primate order  Primates  Apes (gibbons, orangutans, gorillas, and chimpanzees)  Evolved from a line of Old-World monkeys  Have long arms, grasping hind feet specialised for arboreal travel  Opposable thumbs  Emergence of Humankind  Of the family hominins which has two genera: Australopithecus and Homo  Australopithecus  Believed to evolve 6mya in Africa from a line of apes  Homo has 2 species: Homo erectus (extinct) and Homo sapiens (us)  Thought to evolve from Australopithecus 2mya  Then we came along, Homo sapiens: Large brain, upright posture, and free hands with opposable thumbs  Some evolution terminology:  Exaptations- evolved to form one function and were co-opted to form another  I.e.- bird wings; evolved initially with the purpose of walking  Homologous-structures similar due to evolutionary origin  Analogous- structures similar without evolutionary origin due to convergent evolution (similar evolution in unrelated species due to environmental similarities)  Evolution of the Human Brain  The idea that the size of the brain is related to intelligence ran into two problems:  1. Humans are considered the most intelligent species but don’t have the largest brains  2. The size of the brains of intellectuals are not seen as having a larger size or weight than the average human brain  Therefore, human intellect has no correlation to brain size or weight  Then, it was proposed that the brain weight as a percentage of total body weight may be a better measure for intellect  Humans (2.33%) therefore surpassed elephants (.20%), only to be surpassed by the shrew (3.33%)  A new approach: Comparing the evolution of brain regions  The evolution of the brain stem (reflex activity) and the cerebrum (more complex adaptive processes) should be considered separately  The brain has increased in size during evolution, but most of the increase has been in the cerebrum  An increase in the number of convulsions (folds on the cerebral surface) has greatly increased the column of the cerebral cortex (the outermost layer of cerebral tissue)  Some similarities between all brains:  All constructed of neurons and neural structures  The neural structures in the brain structure of one species can be found in the brain structure of related species  Understanding Mate Bonding  Most species have promiscuous mating habits (indiscriminate copulation by members of both sexes), but some species form mating bonds (enduring mating relationships)  Most mammals form mating bonds. Why?  Trivers’ theory (1972): Since females give birth to few, helpless, slow- developing young at a time, they must remain with the young for a relatively lengthy period of time. To ensure the spreading of one’s genes, males may stay with the female during this period. Therefore, natural selection promoted mammalian males who had a tendency to bond with the females with which they have copulated.  The most prevalent form of mate bonding is polygyny – one male forms mating bonds with several females  According to current theory, tendency to establish mating bonds with only the fittest males evolved in females of many mammalian species  Polyandry is the mating arrangement in which one female mates with more than one male  This does not occur in mammals, it only occurs when there is a shortage of females and surplus of males in a species  About 4% of mammals are monogamous, which is a mate-bonding pattern in which enduring bonds are formed between one female and one male  Thought to evolve in a species in which a female can raise more young (or fir young) if she had undivided help.  Are humans monogamous?  Part 2: Fundamental Genetics  Mendelian Genetics  Mendel studied reproduction and genetics in pea plants and came up with these things:  Dichotomous traits- traits that occur in one form or another, never in combination  True-breeding lines- breeding lines in which interbred members always produce offspring with the same trait  Dominant trait vs. recessive trait  Phenotype- an organism’s observable traits  Genotype- the genetic material passed on to the next generation  Gene- each inherited factor  Allele- two genes that control the same trait  Homozygous- when an organism possesses two of the same alleles  Heterozygous- when an organism possesses two different alleles  Chromosomes: Reproduction and Recombination  Chromosomes- threadlike structures in the nucleus of the cell, where the alleles lie  Humans have 23 chromosome pairs  Gametes- egg cells and sperm cells, produced by the cell division process called meiosis  Zygote- a fertilised egg cell  Genetic recombination and mitosis as well  Chromosome: Structure and Replication  Each chromosome has a double-stranded molecule of deoxyribonucleic acid (DNA), with each strand being a sequence of nucleotide bases attached to a chain of phosphate and deoxyribose. There are 4 nucleotide bases:  adenine (A), thymine (T), guanine (G), and cytosine (C)  Note that A and T attract to each other, while G and C attract to each other  Replication- 2 DNA strands unwind, then the exposed nucleotide bases attract their respective partners and voilá, there are two identical DNA strands where there used to be one  Errors in the replication process  Can lead to Down Syndrome  Accidental alterations, called mutations, which are often bad or neutral for the organism’s fitness, but occasionally positive  Sex Chromosomes and Sex-Linked Traits  Autosomal chromosomes- the majority of chromosomes, which come in pairs  Sex chromosomes- the pair of chromosomes that determine an individual’s sex  [Mammals] There are 2 X chromosomes in females, and 1 X and 1 Y chromosome in males  Sex-linked traits are traits that are influences by genes on the sex chromosome  The Genetic Code and Gene Expression  Structural genes are genes that contain the
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