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Transcript
Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 16
How Populations
Evolve
Lecture Outline
See separate FlexArt PowerPoint slides
for all figures and tables pre-inserted into
PowerPoint without notes.
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Outline
• 16.1 Genes, Populations, and Evolution
• 16.2 Natural Selection
• 16.3 Maintenance of Diversity
2
16.1 Genes, Populations, and
Evolution
• A population is all the members of a single
species occupying a particular area at the same
time.
• Diversity exists among members of a population.
• Population genetics is the study of this
diversity in terms of allele differences.
 Evaluates the diversity of a population by studying
genotype and phenotype frequencies over time
3
Genes, Populations, and
Evolution
• In the 1930s, population geneticists began to
describe variations in a population in terms of
alleles
• Microevolution pertains to evolutionary
changes within a population.
 Various alleles at all the gene loci in all individuals
make up the gene pool of the population.
 The gene pool of a population can be described in
terms of:
• Genotype frequencies
• Allele frequencies
4
The Genetic Basis of Body
Color in the Peppered Moth
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Phenotype:
Genotype:
DD
homozygous
Dd
heterozygous
dd
homozygous
Alleles:
D
D
d
D
d
d
chromosome
5
Genes, Populations, and
Evolution
• Allele Frequencies
 The proportion of each allele within a population’s gene pool.
 Frequencies of the dominant and recessive allele must add up to
1.
• This relationship is described by the expression p + q = 1
• p is the frequency of one allele and q is the frequency of the other
 Microevolution involves a change in these allele frequencies
within populations over time.
• If the gene frequencies do not change over time, microevolution has
not occurred.
6
How Hardy-Weinberg Equilibrium
Is Estimated
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Gene pool:
Allele frequencies:
Population = 25 moths, 50 alleles
Equilibrium
genotype frequencies:
d = 40
D = 10
DD
DD
Dd
Dd
Dd
D D D D D d d d
D
Dd
Dd
Dd
Dd
Dd
D D D D d d d d
dd
d
P2 + 2pq + q2 = 1
p+q=1
Dd
dd
Dd
dd
dd
dd
d d d d d d d d
p = frequency of D
p = frequency of D
q = frequency of d
q = frequency of d
Frequency of D
Frequency of DD
p = 10/50 alleles
= 0.20
p2 = freq D2
= (0.2)(0.2)
dd
dd
dd
dd
d d d d d d d d
Frequency of d
q = 40/50 alleles
Frequency of Dd
= 0.80
2pq = 2(freq D x freq d)
= 2(0.20 x 0.80)
dd
dd
dd
dd
d d d d d d d d
= freq d 2
= (0.08)(0.80)
dd
dd
dd
d d d d d d d d
= 0.32
Frequency of dd
q2
dd
= 0.04
1.00
= 0.64
1.00
7
Genes, Populations, and
Evolution
• The Hardy-Weinberg Equalibrium states
that:
 Allele frequencies in a population will remain
constant assuming:
• No Mutations
• No Gene Flow
• Random Mating
• No Genetic Drift
• No Selection
8
Hardy-Weinberg Equilibrium
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
eggs
0.20 D
0.80 d
sperm
0.20
D
0.04 DD
0.16 Dd
0.16 Dd
Offspring
0.64 dd
0.80
d
9
Genes, Populations, and
Evolution
• Hardy-Weinberg Equilibrium:
 Required conditions are rarely (if ever) met
 Deviations from a Hardy-Weinberg equilibrium
indicate that evolution has taken place
• Analysis of allele changes in populations over time
determines the extent to which evolution has occurred.
10
11
Mechanisms of Microevolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
F1 generation
Allele
frequencies:
F2 generation
Genotype
frequencies:
Allele
frequencies:
Genotype
frequencies:
DD
D
d
0.20 D
0.80 d
D
Dd
dd
Conclusion:
DD
If...
d
Random mating
No selection
No migration
No mutation
p2 + 2pq + q2 = 1
0.20 D
...then we expect
DD = 0.04
Dd
dd
p2 + 2pq + q2 = 1
No change in allele frequencies
No change in genotype frequencies
Evolution has not occurred
DD = 0.04
0.80 d
Dd = 0.32
Dd = 0.32
dd = 0.64
dd = 0.64
DD
If...
D
Nonrandom mating
Dd
...then we observe
dd
d
X
DD
X
Dd
X
p2 + 2pq + q2 = 1
DD = 0.10
0.80 d
or
Dd
0.20 D
No change in allele frequencies
Genotype frequencies change
Evolution has not occurred
Dd = 0.20
DD
dd = 0.70
dd
If...
Selection
...then we observe
d
Dd
D
0.80 D
0.20 d
DD
p2 + 2pq + q2 = 1
Allele frequencies change
Genotype frequencies change
Evolution has occurred
DD = 0.64
Dd = 0.32
dd = 0.04
12
Genes, Populations, and
Evolution
• Causes of Microevolution
 Genetic mutations
• The raw material for evolutionary change
• Provide new alleles
• Some mutations might be more adaptive than
others
– Ex: Genetic mutations affecting pigment color in
peppered moths have provided the variation needed for
natural selection to occur
13
Animation
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
Genes, Populations, and
Evolution
• Causes of Microevolution
 Gene Flow (gene migration)
• Movement of alleles between populations when:
– Gametes or seeds (in plants) are carried into another
population
– Breeding individuals migrate into or out of population
• Continual gene flow reduces genetic divergence
between populations
15
Gene Flow
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
gene flow
Pisum arvense
Pisum sativum
Genes, Populations, and
Evolution
• Causes of Microevolution
 Genetic Drift
• Changes in the allele frequencies of a population due to
change rather than selection by the environment
• Does not necessarily lead to adaptation to the environment
• Occurs by disproportionate random sampling from population
– Can cause the gene pools of two isolated populations to
become dissimilar
– Some alleles are lost and others become fixed (unopposed)
• Likely to occur:
– After a bottleneck
– When severe inbreeding occurs, or
– When founders start a new population
• Stronger effect in small populations
17
Genetic Drift
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
10% of
population
natural disaster kills
five green frogs
20% of
population
18
Animation
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
19
Genes, Populations, and
Evolution
• Bottleneck Effect
 A random event prevents a majority of
individuals from entering the next generation
 The next generation is composed of alleles
that just happened to make it
20
Genes, Populations, and
Evolution
• Founder Effect
 When a new population is started from just a
few individuals
 The alleles carried by population founders are
dictated by chance
 Formerly rare alleles will either:
• Occur at a higher frequency in the new population,
or
• Be absent in new population
21
Bottleneck and Founder Effects
Change Allele Frequencies
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
b.
c.
Original population
gene pool = 3,800 alleles*
d.
Remnant population
gene pool = 90 alleles*
11%
13%
8%
26%
44%
45%
53%
*1 marble = 10 alleles
Genes, Populations, and
Evolution
• Nonrandom Mating
 When individuals do not choose mates
randomly
• Assortative mating:
– Individuals select mates with the same phenotype with
respect to a certain characteristic
– Individuals reject mates with differing phenotype
– Increases the frequency of homozygotes for certain loci
23
16.2 Natural Selection
• Adaptation of a population to the biotic and
abiotic environment
 Requires:
• Variation - The members of a population differ from
one another
• Inheritance - Many differences are heritable
genetic differences
• Differential Adaptiveness - Some differences affect
survivability
• Differential Reproduction – Some differences affect
the likelihood of successful reproduction
24
Natural Selection
• Results in:
 A change in allele frequencies of the gene pool
 Improved fitness of the population
• Major cause of microevolution
25
Natural Selection
• Most traits are polygenic - variations in the trait
result in a bell-shaped curve
• Three types of selection occur:
 (1) Directional Selection
• The curve shifts in one direction
– Bacteria become resistant to antibiotics
– Increasing body size in horse evolution
26
Natural Selection
• Three types of selection occur (cont):
 (2) Stabilizing Selection
• The peak of the curve increases and tails decrease
• Example - human babies with low or high birth
weight are less likely to survive
 (3) Disruptive Selection
• The curve has two peaks
• Example – British land snails vary because a wide
geographic range causes selection to vary
27
Three Types of Natural Selection
Number of Individuals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Phenotype Range
Number of Individuals
stabilizing selection
Peak narrows.
a.
