PSYC271 Chapter 2: Evolution, Genetics, and Experience
2.1 Thinking about Biology of Behaviour: Dichotomies to Interactions
Biological or Psychological?
Cartesian Dualism: Philosophical position of Descartes, arguing that the
universe is composed of 2 elements: physical matter and human mind
Physical matter follows natural laws and is thus a suitable object for
scientific investigation. Human mind does not follow natural laws and is
thus an area for the church (1400s)
Gave rise to the distinction between biology and psychology (Dichotomy)
Inherited or Learned?
Behaviourism (Watson) committed to nurture
Instinctive Behaviours: behaviour occurring in all like members of a species,
even when there seems to have been no opportunity for them to have been learned
Issues With Thinking About Biology of Behaviour through Traditional Dichotomies
2 lines of evidence against physiologicalorpsychological thinking (the
assumption that some aspects of human psychological functioning are so complex
that they could not possibly be the product of a physical brain):
Even the most complex psychological changes (Ex. selfawareness,
emotion, etc.) can be produced by damage to, or stimulation of, parts of
the brain ▯showed that brain responsible for everything
Discovery that some species (primates) possess abilities that were once
thought to be solely human ▯showed that brains responsible for
psychological events, not god
Ex. Red dotmirror selfawareness test with chimps
Asomatognosia: deficiency in the awareness of parts of one’s own body.
Usually a result of damage to right parietal lobe (losing awareness of left side of
When looking at the naturenurture debate, it is important to study how the 2
interact together, not how much of each factor influences the behaviour.
Neurons become active long before they are fully developed.
The subsequent course of their development (ex. Number of connections they
form or whether or not they survive) depends greatly on their activity, much of
which is triggered by external experiences
Experience continuously modifies genetic expression
All behaviour is the product of interactions among 3 factors:
Organism’s genetics, which are a product of evolution
Organism’s perception of the current situation
2.2 Human Evolution
Darwin’s theory of natural selection/evolution. Presented 3 kinds of evidence:
Documented the evolution of fossil records through progressively more recent
geological layers Described striking similarities among living species, suggestion evolution from
a common ancestor (ex. Human arm vs. bat’s wing)
Pointed to the major changes that had been brought about in domestic plants and
animals by programs of selective breeding
Grant (1991) observed rapid evolution in progress among finches. After as short as 18
months, a drought had left only larger seeds on the island and increased beak size of
finches on the Galapagos islands
Fitness: ability of organism to survive and contribute its genes to the next generation
Evolution accepted by everyone who understands its evidence, and science.
Evolution and Behaviour:
Males of many species establish a hierarchy of social dominance through
combative encounters with other males
Differences between males closer to the bottom of the hierarchy tend to
be barely noticeable ▯learn that they can’t win and stop trying
Males higher up on the hierarchy copulate with more females and are
more likely to pass on their genes
Males higher up also more likely to produce healthier/more successful
Courtship often precedes copulating. A signal (visual, olfactory, etc) is
sent to the potential partner, partner responds, copulation ensues. If one
partner doesn’t respond as necessary, copulation is unlikely.
Species: Can breed with one another
Conspecifics: members of the same species
If courting display not properly performed, no copulation, and no genes
passed on. If several organisms of the same species develop new courting
approaches, they will only breed with each other, until they start to
genetically differ from the original group.
Course of Human Evolution:
Evolution of Vertebrates:
Complex multicellular waterdwelling organisms first appeared on the
earth about 600 million years ago. 150 million years later evolved into:
Chordates: animals with dorsal nerve cords
25 million years later, developed spinal cords.
Vertebrates: Chordates that posses a spinal cord
The first vertebrates were bony fish. Today, there are 7 classes of
vertebrates (including fish, mammals, birds, reptiles, etc)
Evolution of Amphibians:
About 410 million years ago, body fish started to venture out of water
Fish who could survive out of water for extended periods of time had
several advantages: could escape from stale pools into nearby fresh water,
could take advantage of terrestrial food sources
These advantages led to evolution of legs and lungs. Amphibians began
to appear 400 million years ago. Amphibians ▯frogs, toads, salamanders. All must live underwater at
youth; can only survive above water in adulthood.
Evolution of Reptiles:
Reptiles ▯snakes, lizards, turtles
Evolved from amphibians 300 million years ago.
Adaptations ▯scales and shellcovered eggs. This reduced reliance on
watery habitats (scales reduce water loss)
Don’t have to live underwater in first stage of life
Evolution of Mammals:
About 180 million years ago
Females of this new class fed young with secretions from mammary
glands. Eventually, young began to develop within watery environments
within the females, rather than eggs.
Platypus is one of very few surviving mammals that lays eggs
Developing inside mother advantages: environmental stability, and long
term security. Allowed for more complex developments to unfold
Today, 20 different orders of mammals. Humans belong to the ‘primate’
order. Although primates difficult to categorize because they do not share
a single characteristic that distinguishes them all from other animals.
