Download CHAPTER 24 LECTURE SLIDES Prepared by Brenda Leady

CHAPTER 24
LECTURE
SLIDES
Prepared by
Brenda Leady
University of Toledo
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Population genetics
Study of genes and genotypes in a
population
 Want to know extent of genetic variation,
why it exists, how it is maintained, and
how it changes over the course of many
generations
 Helps us understand how genetic variation
is related to phenotypic variation

2
Gene pool
All of the alleles for every gene in a given
population
 Study genetic variation within the gene
pool and how variation changes from one
generation to the next
 Emphasis is often on variation in alleles
between members of a population

3
Population
Group of individuals of the same species
that occupy the same environment and
can interbreed with one another
 Some species occupy a wide geographic
range and are divided into discrete
populations

4
Genes Are Usually Polymorphic

Polymorphism – many traits display variation within
a population
 Due



to 2 or more alleles that influence phenotype
Polymorphic gene- 2 or more alleles
Monomorphic – predominantly single allele
Single nucleotide polymorphism (SNPs)
 Smallest type of
 Most common –
genetic change in a gene
90% of variation in human gene
sequences


Large, healthy populations exhibit a high level of
genetic diversity
Raw material for evolution
Allele and genotype frequencies

Related but distinct calculations
7
Example



Allele frequency of CW
100 4 o’clock plants
49 red-flowered
 CRCR


42 pink-flowered
 CRCW

9 white-flowered

1.0 - 0.3 = 0.7 frequency of CR
Genotype frequency of CWCW
 CWCW
8
Hardy-Weinberg equation
9
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Generation 1
CRCR
Genotypes
CRCW
CR = 0.7
Allele and gamete frequencies
CWCW
CW = 0.3
Generation 2
CR
CW
0.7
0.3
CR
0.7
CRCR (p2)
(0.7)(0.7) = 0.49
CRCW (pq)
(0.7)(0.3) = 0.21
p2
+ 2pq
+ q2
=1
0.49 + 2(0.21) + 0.09 = 1
CW
0.3
CRCW (pq)
(0.7)(0.3) = 0.21
CWCW (q2)
(0.3)(0.3) = 0.09
Frequency of CRCR genotype (red flowers)
=
(0.7)2 = 0.49
Frequency of CRCW genotype (pink flowers) = 2(0.7)(0.3) = 0.42
Frequency of CWCW genotype (white flowers) =
(0.3)2
= 0.09
1.00
10

Conditions…
 No new mutations occur
 No natural selection occurs
 The population is so large that
allele
frequencies do not change due to random
sampling error
 No migration occurs between different
populations
 Random mating
In reality, no population meets these
conditions
 If frequencies are not in equilibrium, an
evolutionary mechanism is at work

11
Microevolution


Changes in a population’s gene pool from
generation to generation
Change because…
 Introduce
new genetic variation (mutations, gene
duplication, exon shuffling, horizontal gene transfer)

Not a major factor dictating allele frequencies
 Evolutionary
mechanisms that alter the prevalence of
an allele or genotype (natural selection, random
genetic drift, migration, nonrandom mating)

Potential for widespread genetic change
12
Natural selection
Process in which beneficial traits that are
heritable become more common in
successive generations
 Over time, natural selection results in
adaptations

 Changes
in populations of living organisms
that promote their survival and reproduction in
a particular environment
13

Reproductive success
 Likelihood
of an individual contributing fertile
offspring to the next generation
 Attributed to 2 categories of traits
Certain characteristics make organisms better
adapted to their environment and more likely to
survive to reproductive age
 Traits that are directly associated with
reproduction, such as the ability to find a mate and
the ability to produce viable gametes and offspring

14
Modern description of natural selection
1.
2.
3.
4.
Within a population, allelic variation arises from random
mutations that cause differences in DNA sequences
Some alleles encode proteins that enhance an
individual’s survival or reproductive capability compared
to other members of the population
Individuals with beneficial alleles are more likely to
survive and contribute their alleles to the gene pool of
the next generation
Over the course of many generations, allele frequencies
of many different genes may change through natural
selection, thereby significantly altering the
characteristics of a population
15
Fitness
Relative likelihood that a genotype will
contribute to the gene pool of the next
generation as compared with other
genotypes
 Measure of reproductive success
 Hypothetical gene with alleles A and a

 AA,
Aa, aa
16

Suppose average reproductive successes
are…
 AA
produces 5 offspring
 Aa produces 4 offspring
 aa produces 1 offspring

Fitness is W and maximum is 1.0 for
genotype with highest reproductive ability
 Fitness
of AA: WAA = 5/5 = 1.0
 Fitness of Aa: WAa = 4/5 = 0.8
 Fitness of aa: Waa = 1/5 = 0.2
17
Mean fitness of population
Average reproductive success of members
of a population
 As individuals with higher fitness values
become more prevalent, natural selection
increases the mean fitness of the
population

18
Natural selection patterns
Directional selection
 Stabilizing selection
 Disruptive/Diversifying selection
 Balancing selection

19
Directional selection
Individuals at one extreme of a phenotypic
range have greater reproductive success
in a particular environment
 Initiators

 New
allele with higher fitness introduced
 Prolonged environmental change
20
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Population of mice in a dimly lit forest
Number of
individuals
Light fur
Many generations
Dark fur
Many generations
Number of
individuals
Light fur
(a) An example of directional selection
Dark fur
(b) Graphical representation of directional selection
21
Stabilizing selection
Favors the survival of individuals with
intermediate phenotypes
 Extreme values of a trait are selected
against
 Clutch size

