PSYA01H3 Chapter Notes -Branching Process, Natural Selection, Probability Distribution
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PSY Exam Prep – Chap 3
Alfred Russell Wallace devised the theory of natural selection at about the same time that
The voyage of the Beagle
•A 5 year journey for research
•Examined organisms from many different places. Especially Galapagos Islands
Darwin’s thinking about evolution can be traced to 4 insights:
•Species are not fixed- change over time
•Evolution is a branching process- all species came from 1 ancestor
•Evolution is continuous with gradual changes
•Evolution is based on natural selection
Variation and Competition determine reproductive success
•Variation: the differences found across individuals of any given species (genetic,
•Competition: different animals/species competing for food, mates, territory
Although evolution occurs over the long run, natural selection can produce important changes in
the short run- on the space of only a few years. Phenotypic variation (differences in beak size)
can produce important selective advantages that affect survival.
Natural selection works because the members of any species have different phenotypes/ because
these phenotypes are caused by different genotypes, successful individuals will pass on their
genes to the next generation. Over time, competition for food and resources will allow only the
best adapted phenotypes to survive, thereby producing evolutionary change.
Darwin’s natural selection could not explain inheritance
•6 years later Darwin’s famous book, Gregor Mendel conducted experiments with pea
Discovery of the structure of DNA: James Watson and Francis Crick
There are no genes for behaviour, only genes for physical/physiological processes that relate to
behaviour. Ex: no gene for learning, but there is a gene that controls Dopamine production-
which is known chemical linked to the learning process.
Protein synthesis is under polygenetic control: influences by many pairs of genes, not just one.
Sexual reproduction increases a species’ ability to adapt to environmental changes. Offspring are
more genetically diverse and may have genes that enable them to survive in a new environment.
These genes manufacture proteins that give rise to physical and behavioural differences that
allow the offspring to be more successful.
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