Biol 121- 2010.03.15- Evolution- Allele Frequencies in Populations (Ch. 25).docx

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22 Apr 2012
Biol 121 225
Freeman 503-508, 511-520 Mar 15, 10
Allele Frequencies in Populations
-a population is a group of individuals from the same species that live in the
same area at the same time (and interbreed)
Population genetics
-population genetics looks at:
-contribution of alleles to a population
-quantification of genetic variation in a population
-measurement of changes in a population over time = evolution
Four mechanisms that shift allele
frequencies in populations
1) Natural selection increases frequency of certain alleles the ones that
contribute to success in survival and reproduction
2) Genetic drift causes allele frequencies to change randomly, and in some
cases drift may even cause alleles that decrease fitness to increase in fq.
3) Gene flow occurs when individuals leave the population and join another and
breed. Allele frequencies may change when gene flow occurs b/c arriving
individuals introduce alleles while departing individuals remove alleles
4) Mutation modifies allele frequencies by continually introducing new alleles
the alleles created by mutation may be beneficial, detrimental, or have no
effect on fitness
-natural selection is not the only agent responsible for evol
-each of these four processes have diff consequences
-natural selection is the only mechanism that acting alone can result in
-the other three (mutation, gene flow, drift) do not favour certain alleles over
-mutation and drift introduce a nonadaptive component into evol
Agrostis tenuis growing on heavy
metals on an abandoned mine in
-A. tenuis is a plant species and some of its individuals can grow on heavy
metals such as in an abandoned mine site in Wales the population has
adapted and evolution has occurred
-therefore, a change in allele frequency has occurred
Gene pool
-all the gametes of a single generation in a population
-can put all the genes together in a ‘pool’ and calculate the expected frequency
of particular alleles if pulled out gametes at random
p and q
p = frequency of dominant allele
q = frequency of recessive allele
q² = frequency of homozygous recessive, 2pq = fq of heterozygous
p + q = 1
p^2 + 2pq + q^2 = 1
Hardy-Weinberg (HW) principle or
-HW describes a non-evolving population
-ie. The frequency of alleles and genotypes in the gene pool remain constant
-sexual recombination through meiosis and random fertilization do not change
the population’s genetic structure
Assumptions of HW theorem (5)
1) No natural selection at the gene in question (no advantage of an allele)
2) No genetic drift (random change in allele frequency)
3) No gene flow (immigration = emigration = 0)
4) No mutation (no new alleles)
5) Random mating (no sexual selection)
Limitations of HW theorem (2)
1) Assumptions (above 5) are rarely true in a natural population
2) Usually, the allele frequency changes from one generation to the next
Applications of HW theorem (2)
1) Modeling being able to predict the allele frequencies
2) Null model works as null hypothesis if not met, then evol is occurring
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Biol 121 225
Freeman 503-508, 511-520 Mar 15, 10
Non-Directional (Random) Changes in Allele Frequencies
Genetic Drift
-Genetic drift causes allele frequencies to change randomly, and in some cases
drift may even cause alleles that decrease fitness to increase in fq.
-there are two types of this
-genetic drift is random with respect to fitness the allele fq changes are not
adaptive; and genetic drift is most pronounced in small pop
-genetic drift tends to decrease genetic diversity over time, as alleles are
randomly lost or fixed
Two types of Genetic Drift
1) Founder Effect
2) Bottleneck Effect
1) Founder Effect
-when a group of individuals emigrate to a new geographic area and establishes
a new population
-if the group is small enough, allele frequencies in the new population are
almost guaranteed to be diff from those in the old, due to sampling error
-a change in allele frequencies that occurs when a new pop is established is
called a founder effect
-e.g. Hurricanes swept through the Carribean region in 1995, and a large raft
floated onto Anguilla with green iguana and they had different allele
frequencies than the original pop because only 15 individuals arrived
2) Bottleneck effect
-if a large pop experiences a sudden reduction in size, population bottleneck is
said to occur survivors are random
-caused by disease outbreaks, floods, fires, storms, etc
-a genetic bottleneck is a sudden reduction in the number of alleles in a
-the bottleneck effect can cause a decrease in genetic variation by elimination
of some alleles
-drift occurs during genetic bottlenecks causing a change in allele fq
-e.g. Pingelap Atoll in South Pacific only 20 ppl out of several thousand
survived a typhoon and famine in 1775 one of these ppl had CNGB3 loss of
function recessive allele for colour vision which is normally rare but the
resulting population even today has 1/20 ppl with achromatopsia
Gene flow
-movement of alleles from one population to another
-occurs when individuals leave one pop and join another and breed
-as an evolutionary mechanism, gene flow usually equalizes allele frequencies
b/t the source pop and the recipient pop
-when alleles move from one pop to another, the populations tend to become
more alike
-gene flow is random with respect to fitness arrival/departure of alleles can
increase or decrease the average fitness, depending on the situation
-but a movement of alleles b/t pop tends to reduce genetic differences b/t
e.g. Prairie lupine populations on Mt. St. Helens in Washington state
-after Mt exploded, new populations emerged
-the oldest individuals in the new pops had different allele fq than source
-however, as pop got older, allele fq became progressively more like those in
the oldest pop (source pop)
-gene flow can increase genetic diversity in a recipient pop if new alleles arrive
but can also decrease genetic variation in the source pop if alleles leave
-where do entirely new alleles come from?
-mutations occur when DNAP makes an error in DNA replication, resulting in
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