-
.
Chapter 23: Population Genetics
(Microevolution)
Microevolution is a change in allele frequencies or genotype
frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg
theorem
Microevolution is deviation from Hardy-Weinberg equilibrium
Genetic variation must exist for natural selection to occur
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
• Explain what terms in the Hardy-
Weinberg equation give:
– allele frequencies (dominant allele,
recessive allele, etc.)
– each genotype frequency (homozygous
dominant, heterozygous, etc.)
– each phenotype frequency
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
Chapter 23: Population Genetics
(Microevolution)
Microevolution is a change in allele frequencies or genotype
frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg
theorem
Microevolution is deviation from Hardy-Weinberg equilibrium
Genetic variation must exist for natural selection to occur
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
Microevolution is a change in allele frequencies or
genotype frequencies in a population over time
population – a localized group of individuals capable of
interbreeding and producing fertile offspring, and that are more or
less isolated from other such groups
gene pool – all alleles present in a population at a given
time
phenotype frequency – proportion of a population with a given
phenotype
genotype frequency – proportion of a population with a given
genotype
allele frequency – proportion of a specific allele in a
population
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a change in allele frequencies or
genotype frequencies in a population over time
allele frequency – proportion of a specific allele in a
population
diploid individuals have two alleles for each gene
if you know genotype frequencies, it is easy to calculate allele
frequencies
example:
population (1000) = genotypes AA (490) + Aa (420) + aa (90)
allele number (2000) = A (490x2 + 420) + a (420 + 90x2) = A
(1400) + a (600)
freq[A] = 1400/2000 = 0.70
freq[a] = 600/2000 = 0.30
note that the sum of all allele frequencies is 1.0 (sum rule of
probability)
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
Chapter 23: Population Genetics
(Microevolution)
Microevolution is a change in allele frequencies or genotype
frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg
theorem
Microevolution is deviation from Hardy-Weinberg equilibrium
Genetic variation must exist for natural selection to occur
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
• Explain what terms in the Hardy-
Weinberg equation give:
– allele frequencies (dominant allele,
recessive allele, etc.)
– each genotype frequency (homozygous
dominant, heterozygous, etc.)
– each phenotype frequency
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Make up and do some pop gen problems.
Suggestions: Start with a population of either 100 or 10,000
individuals. Have the
number of individuals with the recessive phenotype be the square
of a
whole number (so that q2 is easy to solve). Then answer the
frequency
questions below.
What is the frequency in the population of:
• the recessive phenotype?
• the dominant phenotype?
• the dominant allele?
• the recessive allele?
• homozygous recessive individuals?
• homozygous dominant individuals?
• heterozygous individuals?
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
The Hardy-Weinberg Theorem
the Hardy-Weinberg theorem describes the frequencies of
genotypes in a population based on the frequency of occurrence of
alleles in the population that is in a state of genetic equilibrium
(that is, not evolving)
the usual case for calculations: if allele “A” is dominant to
“a”, and they are the only two alleles possible at the A-locus,
then
p = freq[A] = the frequency of occurrence of the A-allele in the
population
q = freq[a] = the frequency of occurrence of the a-allele in the
population
Then p + q = 1 (following the sum rule for probability)
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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The Hardy-Weinberg Theorem
Allele associations follow the product rule for probability, so
you multiply to predict the genotype frequencies:
( p + q ) x ( p + q ) = p2 + 2 pq + q2
p2 = frequency of homozygous dominant genotypes
2 pq = frequency of heterozygous genotypes
q2 = frequency of homozygous recessive genotypes
note that ( p + q ) x ( p + q ) = 1 x 1 = 1, so
p2 + 2 pq + q2 = 1
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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Hardy-Weinberg Equilibrium
if the Hardy-Weinberg theorem can be used to accurately predict
genotype frequencies from allele frequencies for a population
then…
the population is in Hardy-Weinberg equilibrium or genetic
equilibrium
in such cases you can use data from one generation to predict
the allele, genotype, and phenotype frequencies for the next
generation
such populations are not evolving, but are static instead
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
Make up and do some pop gen problems.
Suggestions: Start with a population of either 100 or 10,000
individuals. Have the
number of individuals with the recessive phenotype be the square
of a
whole number (so that q2 is easy to solve). Then answer the
frequency
questions below.
What is the frequency in the population of:
• the recessive phenotype?
• the dominant phenotype?
• the dominant allele?
• the recessive allele?
• homozygous recessive individuals?
• homozygous dominant individuals?
• heterozygous individuals?
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
• Describe the assumptions of the Hardy-
Weinberg equilibrium model.
