Download Ch. 16 - Evolution of Populations

Chapter 16
What Darwin didn’t know….
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◦
How traits were inherited
What caused variations
Mutations – change in the DNA
Sexual reproduction
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
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◦
“gene shuffling”
crossing-over
Sexual reproduction by itself does not cause evolution to
occur…..
◦ Gene pool - All the genes (alleles) in a given
population.
 Population - A group of individuals of the same
species that interbreed.
◦ Relative Frequency - The number of times an allele
appears in a population relative to other alleles.
Sample Population
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
Frequency of Alleles
allele for
brown fur
allele for
black fur
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Evolution happens when the relative
frequency of alleles in a population changes.
◦ What can cause relative frequencies of alleles to
change?

Single-gene traits have 2 or 3 phenotypes.
◦ Natural selection can lead to changes in allele
frequencies.
Frequency of Phenotype
(%)
100
80
60
40
20
0
Widow’s peak
Phenotype
No widow’s
peak

Polygenic traits are more complicated; there
are many phenotypes.
◦ Natural selection affects the distributions of
phenotypes.
Frequency of Phenotype
Phenotype (height)
“normal distribution”
Key
Directional Selection
Food becomes scarce.
Low mortality,
high fitness
High mortality,
low fitness
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Disruptive Selection
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Low mortality,
high fitness
Number of Birds
in Population
Key
Largest and smallest seeds become more
common.
Beak Size

Natural selection is not the only source of
evolutionary change.
◦ Small populations migrating to a new habitat can
quickly cause a change in allele frequencies.
 This results in a small number of individuals having a
profound effect on gene frequencies.
 The is called genetic drift or the “founder effect”
 Darwin’s finches are an example of genetic drift.
Sample of
Original Population
Founding Population A
Founding Population B
Descendants

1.
2.
3.
4.
5.
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Five conditions for genetic equilibrium (HardyWeinberg principle):
Random mating
Very large population
No migration in or out
No mutations
No natural selection
All five conditions can’t exist at once, so
evolution must happen.

Formation of a new species.
1. Isolating mechanisms separate a founding
population
 Geographic isolation
 Behavioral isolation
 Temporal isolation
2. Natural selection and genetic drift causes
changes in the gene pool
3. Members of the two populations can no longer
interbreed and are reproductively isolated
Ex: Darwin’s finches
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species