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PSYB65 :Human Brain and Behavior
Evolution of Humans
Evolutionary Psychology: attempts to apply the principles of evolution to human
behavior to figure out what it means to be human.
It is difficult to examine behavior through a fossil however, a male skeleton would
show more fracture and dents, especially on the left side, compared to females
indicating the males did the fighting and were injured by a right-handed
Charles Darwin: assisted by Wallace, published the Origin of Species, which presents
the theory of evolution.
Tells us that all living creatures have been and continue to be the subject to
Evolutionary theory provides important insights into behaviors that also have
been subject to selection pressures
He sailed on the HMS Beagle and travelled to the Galapagos Islands for 5 years
examining the finches there for isolation, different beak size and function.
Using Linnaeus’ classification supported ideas of evolution
William Smith’s idea that the Earth was older than believed and that some
species have changed or gone extinct
Lyell’s idea that geological processes are still continuing, and
Thomas Malthus’s idea that populations grow exponentially until it reaches
carrying capacity they could propose the idea of natural selection.
1. Linnaeus Classification of organisms based on structure; observed that there
were communalities in structure among related species
2. Lyell & Smith : supported the notion that the earth was far older than previously
thought ; observed fossils and observed that some species change whereas
others extinct and that geological processes are still continuing
3. Thomas Malthus observed that food supplies affect population; survival of the
Historical Theory of Evolution
Variation: the differences in morphology that are characteristic of each individual.
Genetic variation involves differences in traits or alleles (form of a gene), caused
by meiosis, mutation, transposable elements, crossing over, etc.
An individuals phenotype is an expression of the genotype and environment
interacting, however the phenotype can change with the environment and
genotype only with mutation.
1 University of Toronto 2
PSYB65 :Human Brain and Behavior
Inheritance: passing differences in morphology from one generation to the next.
Differential Reproduction: organisms that are best suited for an environment will be
able to better survive and have a greater fitness than those that don’t.
Natural Selection: proposed by Darwin, the mechanism causing change over time.
This idea involves all individuals being unique and that any trait causing
reproductive advantage will be passed on and therefore magnified in the
It is the competition for survival and reproduction.
If such a trait exists and is selected for it is an adaptation and helps solve a
problem with the physical/chemical/social/developmental environment.
Sexual Selection: another mechanism of evolution, intersexual which involves an
organism selecting a mate based on specific traits (color, size, call, age etc), and
Intrasexual which involves members of the same gender competing for mates.
[Modern Theory of Evolution] also known as the modern synthesis, explains new
ideas that were not present in Darwin’s original theory of evolution.
This includes knowledge that traits are passed on by genes made of DNA and
that change occurs after a mutation.
Genes cause change at a molecular level but evolution occurs on a level of
populations and gene frequency, natural selection, genetic drift, and gene flow
(movement of genes in a population).
The similarities for there to be common features, such as eyes and bipedal gait
(walking upright on two legs) among members of our species
1. Certain environments select certain phenotypes and phenotypes
are an expression of the genotypes interacting the environment
2. Although genes occur at the level of the individual, evolutionary
change occurs at the level of population
3. Species represent different gene pools, rather than
fundamentally unique groups
Polygenic Traits: when multiple genes, which may be on different chromosomes, affect
a trait, for example eye color. These traits do not follow a simple dominant recessive
Evolution of the Nervous System
We must be cautious about directly implicating brain size with increased
intelligence “or humanness”
Although humans do not have the largest brains in the animal kingdom, we tend
to have one of the largest brains for the size of our body
Endocast: a mold of the inside of the skull, to allow for inspection of sulci and g