5 different categories of primate: newworld monkeys, oldworld
monkeys, prosimians, apes, hominins (humans)
Chimps are the closest human relative ▯99% genes identical
Emergence of Humankind:
Hominins: primate order that includes humans
The human family is composed of 2 genera (plural of genus): Homo +
Homo is composed of 2 species: Homo Erectus (extinct) & Homo
Some fossil evidence found for each of these genera/species
Belief that australopithecines evolved about 6 million years ago in
Africa, from a line of apes (small brains, 3 ft tall, walk upright)
Homo species evolved from one species of Australopithecus about 2
million years ago. Coexisted in Africa until Australopithecus died out.
Brains bigger, but smaller than human .Used fire & tools.
200,000 years ago, Homo replaced by modern humans in Africa. About
50,000 years ago, humans began to migrate out of Africa.
Big 3 human attributes: large brain, upright postures, free hands with
Most human accomplishments are of recent origin (writing, painting,
Common Misconceptions About Human Evolution:
Evolution does not proceed in a single line. Not a ladder, but a dense bush
Humans have no evolutionary supremacy. We are the last surviving species of a
family (hominins) that has existed for only a blip of evolutionary time Evolution not always slow and gradual. Can occur rapidly with adaptive genetic
mutations or sudden environmental changes
Few products of evolution have survived to the present day – Fewer than 1% of
all known species are still in existence.
Evolution does not lead to perfection. Creates solutions to environmental
problems, but they are by no means the perfect design of a species.
Not all existing behaviours or structures are adaptive. Some are byproducts of
evolution that serve no adaptive purpose, called spandrels
Behaviours/structures that were once adaptive might become
nonadaptive, or maladaptive if environment changes
Not all existing adaptive characteristics evolved to perform their current
function. Exaptations (ex. Bird wings – limbs first evolved for the purpose of
Similarities among species fo not necessarily mean that the species have
common evolutionary origins.
Homologous: structures that are similar because they have common
Analogous: Similar structures that don’t have common evolutionary
Convergent Evolution: evolution in unrelated species. Similar solutions
to the same environmental demands, resulting in analogous structures
Evolution of the Human Brain:
Early research assumed that brain size and intellectual capacity were closely
related. Problems with this:
Humans don’t have the biggest brains (compared to whales, etc)
The brain sizes of highly ranked intellectuals were no bigger than ours
No clear relationship between brain size & intelligence. Also no relationship
between brain weight to body weight ratio (shrew example)
More reasonable approach: study evolution of different brain regions
Brain Stem: regulates reflex activities that are critical for survival
(HR, respiration, blood glucose levels)
Cererbum: involved in more complex processes like learning,
perception, and motivation
Comparing species’ brains makes 3 important points:
1. Brain has increased size over evolution
2. Most of the size increase can occurred in the cerebrum
3. Increase in convolutions (folds on cerebrum surface) has greatly
increased the volume of the cerebral cortex (outer layer of cerebrum)
Fundamental similarities between species brains:
All constructed of neurons
Neural structures found in brain of one species can usually be found in
the brains of other species. Also similar structures tend to perform similar
Evolutionary Psychology: Understanding Mate Bonding
In most species, mating is promiscuous Most mammals tend to form mating bonds ▯due to the fact that female
mammals give birth to small numbers of helpless, slowdeveloping offspring. It is
adaptive for the male to stick around and help with upbringing of offspring to
ensure their survival. ▯Similarly, selective pressures encourage females to act in
ways that will ensure a male will bond to them.
Polygyny: 1 male forms mating bonds with more than 1 female. Most common
mating arrangement in mammals.
Females tend to mate with only the fittest males. Since they can only
bear small numbers of offspring, females place emphasis on choosing fit
males that will ensure to provide resources during rearing, helping
offspring to survive. At the very least, the fittest males provide the highest
chances of providing the most resources.
Polygyny consists of selective bonding by females and nonselective
bonding by males (males try to have as many offspring as possible
because they can.)
Polyandry: mating arrangement in which one female forms mating bonds with
more than one male.
Nonoccurring in mammals. Occurs only in species where male
contributes more to reproduction than female (ex. Sea horse).
Large body size & aggression evolved in male mammals because females tend
to be more selective towards males with these features.
Monogamy: mating arrangement in which enduring bonds are formed between
1 male and 1 female.
About 4% of mammals
Females in the species using this mating approach developed behavioural
patterns designed to keep males bonded to them. (Ex. Not copulating with
a male until he has been with her for a given amount of time, driving other
females away from their males) ▯This made it harder for males to copulate
with many females, so the tendency in males to stick to one female to try
to ensure their offspring survive developed
Thinking About Evolutionary Psychology: