Download Sexual Selection - Cathedral High School

Chapter 16: pp. 283 - 298
BIOLOGY
10th Edition
How Populations
Evolve
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10% of
population
natural disaster kills
five green frogs
20% of
population
PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor
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1
DARWINIAN EVOLUTION
MEETS GENETICS
Darwin = no genetics
Today = The modern
synthesis
connects Darwin’s theory
of natural selection with
population genetics
INHERITANCE
2 genes inherited
different genes may be same
or different alleles
alleles determine genotype
genotype and environment
determines phenotype
natural selection acts on
phenotypes
POPULATION GENETICS

In 1930s population geneticists described 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.

Gene pool of a population:

Genotype

Allele frequencies
Industrial Melanism and
Microevolution
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Early observation
36% dark-colored phenotype
Later observation
64% dark-colored phenotype
10
Frequency of Gametes Calculation

From genotype frequencies, the allele and
gamete frequencies can be calculated
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genotypes
DD
frequency
From ofgenotype
frequencies,
genotypes
Dd
dd
the
allele and0.64
0.32
ingamete
the population
frequencies can be calculated
0.04
frequency of alleles and
gametes in the population
0.04 + 0.16
0.16 + 0.64
0.20 D
0.80 d
7
Hardy-Weinberg Equilibrium
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F1 generation
Genotypes:
DD
Dd
dd
Genotype frequencies:
0.04
0.32
0.64
Allele and gamete frequencies:
D = 0.20
d = 0.80
eggs
F2 generation
0.20 D
0.80 d
sperm
0.20
D
0.04 DD
0.16 Dd
0.16 Dd
0.64 dd
0.80
d
Offspring
Genotype frequencies:
0.04 DD + 0.32 Dd + 0.64 dd = 1
p + 2pq + q = 1
2
2
p2 = frequency of DD genotype (dark-colored) = (0.20)2
= 0.04
2pq = frequency of Dd genotype (dark-colored) = 2(0.20)(0.80)
= 0.32
q2 = frequency of dd genotype (light-colored) = (0.80)2
= 0.64
1.00
9
Hardy-Weinberg Theorem
describes and predicts genotype and allele frequencies in a non-evolving
population (equilibrium)
Evolution = change in allele frequencies in a population
– hypothetical: what conditions would NOT cause allele frequencies to change?
– non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
So, what is the point of discussing
Hardy Weinberg?
• The Hardy-Weinberg formulas allow scientists to
determine whether evolution has occurred.
• Any changes in the gene frequencies in the
population over time can be detected.
• It allows us to compare how drastic or not changes
are compared to what it could have been without
evolution
• Then compare to other organisms
Hardy-Weinberg Equation
p2 + 2pq + q2 = 1 and p + q = 1
•p = frequency of the dominant allele in the population
q = frequency of the recessive allele in the population
•p2 = percentage of homozygous dominant individuals
q2 = percentage of homozygous recessive individuals
2pq = percentage of heterozygous individuals
G.H. Hardy
mathematician
W. Weinberg
physician
Using Hardy-Weinberg equation
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
population:
100 cats
84 black, 16 white
How many of
each genotype?
p2=.36
BB
2pq=.48
Bb
q2=.16
bb
Must assume population is in H-W equilibrium!
What are the genotype frequencies?
5 Causes of evolutionary change
Genetic Drift
Gene Flow
Mutation
Non-random mating
Selection
1. GENETIC DRIFT
• Genetic drift:
changes in the gene
pool of a small
population due to
chance (usually
reduces genetic
variability)
• types = bottleneck
effect and founder
effect
GENETIC DRIFT
Bottleneck Effect
• type of genetic drift
resulting from a
reduction in population
(natural disaster) such
that the surviving
population is no longer
genetically
representative of the
original population
Ex: Tay Sachs
Jewish Middle
Ages
GENETIC DRIFT
Founder Effect
Ex: Polydactyly in Amish
• a cause of genetic drift attributable to
colonization by a limited number of
individuals from a parent population
– just by chance some rare alleles may
be at high frequency;
others may be missing
– skew the gene pool of
new population
• human populations that
started from small group
of colonists
• example:
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
17
2. GENE FLOW
• genetic exchange due to
the migration of fertile
individuals or gametes
between populations
(reduces differences
between populations)
• seed & pollen distribution by
wind & insect
• migration of animals
3. NON RANDOM MATING
• Individuals choose
their mates
• Sexual selection
• inbreeding and
assortive mating (both
shift frequencies of
different genotypes)
4. MUTATIONS
• Mutation creates