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Hardy-Weinberg Equilibrium
the assumptions of this model are:
large population size (due to statistical constraints, to
minimize genetic drift)
no migration – no exchange of alleles with other populations (no
gene flow)
no mutations of the alleles under study occur
random mating of all genotypes
no natural selection
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
• Describe the assumptions of the Hardy-
Weinberg equilibrium model.
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
Chapter 23: Population Genetics
(Microevolution)
Microevolution is a change in allele frequencies or genotype
frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg
theorem
Microevolution is deviation from Hardy-Weinberg equilibrium
Genetic variation must exist for natural selection to occur
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
• Describe conditions that can keep
populations from establishing or
maintaining genetic equilibrium.
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
if allele and/or genotype frequencies in a population change
over time, then it is by definition evolving (evolution means
changing over time), undergoing microevolution
things that can cause microevolution
small population size: genetic drift
migration – gene flow; individuals leave and/or join a
population
mutations
nonrandom mating
natural selection
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
consequences of small population size: genetic drift
Consider taking a small sample of individuals from a larger
population
If only two individuals were picked they almost certainly won’t
reflect the allele frequency in the larger population (in many
cases, they can’t even possibly do so).
The same holds true for 3, 4, or 5 individuals.
As the selected sample gets larger it becomes more likely that
the sample reflects the allele frequency in the larger
population.
Mating to produce the next generation is effectively sampling
the population
It takes a very large sampling size (thousands) to have a good
chance of the sample essentially matching the allele frequencies
and genotype frequencies of the population.
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
genetic drift is a change in gene frequencies of populations
because of small population size
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
genetic drift tends to decrease genetic variation within a
population
genetic drift tends to increase genetic variation between
populations
NOTE: genetic drift places a major factor in evolution,
especially when populations are split, but does NOT involve natural
selection
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
consequences of small population size: genetic drift
two general mechanisms lead to small population sizes
genetic bottlenecks are created by dramatic reduction in the
population size – endangered species face a genetic bottleneck on a
species-wide scale, and suffer lasting effects even if population
size later recovers
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium consequences of small
population size: genetic drift
two general mechanisms lead to small population sizes
founder effect – when a new population is established, typically
only a few individuals (founders) are involved in colonizing the
new area, essentially an “isolation bottleneck” for the new
population; this is common for islands
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
migration – when individuals leave or join a population
migrating individuals carry their alleles with them (gene flow),
usually resulting in changes in allele frequencies
gene flow tends to decrease genetic variation between
populations
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
mutations increase variation in the gene pool of a species
remember that mutations may be neutral, harmful, or
beneficial
even at the risk of harmful effects, mutations are necessary to
increase variation in the population so that natural selection can
produce organisms more suited to their environment
http://www.auburn.edu/academic/classes/biol/1020/bowling/http://cnews.canoe.ca/CNEWS/Science/2004/06/24/512617.html
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Microevolution is a deviation from
Hardy-Weinberg equilibrium
nonrandom mating
if individuals do not mate at random, then Hardy-Weinberg
equilibrium is not achieved
the most common cases of nonrandom mating involve inbreeding –
mating between individuals of similar genotypes, either by choice
or due to environmental factors such as location
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium inbreeding does not change allele
frequencies, but increases
the frequency of homozygous genotypes
inbreeding depression is seen in some cases, where inbred
individuals have lower fitness that non-inbred individuals
fitness – relative ability of a genotype to contribute to future
generations
fertility declines and high juvenile mortality associated with
“unmasking” harmful recessive alleles can reduce fitness for inbred
individuals
hybrid vigor also leads to higher relative fitness for
hybrids
self-fertilization is the most extreme case of inbreeding
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
assortive mating – a type of nonrandom mating where mates are
(sexually) selected based on phenotypes – really is an aspect of
natural selection
positive assortive mating – selection for the same phenotype;
works like inbreeding for the genes governing that phenotype, and
for loci closely linked to those genes
negative assortive mating – selection for the opposite
phenotype
less common than positive assortive mating
leads to a decrease in homozygous genotypes for the genes
governing the selected phenotype, and for loci closely linked to
those genes
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
natural selection changes allele frequencies in a way that leads
to adaptation to the environment
fitness is the ability of an organism to compete successfully in
passing its alleles on to the next generation (in a vessel that can
continue that process)
populations undergoing natural selection are evolving, with
alleles that contribute to better fitness increasing in frequency
over successive generations
natural selection only operates based on the current environment
– as conditions change, different alleles will be selected for
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
sexual selection (mate choices based on inherited
characteristics) is an aspect of natural selection
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
• Describe conditions that can keep
populations from establishing or
maintaining genetic equilibrium.
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
• Explain three main types of natural
selection.
