Download Hardy-Weinberg Principle

Ch 26 Introduction
• There are four mechanisms that shift allele frequencies in
populations:
1. Natural selection increases the frequency of those alleles
that contribute to reproductive success in a particular
environment.
2. Genetic drift causes allele frequencies to change randomly.
3. Gene flow occurs when individuals leave one population,
join another, and breed.
4. Mutation modifies allele frequencies by continually
introducing new alleles.
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The Hardy-Weinberg Principle
• To study how the four evolutionary processes affect populations, in
1908 G. H. Hardy and Wilhelm Weinberg developed a
mathematical model to analyze the consequences of matings among
all of the individuals in a population.
• Hardy and Weinberg wanted to know what happened in an entire
population, when all of the individuals—and thus all possible
genotypes—bred.
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Deriving the Hardy-Weinberg Principle
• a simple situation:
– a gene with two alleles, A1 and A2.
• frequency of A1 = p and the frequency of A2 = q.
– Because there are only two alleles, p + q = 1.
• three possible genotypes :
– A1A1, A1A2, and A2A2.
– The frequency of the A1A1 genotype is p2.
– The frequency of the A2A2 genotype is q2.
– The frequency of the A1A2 genotype is 2pq.
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Deriving the Hardy-Weinberg Principle
• a simple situation:
– a gene with two alleles, A1 and A2.
• frequency of A1 = p and the frequency of A2 = q.
– Because there are only two alleles, p + q = 1.
• three possible offspring genotypes :
– A1A1, A1A2, and A2A2.
– The frequency of the A1A1 genotype is p2.
– The frequency of the A2A2 genotype is q2.
– The frequency of the A1A2 genotype is 2pq.
– these frequencies cover all possible genotypes, so they sum to 1
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Deriving the Hardy-Weinberg Principle
• If the population is in Hardy-Weinberg equilibrium, when allele
frequencies are calculated for this new generation, the frequency of
A1 is still p and the frequency of A2 is still q.
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© 2011 Pearson Education, Inc.
The Hardy-Weinberg Principle
• The Hardy-Weinberg principle makes two fundamental claims:
1. If the frequencies of alleles A1 and A2 in a population are
given by p and q, then the frequencies of genotypes A1A1,
A1A2, and A2A2 will be given by p2, 2pq, and q2 for generation
after generation.
2. When alleles are transmitted via meiosis and random
combination of gametes, their frequencies do not change over
time. For evolution to occur, some other factor or factors must
come into play.
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© 2011 Pearson Education, Inc.
Important Assumptions of the Hardy-Weinberg Model
• For a population to conform to the Hardy-Weinberg principle, none
of the four mechanisms of evolution can be acting on the
population.
1. No natural selection.
2. No genetic drift or random allele frequency changes.
3. No gene flow.
4. No mutation.
• In addition, the model assumes that mating is random with respect
to the gene in question. Thus, here are the five assumptions that
must be met:
5. Random mating.
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Hardy-Weinberg Principle: A Null Hypothesis
Biologists often want to test whether natural selection is acting on
a particular gene, nonrandom mating is occurring, or one of the
other evolutionary mechanisms is at work. In addressing questions
like these, the Hardy-Weinberg principle functions as a null
hypothesis.
• When genotype frequencies do not conform to Hardy-Weinberg
proportions, evolution or nonrandom mating is occurring in that
population.
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Types of Natural Selection
• Natural selection occurs in a wide variety of patterns (see
Directional, Stabilizing, Disruptive selection).
• Genetic variation refers to the number and relative frequency of
alleles that are present in a particular population.
• Maintaining genetic variation is important because lack of variation
can make populations less able to respond successfully to changes
in the environment.
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Directional Selection
• Directional selection is a pattern of natural selection that increases
the frequency of one allele.
• This type of selection reduces a population’s genetic diversity over
time.
• If directional selection continues over time, the favored alleles
eventually become fixed, reaching a frequency of 1.0, or 100%.
Disadvantageous alleles will be lost, reaching a frequency of 0.0.
– When disadvantageous alleles decline in frequency, purifying
selection is said to occur.
