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Evolution of Populations
Chapter 16
What’s Today All About?

Essential Question:

How can genetic
variation play a role in
major evolutionary
changes?
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Objectives:
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Describe how natural
selection affect singlegene and polygenic
traits.
Describe genetic drift
Identify the five
conditions needed to
maintain genetic
equilibrium
Factors involved in the
formation of a new
species
Evolution as Genetic Change

Natural selection never acts directly on
genes—acts on the entire organism
Natural Selection on Single-Gene
Traits

Can lead to changes in allele frequencies
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Example: Black lizards absorb more
light warm up faster feed faster move
faster
Natural Selection on Polygenic
Traits
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Natural selection (best adapted) affects the
phenotype distribution in a population in
three different ways:
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Directional selection
Stabilizing selection
Disruptive selection
Directional Selection

When individuals at one end of the curve have a
higher fitness than individuals in the middle or at
the other end
Key
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Stabilizing Selection

Low mortality,
high fitness
Key
High mortality,
low fitness
Selection
against both
extremes
keep curve
narrow and in
same place.
Birth Weight
When individuals
near the center of the
curve have higher
fitness than
individuals at either
end of the curve
Disruptive Selection

When individuals at the upper and lower
ends of the curve have higher fitness than
individuals near the middle
Low mortality,
high fitness
High mortality,
low fitness
Beak Size
Population splits
into two subgroups
specializing in
different seeds.
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Speciation

New species can develop because of
natural selection and chance events that
cause reproductive isolation.

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Behavioral isolation—different bird songs
Geographic isolation—separated by rivers,etc
Temporal isolation—reproduce at different
times
Genetic Drift
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Random change in allele frequency
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Small populations that carry a particular trait
may leave more descendants than other
individuals, just by chance.
Chance occurrences of this type can cause
allele to become common in a population
Founder Effect

Situation in which allele frequencies
change as a result of the migration of a
small subgroup of a population
Evolution Vs. Genetic Equilibrium


Scientist want to know if under any
conditions evolution will NOT occur.
Hardy-Weinberg Principle—allele
frequencies will remain constant unless one
ore more factors cause those frequencies to
change
Maintaining Genetic Equilibrium
1.
2.
3.
4.
5.
Random Mating
Large Population
No Movement in or out
No Mutations
No Natural Selection
Process of Speciation

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Chance events and natural selection create
new species  speciation
To evolve a new species, gene pools of two
populations must become separated for
them to become new species

Become reproductively isolated from one
another
Three Types of Isolation

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Behavioral isolation—different bird songs
Geographic isolation—separated by
rivers,etc
Temporal isolation—reproduce at different
times