Cindy Zhu, PSYC271 Fall 2011 Page 1 of10
Chapter 2: Evolution, Genetics, and Experience
• Zeitgeist: the general intellectual climate of our culture, which affects the way we think,
including about the biological bases of behaviour which may be inconsistent with facts
2.1 Thinking about the Biology of Behaviour: From Dichotomies to Interactions
Is it physiological, or is it psychological?
• Idea of this clear dichotomy has a long history in many cultures: in Western culture rose
to prominence after a 17 century conflict between science and the Roman Church.
• Descartes advocated philosophy of universe made of two parts: 1) physical matter
(according to laws of nature, scientific investigation, including the brain) and 2) mind (soul,
self, spirit which controls human behaviour obeys no natural law; mind is separate from the
brain; only humans have minds).
• This is Cartesian Dualism, and being sanctioned by the Church became widely
• Consequences: beliefs in spirit world, ghosts, psychokinesis, life after death, telepathy,
clairvoyance. Where’s the evidence? Research in parapsychology in 1930s-40s yielded
Is it inherited, or is it learned?
• Nature-nurture issue: early
experimental psychologists focused on
nurture side, including John B. Watson,
father of behaviourism.
• Ethology in Europe led to focus on
instinctive behaviours and emphasis on
the role of nature.
Problems with Thinking about Biology of
Behaviour in Traditional Dichotomies
• Difficulty with Physiological/Psychology
Thinking: evidence against
o Brain damage affects mental
experience: Oliver Sack’s account
of “the man who fell out of bed” due
to asomatognosia, a deficiency in
awareness of parts of the body
caused by right parietal lobe
o Brain stimulation affects
mental experience: Wilder Penfield
when treating epilepsy with surgery,
and Robert Heath when treating Cindy Zhu, PSYC271 Fall 2011 Page 2 of10
schizophrenia and depression – lightly stimulate parts of exposed brain with
electricity, and reported patient experience.
o Drug s affect mental experience: alcohol (elevated mood, impaired judgement,
short attention); cocaine; mescaline (visual hallucinations); marijuana (mood
changes, perception and motivation)
o Mental experiences affect brain images: Looking at fMRI images of different
expressions of disgust, different intensity of activation at right interior insula (behind
cortex of temporal/frontal lobe) – shows mental experience is correlated with brain
o Nonhuman species have psychological abilities too: G.G. Gallup’s research
on self-awareness in chimpanzees: when confronted with a mirror image of itself,
can become the object of its own attention (groom and inspect parts of their body,
making faces and unusual postures). Test with being painted red on its forehead and
ear, showed mark-directed responses to touch and inspect.
Nature-or-Nurture Thinking Runs Into Difficulty
• Factors other than genetics and learning influence behavioural development: fetal
environment, nutrition, stress, sensory stimulation broadening concept of nurture, then
thinking about which are influenced by nature and which by nurture, instead of
abandoning the dichotomy and focusing on the interaction between genetics and
• 1) Neurons become active long before being fully developed; 2) subsequent course of
their development depends greatly on their activity (triggered by external experience); 3)
experience continuously modifies genetic expression
• Interaction between three factors: 1) genetic endowment, product of evolution; 2)
experience; 3) perception of current situation
2.2 Human Evolution
• Darwin: On the Origin of Species (1859) was not first to suggest that species evolve
(undergo gradual orderly change) from pre-existing species, but first to amass a large
body of supporting evidence to suggest how evolution occurs. Why are some traits
passed down, and not others?
o 1) Documented evolution of fossil records through geological layers
o 2) Described striking structural similarities among living species suggesting
evolution from common ancestors (homologous structures)
o 3) Pointed to major changes in domestic plants and animals by selective
breeding, e.g. dogs and their slightly different alleles
o 4) Direct observation of rapid evolution (Grant and Galapagos finches)
o Supporting evidence is abundant, various, ever increasing and easily available.
• Natural selection: heritable traits which are associated with high rates of survival and
reproduction will be the most likely ones to be passed on. Over generations, this leads to
species better adapted to their niche. Fitness is the ability of an organism to survive and
contribute its genes to the next generation. Cindy Zhu, PSYC271 Fall 2011 Page 3 of 10
Evolution and Behaviour
• Important behaviours: find food, avoid predation, defend one’s young, etc.
• Species: a group of organisms reproductively isolated from other organisms; may form
new species when a reproductive barrier occurs until cross-fertilization becomes
impossible – such as geographic or behavioural.
• Social dominance: established among males through combative encounters (posturing,
threatening, fighting) in a hierarchy. Important because dominant males copulate more
and are more effective in passing on their genes. High-ranking females (chimpanzees)
produce more and healthier, longer-living offspring (due to access to productive food
• Courtship display: Interchanging and reacting to signals between the sexes, until
copulation occurs. Developing different courtship displays leads to behavioural
reproductive barrier between themselves and rest of conspecifics (members of the
same species) since only the right exchange of displays leads to reproduction; may lead
to evolution of a new species.
Course of Human Evolution
• Evolution of Vertebrates: complex water-dwelling multicellular organisms first appeared
600 MYA, and first chordates (with dorsal nerve cords) appeared 450 MYA. First
chordates with protective spinal bones – vertebrates – evolved 425 MYA and were
primitive bony fishes. There are links between fish and land vertebrates with scales,
teeth and gills, but also primitive wrists and finger bones (dated around 375 MYA).