 Too
many eggs and offspring die due to lack
of care and food
 Too few eggs does not contribute enough to
next generation
22
23
Disruptive/Diversifying selection
Favors the survival of two or more different
genotypes that produce different
phenotypes
 Likely to occur in populations that occupy
heterogeneous environments
 Members of the populations can freely
interbreed

24
Contaminated soil
soil
Contaminated
Agrostis
Agrostis capillaris
capillaris
Number of individuals
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Metal sensitive
Metal resistant
Many generations
Number of individuals
(a) Growth of Agrostis capillaris on contaminated soil
Metal sensitive
(b) Graphical representation of disruptive selection
Metal resistant
a: © Courtesy Mark McNair/University of Exeter
25
Balancing selection
Maintains genetic diversity
 Balanced polymorphism

 Two
or more alleles are kept in balance, and
therefore are maintained in a population over
the course of many generations

2 common ways
 For

a single gene, heterozygote favored
Heterozygote advantage – HS allele
 Negative

frequency-dependent selection
Rare individuals have a higher fitness
26
27
Sexual selection
Form of natural selection
 Directed at certain traits of sexually
reproducing species that make it more
likely for individuals to find or choose a
mate and/or engage in successful mating
 In many species, affects male
characteristics more intensely than it does
female

28

Intrasexual selection
 Between
members of the same sex
 Horns in male sheep, antlers in male moose, male
fiddler crab enlarged claws
 Males directly compete for mating opportunities or
territories

Intersexual selection
 Between
members of the opposite sex
 Female choice
 Often results in showy characteristics for males
 Cryptic female choice

Genital tract or egg selects against genetically related sperm

Inhibits inbreeding
29
30






Explains traits that decrease survival but increase
reproductive success
Male guppy (Poecilia reticulata) is brightly colored
compared to the female
Females prefer brightly colored males
In places with few predators, the males tend to be
brightly colored
In places where predators are abundant, brightly colored
males are less plentiful because they are subject to
predation
Relative abundance of brightly and dully colored males
depends on the balance between sexual selection, which
favors bright coloring, and escape from predation, which
favors dull coloring
31
Seehausen and van Alphen Found That Male
Coloration in African Cichlids Is Subject to Female
Choice

Cichlidae have over 3,000 species
 More



different species that any other vertebrate species
Complex mating and brood care
Female play important role in choosing males with
particular characteristics
Pundamilia pundamilia and Pundamilia nyererei
 In
some locations, they do not readily interbreed and
behave like two distinct biological species
 In other places they behave like a single interbreeding
species with two color morphs
 They can interbreed to produce viable offspring
Hypothesized that females choose males
for mates based on male’s coloration
 Male were in glass enclosures to avoid
direct competition
 Goal to determine which of 2 males a
female would prefer
 Females’ preference for males dramatically
different under different lights
 Mating preference lost under
monochromatic light
 Sexual selection followed a diversifying
mechanism

Genetic drift
Changes allelic frequency due to random
chance
 Random events unrelated to fitness
 Favors either loss or fixation of an allele

 Frequency

reaches 0% or 100%
Faster in smaller populations
36
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1.0
N = 10
All BB
Frequency of B allele
0.75
N = 1,000
0.5
N = 10
0.25
0.0
0
10
20
30
Generations
40
50
All bb
37
Bottleneck
Population reduced dramatically and then
rebuilds
 Randomly eliminated members without
regard to genotype
 Surviving members may have allele
frequencies different from original
population
 Allele frequencies can drift substantially
when population is small
 New population likely to have less genetic
variation

38
39
Founder effect
Small group of individuals separates from
a larger population and establishes a new
colony
 Relatively small founding population
expected to have less genetic variation
than original population
 Allele frequencies in founding population
may differ markedly from original
population

40
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41
Neutral theory of evolution


Non-Darwinian evolution
Neutral variation
 Much
of the variation seen in natural populations is
caused by genetic drift
 Does not preferentially select for any particular allele


Most genetic variation is due to the accumulation
of neutral mutations that have attained high
frequencies due to genetic drift
Neutral mutations do not affect the phenotype so
they are not acted upon by natural selection
42
Main idea is that much of the modern
variation in gene sequences is explained
by neutral variation rather than adaptive
variation
 Sequencing data supports this idea
 Nucleotide substitutions much more likely
in 3rd base of codon (usually doesn’t
change amino acid) than 1st or 2nd (usually
does change amino acid)
 Changing the amino acid is usually
harmful to the coded protein

43
44
Migration
Gene flow occurs when individuals migrate
between populations having different allele
frequencies
 Migration tends to reduce differences in
allele frequencies between the 2
populations
 Tends to enhance genetic diversity within
a population

45
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Population 1
Geographic barrier
Population 2
Allele
variant 2
Allele
variant 1
Western deer
population
Mountain range
Eastern deer
population
Pass
46
Nonrandom mating

One of the conditions required to establish
the Hardy-Weinberg equilibrium is random
mating
 Individuals
choose their mates irrespective of
their genotypes and phenotypes

Forms of nonrandom mating
 Assortative/disassortative
 Inbreeding
47

Assortative mating
 Individuals
with similar phenotypes are more
likely to mate
 Increases the proportion of homozygotes

Disassortative mating
 Dissimilar
phenotypes mate preferentially
 Favors heterozygosity
48

Inbreeding
 Choice
of mate based on genetic history
 Does not favor any particular allele but it does
increase the likelihood the individual will be
homozygous
 May have negative consequences with regard
to recessive alleles
 Lower mean fitness of a population if
homozygous offspring have a lower fitness
value
 Inbreeding depression
49
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