PSYCH 3F03 Chapter Notes - Chapter 1-4: Squirrel Monkey, Follicular Phase, Sifaka

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PSYCH 3F03 – Readings
1,1 – (Gregory, T.R 2009): Understanding Natural Selection:
Essential Concepts and Common Misconceptions.
Introduction
Natural selection  non-random dierence in reproductive
output among replicating entities, often due indirectly to
dierences in survival in a particular environment
oMain process responsible for complexity and adaptive
intricacy of the living world
Concepts of evolution by natural selection are far more dicult
for students to grasp than most biologists imagine
The Basis and Basics of Natural Selection
First explanation of natural selection was by Darwin in the Origin
of Species, which can be broken down into 5 direct observations
and 3 associated inferences
oObservations:
Ecology
Populations have the potential to increase
exponentially
Populations generally remain stable once they
reach a certain size
Natural resources are limited
Heredity
Individuals in a population are not identical,
they vary in many characteristics
Many characteristics are heritable
oInferences:
Not all ospring that are produced survive and
reproduce because of struggle for resources
Some individuals are more likely to survive and
reproduce than others because of their heritable
traits
EVOLUTION BY NATURAL SELECTION
dierences in survival and reproduction among
individuals are non-random
with some traits being
passed on at a higher rate than others and
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increasing in proportion
in the population from one
generation to the next
Populations have the capacity to increase in numbers
exponentially
oDarwin used elephants as an illustration of the principle of
rapid population growth
Why don’t species expand exponentially? Because most ospring
that are produced do not survive to produce ospring of their
own
oMost population sizes remain stable because not all survive
As Darwin notes, the discrepancy between number of ospring
produced and the number that can be sustained by available
resources creates a struggle for existence
Variation and Inheritance
Variation is a fundamental requirement for evolutionary change
Relatives are more similar to each other than to unrelated
members of the population
oCritical for natural selection to operate
Back then, the source of variation and mechanism of inheritance
was unknown but now we know its all due to the replication of
DNA sequences
oMutations cause new variations (either detrimental,
bene6cial or neutral)
Non-Random Dierences in Survival and Reproduction
Overproduction and limited resources create a struggle for
existence
Populations dier in terms of how many traits are passed from
parent to ospring
Some individuals possess traits that are key to surviving in
particular environments
The probability of a new variant to be passed on to the next
generation is NON-RANDOM if it impacts survival and
reproductive capabilities
oTwo step process: origin of variation by random mutation
and non-random sorting of variation due to its eects on
survival/reproduction
Forms the basis of natural selection
Darwinian Fitness
The Meaning of Fitness in Evolutionary Biology
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Genotype  gene complement
Phenotype  physical and behavioural features
Darwinian 6tness  measure of the total
reproductive output
of
an organism
with a particular
genotype
oMore ospring = higher 6tness
Survival of the Fittest is Misleading
First problem: in Darwin’s context, 6ttest implied best suited to a
particular environment rather than most physically 6t but this
crucial distinction is overlooked in non-technical usage
Second problem: places undue emphasis on survival
oSurvival is ONLY important in that it aects the number of
ospring produced
Third problem: implies an excessive focus on organisms, when in
fact traits or underlying genes equally can be identi6ed as more
or less 6t than alternatives
Fourth problem: the phrase is misconstrued as being circular
(one leads to the other which leads back to the 6rst)
Fitness can be predicted and measured
Which Traits are the Most Fit?
Relative 6tness of dierent traits depends on current
environment
Fitness refers to reproductive success relative to alternatives
here and now
oNatural selection cannot increase the proportion of traits
solely because they may someday become advantageous
Natural Selection and Adaptive Evolution
Natural Selection and Evolution of Populations
None of the observations or inferences that underlies natural
selection is sucient individually to provide a mechanism for
evolutionary change
Genetic variatbion will not result in natural selection unless there
is some impact on survival or reproduction
Certain traits will
on average
be passed on from one generation
to the next at a higher rate than existing alternatives in the
population
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