Transcript chapter 24 population genetics

CHAPTER 24
POPULATION
GENETICS
Prepared by
Brenda Leady, University of Toledo
Copyright (c) The McGraw-Hill
Companies, Inc. Permission required
for reproduction or display.
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Population genetics
Study of genes and genotypes in a
population
 Want to know extent of genetic variation,
why it exists and how it changes over time
 Helps us understand how genetic variation
is related to phenotypic variation
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Gene pool
All of the genes in a 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
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Population
Group of individuals of the same species
that can interbreed with one another
 Some species occupy a wide geographic
range and are divided into discrete
populations
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Genes in Natural Populations Are
Usually Polymorphic
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Polymorphism – many traits display variation within
a population
 Due
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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
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Large, healthy populations exhibit a high level of
genetic diversity
Raw material for evolution
Allele and genotype frequencies
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Related but distinct calculations
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Example
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49 red-flowered RR
42 pink-flowered Rr
9 white-flowered rr
Allele frequency of r

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1.0 - 0.3 = 0.7 frequency of R
Genotype frequency of rr
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Hardy-Weinberg equation
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Relates allele and genotype frequencies
under certain conditions
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Conditions…
 The
population is so large that allele frequencies do
not change due to random sampling error
 The members of the population mate with each other
without regard to their phenotypes and genotypes
 No migration occurs between different populations
 No survival or reproductive advantage exists for any
of the genotypes—in other words, no natural selection
occurs
 No new mutations occur
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In reality, no population meets these conditions
If frequencies are not in equilibrium, an
evolutionary mechanism is at work
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Microevolution
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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)

Population will not evolve with mutations as the only source
 Evolutionary
mechanisms that alter the prevalence of
an allele or genotype (natural selection, random
genetic drift, migration, nonrandom mating)
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Potential for widespread genetic change
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Selective survival of genotypes that confer
greater reproductive success
 Natural selection acts on
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 Characteristics
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with a survival advantage
Make organisms better adapted, more likely to
survive, greater chance to reproduce
 Favors
individuals that produce viable
offspring
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Modern description of natural selection
1.
2.
3.
4.
Allelic variation arises from random mutations that may
alter the function of the protein.
Some alleles may encode proteins that enhance an
individual’s survival or reproductive success compared to
that of 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

Net result of natural selection is a population that is better
adapted to its environment and/or more successful at
reproduction.
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Darwinian 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
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 AA, Aa,
aa
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Suppose average reproductive successes
are…
 AA 5
offspring
 Aa 4 offspring
 Aa 1 offspring
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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
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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
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Natural selection patterns
Directional selection
 Stabilizing selection
 Disruptive selection
 Balancing selection
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Directional selection
Favors individuals at one extreme of a
phenotypic distribution that have greater
reproductive success in a particular
environment
 Initiators
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 New
favored allele introduced
 Prolonged environmental change
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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
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Disruptive selection
Favors the survival of two or more different
genotypes that produce different
phenotypes
 Likely to occur in populations that occupy
diverse environments
 Members of the populations can freely
interbreed
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Balancing selection
Maintains genetic diversity
 Balanced polymorphism
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 Two
or more alleles are kept in balance, and
therefore are maintained in a population over
the course of many generations
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2 common ways
 For
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a single gene, heterozygote favored
Heterozygote advantage – HS allele
 Negative
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frequency-dependent selection
Rare individuals have a higher fitness
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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
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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
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Intersexual selection
 Between
members of the opposite sex
 Female choice
 Often results in showy characteristics for males
 Cryptic female choice
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Genital tract or egg selects against genetically related sperm

Inhibits inbreeding
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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
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Seehausen and van Alphen Found That Male
Coloration in African Cichlids Is Subject to Female
Choice
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Cichlidae have over 3,000 species
 More
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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
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Random genetic drift
Changes allelic frequency due to random
sampling error
 Random events unrelated to fitness
 Favors either loss or fixation of an allele
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 Frequency

reaches 0% or 100%
Faster in smaller populations
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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
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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
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Neutral theory of evolution
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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
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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
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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 don’t change
amino acid) than 1st or 2nd (usually does
change amino acid)
 Changing the amino acid is usually
harmful to the coded protein
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Selectionists oppose the neutralist theory
 Neutralists argue that most genetic
variation arises from neutral genetic
mutations and genetic drift
 Selectionists argue that beneficial
mutations and natural selection are
primarily responsible
 Both accept that genetic drift and natural
selection both play key roles in evolution
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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
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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
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Assortative mating
 Individuals
with similar phenotypes are more
likely to mate
 Increases the proportion of homozygotes
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Disassortative mating
 Dissimilar
phenotypes mate preferentially
 Favors heterozygosity
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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
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