Phenotype Range
Phenotype Range
directional selection
disruptive selection
Peak shifts.
b.
Two peaks result.
c.
28
Human Birth Weight
(stabilizing selection)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
100
70
50
15
30
20
10
10
7
5
5
Percent Infant Mortality
Percent of Births in Population
20
3
2
.9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5
Birth Weight (in kilograms)
29
Directional Selection
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
After More Time
Number of
Individuals
After Time
Number of
Individuals
Number of
Individuals
Initial Distribution
Body Size
Body Size
Body Size
a.
Hyracotherium
Merychippus
Equus
b.
30
Disruptive Selection
Initial
Distribution
Number of
Individuals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
After
Time
Number of
Individuals
Banding Pattern
After
More Time
Number of
Individuals
Banding Pattern
Banding Pattern
a.
b.
b: © Bob Evans/Peter Arnold/Photolibrary
31
Natural Selection
• Sexual selection - adaptive changes in
males and females lead to an increased
ability to secure a mate.
 Males - increased ability to compete with other
males for a mate
 Females choose to select a male with the best
fitness (ability to produce surviving offspring).
32
Natural Selection
• Female Choice
 Choice of a mate is a serious consideration because
females produce few eggs
• Good genes hypothesis: Females choose mates on the basis
of traits that improve the chance of survival.
• Runaway hypothesis: Females choose mates on the basis of
traits that improve male appearance.
• Male Competition
 Males can father many offspring because they
continuously produce sperm in great quantity.
 Compete to inseminate as many females as possible.
33
Dimorphism
• The drab
females tend to
choose
flamboyant
males as
mates.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
34
Sexual Selection:
Competition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Barbara Gerlach/Visuals Unlimited
35
Sexual Selection:
Competition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
b.
a: © Y. Arthus-Bertrand/Peter Arnold, Inc.; b: © Neil McIntre/Getty Images
36
Natural Selection
• A study of sexual selection among humans
shows that female choice and male competition
apply to humans too
 Women must invest more in having a child than men.
 Men need only contribute sperm
• Generally more available for mating than are women.
 More available men results in competition
 Men also have a choice
• Prefer women who are most likely to present them with
children.
37
16.3 Maintenance of Diversity
• Genetic Variability
 Populations with limited variation may not be able to
adapt to new conditions
 Maintenance of variability is advantageous to the
population
• Only exposed alleles are subject to natural
selection
38
Maintenance of Diversity
• Natural selection causes imperfect adaptations
 Depends on evolutionary history
 Imperfections are common because of necessary
compromises
• The environment plays a role in maintaining
diversity
 Disruptive selection due to environmental differences
promotes polymorphisms in a population
 If a population occupies a wide range, it may have
several subpopulations designated as subspecies
 The environment includes selecting agents that help
maintain diversity
39
Subspecies Help Maintain Diversity
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pantheropsis obsoleta obsoleta
Pantheropsis obsoleta quadrivittata
Pantheropsis obsoleta lindheimeri
Pantheropsis obsoleta rossalleni
Pantheropsis obsoleta spiloides
(E.o. lindheimeri, E.o. quadrivittata): © Zig Leszczynski/Animals Animals/Earth Scenes; (E.o. spiloides): © Joseph Collins/Photo Researchers, Inc.;
(E.o. rossalleni): © Dale Jackson/Visuals Unlimited; (E.o. obsoleta): © William Weber/Visuals Unlimited
40
Maintenance of Diversity
• Recessive alleles:
 Heterozygotes shelter recessive alleles from selection
 Heterozygotes allow even lethal alleles to remain in the
population at low frequencies virtually forever
 Sometimes recessive alleles confer an advantage to
heterozygotes
• The sickle-cell anemia allele is detrimental in homozygote
• However, heterozygotes are more likely to survive malaria
• The sickle-cell allele occurs at a higher frequency in malaria
prone areas
41
Maintenance of Diversity
• Heterozygote Advantage
 Assists the maintenance of genetic, and
therefore phenotypic, variations in future
generations.
 In sickle cell disease heterozygous individuals
don’t die from sickle-cell disease, and they
don’t die from malaria.
42
Sickle Cell Disease
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
malaria
sickle-cell
overlap of both
43