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The Major Forces of Evolution
Chapter 6
Reminders…
Exam 1 next Week
 Study Guide is up
 Zoo day set…......Nov. 5th
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Evolution
Evolution is a change in gene frequency in
a population over time
 What causes this change?
 Natural selection
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◦ Migration (gene flow)
◦ Genetic drift
◦ Mutation
Evolution
The first three result in changes in
frequency of genes in a population by
“redistributing” the existing alleles
 Mutation is the only one to introduce
new information.
 Another way to add new info is through
nonrandom mating
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Natural Selection
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Darwin and Alfred Wallace independently
formulated the theory of evolution
They discussed the struggle for existence,
extinction of species, adaptation and variation
He looked at the work of Malthus, who said
that resources are limited but population
growth is not
Darwin realized that selection acts upon an
individual
This led him to formulate the principle of
natural selection
Natural Selection
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Here are the steps:
◦ More individuals in a population are produced
that can survive
◦ There is variation and some individuals are better
adapted to their environment
◦ Individuals compete for limited resources
◦ The better adapted ones will survive and
reproduce, passing down their genes
◦ This results in gene frequencies changing over time
◦ If populations become isolated, this may result in
enough genetic differences to create new species
Natural Selection
For natural selection to occur, traits must
be:
 Variable: variation of genes is crucial for
selection
 Heritable: traits must be inherited
through genes passed by parents
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Natural Selection
Remember, natural selection acts on
individuals, not species
 Each individual has differential
reproductive success and this results in
a change in gene frequencies as well
 The fitness (reproductive success) of any
variation will change as the environment
changes.
 A result of natural selection is
adaptation
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Mutation
A heritable change in the genetic material
(DNA) is a mutation
 Natural selection, together with mutation,
can change gene frequency more rapidly
 Mutations are random
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Migration (Gene Flow)
Migration occurs constantly among
populations within a species
 Also need reproduction
 If we talk about a population being made
up of genes rather than individuals, we are
referring to a gene pool
 This is all the genes in the population
 As individuals of each species move
among populations, gene flow causes
frequencies to change
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Genetic Drift
One generation’s frequencies may not
accurately represent the frequency of it’s
previous generation’s genes
 Two causes are founder effect and
population bottleneck
 Genetic drift is similar to sampling error
because the smaller the sample, the less
likely it will represent the population at
large
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Genetic Drift
A small sub-population that separates
itself from the rest of the population to
“found” a new population will not
represent accurately the frequency of the
parent population
 this is founder effect
 If there is a drastic reduction in the
number of individuals in a population
 This is population bottleneck
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Genetic Drift
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This can result in
◦ A higher proportion of recessive genes
◦ A greater chance of 2 recessive alleles coming
together
◦ More recessively expressed traits
◦ Loss of genetic diversity
Hardy-Weinberg
Used to predict allele frequencies
 Uses a null hypothesis…tries to say
evolution is not happening, which shows
that it does
 Homozygous recessive is aa or rr
 Homozygous dominant is AA or RR
 Heterozygous is Aa or Rr
 Dominants are always expressed, even in
heterozygous form
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Hardy-Weinberg
Equation:
 p=all dominant alleles
 q=all recessive alleles
 p+q=1, or 100% of population
 p2+2pq+q2 = 1
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Assignment
We will practice Hardy-Weinberg
 Lab 6.1
 Self Test 6.1
 Maybe WS…..
 Test Review
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