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
there are three types of natural selection
stabilizing selection
directional selection
disruptive selection
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
stabilizing selection – occurs in populations well adapted to
their environments, selecting against phenotypic extremes
this is probably the type of selection most commonly faced by
populations
example - human birth weight
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium directional selection – permits
species to adapt
to environmental change by favoring selection of one extreme
over the other
example – peppered moth
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Microevolution is a deviation from
Hardy-Weinberg equilibrium disruptive selection – when more than
one extreme phenotype is
favored over intermediate phenotypes
really a special case of direction selection, where there are
trends in more than one direction
can produce a genetic “split” in a population and thus serve as
a mechanism for speciation
example – pocket mice in the Tularosa Basin of New Mexico
Michael E.N Majerus, Nicholas I Mundy. Mammalian
melanism: natural selection in black and white. Trends in
Genetics Volume 19, Issue 11, November 2003, Pages 585-588
http://www.auburn.edu/academic/classes/biol/1020/bowling/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TCY-49MX3DJ-7&_user=409620&_coverDate=11%2F30%2F2003&_rdoc=2&_fmt=high&_orig=browse&_srch=doc-info%28%23toc%235183%232003%23999809988%23466594%23FLA%23display%23Volume%29&_cdi=5183&_sort=d&_docanchor=&view=c&_ct=15&_acct=C000019518&_version=1&_urlVersion=0&_userid=409620&md5=8f7589fcb6f6bbf11dcd4d753b614f3ehttp://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TCY-49MX3DJ-7&_user=409620&_coverDate=11%2F30%2F2003&_rdoc=2&_fmt=high&_orig=browse&_srch=doc-info%28%23toc%235183%232003%23999809988%23466594%23FLA%23display%23Volume%29&_cdi=5183&_sort=d&_docanchor=&view=c&_ct=15&_acct=C000019518&_version=1&_urlVersion=0&_userid=409620&md5=8f7589fcb6f6bbf11dcd4d753b614f3e
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.
• Explain three main types of natural
selection.
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
Chapter 23: Population Genetics
(Microevolution)
Microevolution is a change in allele frequencies or genotype
frequencies in a population over time
Genetic equilibrium in populations: the Hardy-Weinberg
theorem
Microevolution is deviation from Hardy-Weinberg equilibrium
Genetic variation must exist for natural selection to occur
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
• Discuss the importance of genetic
variation for evolution, and the concept
of neutral variation.
• Give a hypothetical example of how
genetic variation that was once neutral
may no longer be neutral.
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
Genetic variation must exist for
natural selection to occur
the ultimate source of genetic variation is mutations
once variation exists, it can be affected by independent
assortment and genetic recombination during gamete formation
consider the cross AaBb x AaBb – 9 different genotypes arise
this involves only 2 alleles at 2 loci; if there were 6 alleles
possible at just 5 loci, over 4 million genotypes are possible
thus, given that there are thousands of genes in an organism,
and that many alleles are possible at most of these loci, it
becomes clear that in nature there is great genetic variability
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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Genetic variation must exist for
natural selection to occur
the demonstrated presence of two or more alleles at a given
locus is genetic polymorphism
biologists have produced tools for studying the genetic
polymorphism of populations at the molecular level (RFLP, DNA
sequencing, etc.)
these tools can be used to demonstrate and study polymorphism in
populations without necessarily knowing the specific genes
involved
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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genetic variation can be maintained by heterozygote advantage or
hybrid vigor
when either the homozygous dominant or recessive is more suited
to an environment than the heterozygote, the homozygous genotype
will be more likely to be fixed in the population
…but when heterozygous genotypes have advantage over either of
the homozygous genotypes, variation tends to increase in the
population
example - sickle cell anemia and malaria resistance
Genetic variation must exist for
natural selection to occur
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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.
genetic polymorphisms can be maintained due to
frequency-dependent selection
there are cases where the frequency of a given genotype affects
the degree to which it is or isn't selected in the population
example - predator/prey relationships, where individuals with a
rare phenotype may be ignored by a predator, but as they become
more abundant the selective advantage decreases because the
predator is more likely to notice them
Genetic variation must exist for
natural selection to occur
http://www.auburn.edu/academic/classes/biol/1020/bowling/
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much of the genetic variation in a population will produce no
selective advantage – called neutral variation
the role of neutral variation in evolution is debated today
remember that what is neutral in one context may not be neutral
in another context, so as environments change some previously
neutral variation may be acted on by natural selection
Genetic variation must exist for
natural selection to occur
http://www.auburn.edu/academic/classes/biol/1020/bowling/
-
.
• Discuss the importance of genetic
variation for evolution, and the concept
of neutral variation.
• Give a hypothetical example of how
genetic variation that was once neutral
may no longer be neutral.
http://www.auburn.edu/academic/classes/biol/1020/bowling/