• Evolution of Amphibians: 410 MYA first bony fishes venturing to land had advantages of
escaping from stagnant pools to nearby fresh water, and new terrestrial food sources –
led to first amphibians 400 MYA. Larval amphibians must live in the water, only adults
can go to land.
• Evolution of Reptiles: 300 MYA evolved from a branch of amphibians, first to lay shell-
covered eggs and be covered in dry scales (reducing water loss). Unlike amphibians, do
not have to spend first stage of life in pond or lake.
• Evolution of Mammals: 180 MYA mammals evolved from a line of small reptiles – fed
young with secretions of mammary glands, stopped laying eggs (except platypus) and
instead nurture young in utero. This increased survival value with long-term security and
o Around 20 orders of mammals, including primates, which have about a dozen
families. Apes (gibbons, orang-utans, gorillas, chimpanzees) with long arms and
grasping hind feet for arboreal treetop travel, but unlike Old-World monkeys have
no tails and can walk upright for short distances.
• Emergence of Humankind: We are primates of the family hominins. Fossil evidence to
reconstruct the events of our evolution is sparse, but we have some knowledge:
o Australopithecines evolved 6 MYA in Africa from apes, before coming extinct 1
MYA; they were short with small brains, but walked upright. Cindy Zhu, PSYC271 Fall 2011 Page 4 of 10
o First Homo evolved from Australopithecus about 2 MYA, with larger brain cavities
and fire and tool use. 200,000 YA Homo sapiens appeared, and began to migrate
out of Africa 50,000 YA.
o Although attributes of large brain, upright posture and free opposable thumb
have been in Homo for hundreds of thousands of years, most accomplishments
Thinking about Human Evolution
• Evolution does not proceed in a single line – a dense bush rather than a ladder.
• We have little reason to claim evolutionary supremacy as humans. We are the last
surviving species of a family that has existed for only a blip of evolutionary time.
• Evolution does not always proceed slowly – rapid changes can be triggered by sudden
environmental changes or adaptive genetic mutations. Human evolution may have been
accelerated by sudden cooling of earth and decrease in African forests.
• Few products of evolution survive – less than 1% of all known species still exist.
• Evolution is a tinkerer, not an architect, and does not progress to preordained perfection.
Although increases in adaptation are improvements, they are not perfect designs.
• Not all existing structures or behaviours are adaptive – spandrels are incidental non-
adaptive evolutionary by-products (e.g. human belly button).
• Exaptations are characteristics evolve to perform one function later co-opted to perform
another, e.g. bird wings were originally limbs for walking.
• Similarities among species do not necessarily mean common evolutionary origins: can
be homologous or analogous (similar with different evolutionary origin) resulting from
convergent evolution (evolution in unrelated species of similar solutions to same
Evolution of the Human Brain
• Brain size and intellectual capacity are not closely related – humans rank far behind
whales and elephants, and brains of acclaimed intellectuals (Einstein) were not
remarkable. Larger bodies require larger brains to regulate and control, not related to
• What about brain weight as percentage of body weight? We lose to shrews.
• More reasonable approach is to compare evolution of different brain regions: brain stem
regulates reflex activities critical for survival, cerebrum for complex adaptive processes
like learning, perception, and motivation.
• The human brain has increased in size from evolution, most of it in the cerebrum, and
increase in number of convolutions (folds) to increase volume of cerebral cortex.
• May be due to increase in social complexity due to dependent young (food sharing,
cooperative defense, communication).
• Fruit-eaters tend to be above regression line for brain weight/body weight, versus plant
eaters: easier to derive nutrition, requires larger brain activity in terms of planning,
identifying, collecting, etc. Eating plant means less energy going into digestion, more into
brain. Cindy Zhu, PSYC271 Fall 2011 Page 5 of 10
Evolutionary Psychology: Mate Bonding
• Evolutionary Psychology: try to understand human behaviours though consideration of
pressures that led to their evolution, including sex differences in mate bonding.
• Most species are totally promiscuous (mating arrangement in which members of both
sexes indiscriminately copulate with many different partners each mating period). Some
species form mating bonds (enduring mating relationships) with members of the other
• Mate bonding in mammals: small number of young, helpless and slow-developing, so
adaptive for males to stay and help promote successful development of offspring and
pass on his genes. Therefore, natural selection promotes a bond.
• Polygyny: one male forms mating bonds with multiple females – females make greater
contribution to rearing. Females can produce only so many offspring, thus important to
mate with fit males; for males which can sire many offspring, little pressure to be
selective in bonding.
• Polyandry: one female forms mating bonds with multiple males – when male
contribution to reproduction greater than female, such as for seahorses.
• Monogamy: enduring bonds between one male and one female. In species where
female can raise more fit young if she had undivided help from the male. Females have
limited reproductive ability, influencing mate strategies: female behaviours such as
driving other females away and other bond-encouraging behaviours will change the
optimal mating strategy for males – from bonding with many females to bond and stay
with one fit female and being protective.
Thinking about Evolutionary Psychology
• Theories that cannot be tested have little use: good theories of behavioural evolution
have predictions about current behaviours built in.
o E.g. human mate selection: men value youth and attractiveness (fertility) while
women value power and earning capacity; men are more likely than women to
2.3 Fundamental Genetics
• Dichotomous traits: traits that occur in one form or another, never in combination (seed
• Began experiments by usin