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© 2011 Pearson Education, Inc.
Stabilizing Selection
• Stabilizing selection occurs when individuals with intermediate
traits reproduce more than others, thereby maintaining intermediate
phenotypes in a population. There is no change in the average value
of a trait over time, and genetic variation in the population is
reduced.
• An example is the percentage of newborn humans with various
birth weights compared with their mortality rates. Those with birth
weights in the middle of the range were most likely to survive.
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© 2011 Pearson Education, Inc.
Disruptive Selection
• In contrast to stabilizing selection, the pattern of natural selection
called disruptive selection occurs when intermediate phenotypes
are selected against and extreme phenotypes are favored.
• Disruptive selection maintains genetic variation but does not
change the mean value of a trait.
• Disruptive selection can cause speciation, the formation of new
species, if individuals with one extreme of a trait start mating
preferentially with individuals that have the same trait.
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© 2011 Pearson Education, Inc.
Heterozygote Advantage
• Directional selection, stabilizing selection, and disruptive selection
describe how natural selection can act on traits in a single
generation or episode. However, they are not the only patterns of
selection.
• In heterozygote advantage, heterozygous individuals have higher
fitness than homozygous individuals do, thus maintaining genetic
variation in the population.
• Heterozygote advantage is one mechanism responsible for a
balancing selection, in which no single allele has a distinct
advantage.
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Balancing Selection
• Balancing selection also occurs when:
1. The environment varies over time or in different geographic
areas such that certain alleles are favored by natural selection
at different times or in different places. This results in the
maintenance of or increase in overall genetic variation.
2. Certain alleles are favored when they are rare, but not when
they are common—a pattern known as frequency-dependent
selection. As a result, overall genetic variation in the
population is maintained or increased.
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Genetic Drift
• Genetic drift : any change in allele frequencies in a population due
to chance (sampling error).
– occurs in every population, in every generation.
– causes allele frequencies to drift up and down randomly over
time.
– is random with respect to fitness
– can lead to the random loss or fixation of alleles. When random
loss or fixation occurs, genetic variation in the population
declines
– more pronounced in small populations than in large ones.
Given enough time, however, genetic drift can be an important
factor even in large populations.
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© 2011 Pearson Education, Inc.
Experimental Studies of Genetic Drift
• Research on genetic drift in small populations of fruit flies used a
genetic marker—a specific allele that causes a distinctive
phenotype—for leg-bristle morphology.
– This gene has two alleles, one resulting in straight (wild-type)
leg bristles and the other in forked (bent) bristles.
• In 70 of the 96 populations studied, genetic drift caused one allele
to be lost.
• In the laboratory, genetic drift was found to decrease genetic
variation within populations and increase genetic differences
between populations.
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© 2011 Pearson Education, Inc.
Genetic Drift in Natural Populations
• Genetic drift is of great concern to conservation biologists because
the small populations found on nature reserves or in zoos are
especially susceptible to it.
• Genetic drift can be caused by any event or process that involves
sampling error, not just the sampling of gametes that occurs during
fertilization. Two examples are the founder effect and bottlenecks.
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© 2011 Pearson Education, Inc.
Gene Flow
• Gene flow, the movement of alleles from one population to
another, occurs whenever individuals leave one population, join
another, and breed.
• Gene flow equalizes gene frequencies between the source and
recipient populations. In other words, gene flow homogenizes
populations.
• Gene flow is random with respect to fitness, but movement of
alleles between populations always tends to reduce genetic
differences between them.
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© 2011 Pearson Education, Inc.
Gene Flow in Natural Populations
• On the island of Vlieland off the coast of
the Netherlands, birds called great tits
breed in two sets of woodlands.
• Females hatched in the eastern woodland
appear to be more well adapted.
• Gene flow is higher in the western
woodland; thus birds coming from the
mainland are introducing alleles at a
higher rate into the western population.
– These mainland alleles result in
individuals less well-adapted to the
island environment.
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Mutation
• Although most evolutionary mechanisms reduce genetic diversity,
mutation restores genetic diversity by creating new alleles.
• Mutation is random with respect to the fitness of the affected allele.
