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DO NOW
• Why does natural selection act on
the phenotype rather than the
genotype of the organism?
• How does natural selection
determine the different survival of
different organisms?
AIM
• Notes on genotype phenotype
• Genetic drift
• Trait wheel
• Homework
• Notes Chapter 15
• Friday Evolution Project oral presentation
Generic Bell Curve for
Polygenic Trait
Frequency of Phenotype
Section 16-1
Phenotype (height)
Figure 16–2 Relative
Frequencies of Alleles
Section 16-1
Gene Pools
Frequency of Alleles
48%
heterozygous
black
16%
homozygous
black
36%
homozygous
brown
allele for
brown fur
allele for
black fur
Figure 16–3 Phenotypes
for Single-Gene Trait
Section 16-1
Widows peak allele is dominant
Frequency of Phenotype
(%)
100
80
60
40
20
0
Widow’s peak
Phenotype
No widow’s peak
Section 16-2
Genetic Drift
Random change in allele frequency
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Section 16-2
Genetic Drift
In small populations an allele may become popular and common in population
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Section 16-2
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Section 16-2
DO NOW
• . . . All the Help I Can Get
• Natural selection operates on traits in
different
ways.
You
might
be
able
1. Choose an animal that you know something about, such as a deer,
and write
its namewhich
at the top traits
of a sheetnatural
of paper. Then,
divide your
to
predict
selection
paper into two columns, and write the heading Trait in one column and
would
favor
if you think about the
Advantage in
the other.
demands of an organism’s environment.
2. Under Trait, write in several of the animal’s traits.
3. Under Advantage, write in how you think the trait would be helpful to
the animal.
Section 16-2
•
Section Outline
16–2 Evolution as
Genetic Change
A.Natural Selection on Single-Gene
Traits
B.Natural Selection on Polygenic
Traits
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
C.
Genetic Drift
D.
Evolution Versus Genetic
Equilibrium
Natural Selection
on polygenic traits
• There are three ways that natural selection
affects the phenotype of an organism
Section 16-2
Figure 16–6 Graph of
Directional Selection
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Section 16-2
Figure 16–7 Graph of
Stabilizing Selection
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Figure 16–8 Graph of
Disruptive Selection
Section 16-2
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Section Outline
Section 16-3
•
16–3 The Process
of Speciation
A. Isolating Mechanisms
1. Behavioral Isolation
2. Geographic Isolation
3. Temporal Isolation
B. Testing Natural Selection
in Nature
1. Variation
2. Natural Selection
3. Rapid Evolution
C.Speciation in Darwin’s
Finches
1. Founders Arrive
2. Separation of Populations
Go Online
•Links from the authors on restricting
use of antibiotics
•Interactive test
•For links on population genetics, go to
www.SciLinks.org and enter the Web
Code as follows: cbn-5161.
•For links on speciation, go to
www.SciLinks.org and enter the Web
Code as follows: cbn-5163.
Section 16-1
Interest Grabber
• Yes, No, or Maybe
• Some traits, such as a widow’s peak, fall
into neat categories: You either have a
widow’s peak or you don’t. Other traits,
such as height, aren’t so easy to
categorize.
Interest Grabber continued
Section 16-1
• 1. Make a list of physical traits that you
think are influenced by genes.
Then, write next to each trait whether
you have the trait or not
(e.g., a widow’s peak) or whether there
are many variations of the
trait (e.g., hair color).
• 2. Are most of the traits you listed
clear-cut or are they mostly traits