variation = a change in
an organism’s DNA
(gametes; many
generations); original
source of genetic variation
(raw material for natural
selection)
5. NATURAL SELECTION
Fitness depends on
•climate change
•food source availability
•predators, parasites, diseases
•toxins
• only form of microevolution that
adapts a population to its environment
• combinations of alleles
that provide “fitness”
increase in the population
Outcomes of natural selection
Frequency of
individuals
Original
population
Original
population
Evolved
population
Stabilizing selection
Phenotypes (fur color)
Directional selection
Diversifying selection
Figure 13.19
Effects of Selection
DIRECTIONAL
SELECTION
giraffe neck
STABILIZING
SELECTION
human birth weight
DISRUPTIVE
SELECTION
weeds
Disruptive
Selection
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Forested
areas
Low-lying
vegetatio
© Bob Evans/Peter Arnold, Inc.
29
Directional Selection
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No
predation
All guppies
are drab
and small
Amount of Color
Low
predation
above waterfall
High
predation
0
below waterfall
Experimental site
4
Months
8
12
Result
© Helen Rodd
26
Natural Selection
• Selection acts on any trait that affects
survival or reproduction
– predation selection
– physiological selection
– sexual selection
Variation & Natural Selection
• Variation is the raw material for natural
selection
– there have to be differences within population
– some individuals must be more fit than others
Sources of Variation in a Population
Mutations
Sex
Geographic Variation
Diploidy
Polymorphism
Heterozygote Superiority
Frequency Dependent Selection
Evolutionary Neutral Traits
Population Variation
– random changes to DNA
Beak depth
• Mutation
Wet year
Dry year
Dry year
• errors in mitosis & meiosis
1977
Dry year
1980
1982
1984
• environmental damage
• Sex
– mixing of alleles
• recombination of alleles
• new combinations = new phenotypes
– spreads variation
• offspring inherit traits from parent
Beak depth of
offspring (mm)
11
10
9
8
Medium ground finch
8
9
10
11
Mean beak depth of parents (mm)
Independent Assortment
Key
Maternal set of
chromosomesPossibility 1
Paternal set of
chromosomes
Two equally probable
arrangements of
chromosomes at
metaphase I
Possibility 2
Metaphase II
Daughter
cells
Combination 1Combination 2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Combination 3Combination 4
Population variation
• Geographical
variation: differences in
location causing
differences in individuals
partly due to environment
• Diploidy
2nd set of chromosomes
hides variation in the
heterozygote
Population Variation
• Prevention of natural selection’s reduction of
variation
Polymorphism: coexistence of 2 or
more distinct forms of individuals
(morphs) within the same population (light
and dark crabs)
•
1- heterozygote advantage
– AA = normal hemo; malaria
– Aa = normal hemo; malaria resistant
– aa = sickle cell
2- frequency dependent selection (survival &
reproduction of any 1 morph declines if it becomes
too common; i.e., parasite/host)
Population Variation
• Fingerprints - Not all genetic variation may be subject to natural
selection
• Some variations may be neutral, providing no apparent advantage
or disadvantage
– Example: human fingerprints
Figure 13.16
Sexual Selection

Female Choice

Choice of a mate is serious consideration



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

Can father many offspring because they continuously
produce sperm in great quantity.

Compete to inseminate as many females as possible.
30
Sexual Selection

Sexual selection adaptive changes in
males and females to increase ability to
secure a mate.
 Males
- ability to compete
 Females
choose to select a male with the
best fitness (ability to produce surviving
offspring).
31
Sexual Selection
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The drab females
tend to choose
flamboyant
males as mates.
32
Sexual Selection: Competition
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a.
b.
a: © Y. Arthus-Bertrand/Peter Arnold, Inc.; b: © Neil McIntre/Getty Images
33

Sexual Selection in Humans
Study 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
 Men
men = competition
Also Have a Choice
 Prefer
women who are most likely to
35
Natural selection cannot fashion
perfect organisms
• This is due to:
– historical constraints (birds can’t have 4 legs + 2
wings)…descent with modification (not randomly making new
ones)
– adaptive compromises (flexible limbs in humans 
sprains)….structure over flexibility
– chance events (natural disasters)
– selection can only edit existing variations (fittest phenotypes
currently available survive)…are they necessarily the best
traits though?