• Because mutations cause random changes in genes, many of them
result in deleterious alleles, alleles that lower fitness. These alleles
tend to be eliminated by selection.
• Rarely, mutation produces a beneficial allele that should increase in
frequency in a population due to natural selection.
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Mutation as an Evolutionary Mechanism
• Mutation can be a significant evolutionary force in bacteria and
archaea, which have short generation times.
• However, mutation in eukaryotes rarely causes a change from the
genotype frequencies expected under the Hardy-Weinberg
principle.
• Mutation is relatively slow compared with natural selection, genetic
drift, and gene flow.
• Mutation introduces new alleles into every individual in every
population in every generation.
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Take-Home Messages
• Mutation plays an important role in evolution:
1. Mutation is the ultimate source of genetic variation. Crossing
over and independent assortment shuffle existing alleles into
new combinations, but only mutation creates new alleles.
2. Mutation alone is usually inconsequential in changing allele
frequencies at a particular gene.
Each of the four evolutionary forces has different consequences
for allele frequencies.
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© 2011 Pearson Education, Inc.
Nonrandom Mating
• In nature, mating may not be random with respect to any particular
gene in question.
• Two mechanisms that violate the Hardy-Weinberg assumption of
random mating are inbreeding and sexual selection.
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Inbreeding
• Inbreeding (mating between relatives)
– increases the frequency of homozygotes
– reduces the frequency of heterozygotes in each generation.
– does not cause evolution (allele frequencies do not change in
the population as a whole)
• Inbreeding depression is a decline in average fitness that takes
place when homozygosity increases and heterozygosity decreases
in a population.
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© 2011 Pearson Education, Inc.
Inbreeding Depression
• Inbreeding depression results from two processes:
– Many recessive alleles represent loss-of-function mutations. In
heterozygotes these alleles have little or no effect; but
inbreeding increases the frequency of homozygous recessive
individuals and thus the frequency of individuals expressing the
mutation.
– Many genes—especially those involved in fighting disease—
are under intense selection for heterozygote advantage. If an
individual is homozygous at these genes, then fitness declines.
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Inbreeding
• Even though it does not cause evolution directly—because it does
not change allele frequencies—inbreeding can speed the rate of
evolutionary change. More specifically, it increases the rate at
which purifying selection eliminates recessive deleterious alleles
from a population.
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Sexual Selection (a form of natural selection)
Sexual selection occurs when individuals within a population
differ in their ability to attract mates. It favors individuals with
heritable traits that enhance their ability to obtain mates.
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The Fundamental Asymmetry of Sex
• females usually invest more in their offspring than males do.
• There are two broad types of sexual selection: female choice and
male-male competition.
– In other words, females should be choosy about their mates,
while males will have to compete with each other for mates.
• Sexual selection should act more strongly on males than on
females.
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Female Choice for “Good Alleles”
• Females may choose mates on the basis of physical characteristics
that signal male genetic quality.
• zebra finch example
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© 2011 Pearson Education, Inc.
Female Choice for Paternal Care
• In many species, females prefer to mate with males that care for
young or that provide the resources required to produce eggs.
• For example, brown kiwi females make an enormous initial
investment in their offspring—their eggs routinely represent over
15 percent of the mother’s total body weight—but choose to mate
with males that take over all of the incubation and other care of the
offspring.
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© 2011 Pearson Education, Inc.
Male-Male Competition
• Sexual selection is intense in elephant seals and is driven by malemale competition.
• Male elephant seals establish territories, areas that they defend and
can use exclusively.
• Evidence for intense sexual selection in males is indicated by the
fact that variation in reproductive success is high in males, whereas
females have lower variation in reproductive success.
– Males with larger territories father more offspring, and their
alleles rapidly increase in the population.
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© 2011 Pearson Education, Inc.
What Are the Consequences of Sexual Selection?
• Sexually selected traits often differ
sharply between the sexes.
• Sexual dimorphism refers to any
trait that differs between males and
females of the same species.
• Sexual selection violates the
assumptions of the Hardy-Weinberg
principle by causing certain alleles to
increase or decrease in frequency and
resulting in evolutionary change.
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