Download Population Genetics

Population Genetics
Mind Your P’s and Q’s
I. Introduction
• A. Old definition
– 1. Change in the look
of the species over
time in response to a
changing environment
– 2. Difficult to work
with experimentally
– 3. Hard to measure
slow and gradual
change
B. New Defintion of Evolution
• 1. changing allelic frequencies
over time
• 2. represents a marriage of
genetics and evolutionary theory
• 3. new area of biology is born
• 4. population genetics
• 5. allows evolution to be
quantified
• 6. independently arrived at by two
investigators-Hardy and Weinberg
C. Definitions of Population Genetics
• 1. Species
– a group of
organisms made up
of different
populations whose
individuals have
the ability to
interbreed and
produce fertile
offspring
2. Population
• a. localized group of individuals
belonging to the same species
• b. Members of the same
population tend to be more
closely related to eachother
than to other populations
• c. result of inbreeding and
proximity
• d. A population is the smallest
unit of living organisms that
can undergo evolution.
3. Gene pool
• a. sum of all of the
alleles present in a
population
• b. each individual
organism donates two
alleles for each gene as
they are diploid
• c. walk down to gym and
release genes
• d. If homozygous for the
gene-allele is said to be
fixed
4. Allelic frequencies
• a. P = the frequency
of the dominant
allele in the gene
pool
• b. Q = the frequency
of the recessive allele
in the gene pool
• c. Calculate the p
and q values from
the population on
the right.
5. Genotypic frequencies
• a. Construct punnet square
• b. Fishing example
• c. Values total 1
6. Example
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need an example to illustrate-population of students with freckles
freckles is the dominant allele and the condition of no freckles is the recessive allele
F = freckles
f = no freckles
500 individuals in our population and let’s have 20 individuals with no freckles, 160
heterozygous with freckles, and 320 of the individuals with homozygous dominant for
the trait.
we can calculate the percentage of recessive and dominant alleles in the gene pool
500 individuals donate 1000 alleles to the gene pool
20 individuals are ff = 40 alleles are there any other recessive alleles to be concerned
with?
160 Ff = 160 recessive alleles from this source brings up the total to 200 alleles for the
recessive condition
if the recessive is 200 alleles, it must be that the dominant type makes up the rest of
the 1000 which is 800
verify
-320 FF = 640 alleles
-160 Ff = 160 alleles and the grand total is 800 alleles
7. PKU
• one in 10,000 babies is
born with this condition
• knowing this number we
can predict all of the other
frequencies
• this represents q2 = .0001
take the square root of
both sides and you have q
= .01 or 1%
• p would then be equal to
.99 or 99%
• what are the chances of
being a carrier for this
trait?
II. Hardy Weinberg Equilibrium
• A. Definition
• B. Conditions
• 1. Must have very
large population sizes
2. Must be no migration into or out of the population
3. Must be no mutation-or if mutations occur-they must be
equal in the forward as well as in the backward direction
4. Must be random mating
5. Must be no natural selection
6. No natural selection essentially means that all
organisms have the same fitness
C. Value of H-W equilibrium
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Static allelic and genotypic frequencies
Represents a situation of no evolution
Idealized situation
Can compare natural populations to idealized
Zero in on source of evolutionary pressures
III. Forces driving microevolutionary change
Example of microevolution
A. Natural selection
• Hair color comes in two varieties the dominant white color (R)
and the recessive red color (r)
• Make up the gene pool with 50 red pieces of plastic
representing the recessive and 50 white which represent the
dominant allele
• What are the initial p and q values for this population
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1.
p = .5
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2.
q = .5
2. Genotypic frequencies
• a. Homozygous dominant
• b. Heterozygous
• c. Homozygous recessive
3. Conditions change-new
predator arrives
• the predator is a hawk who
has keen vision
• he can see the red rabbits
very well and completely
depletes their number
every generation
• this would represent total
selection against the red
color
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13 WW 25WR 12RR
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12 red rabbits are
annihilated this generation
4. Recalculate p and q
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compute the number of surviving rabbits = 38 rabbits
how many alleles are in the gene pool = 38 x 2 = 76
of those 76 alleles, how many are the recessive type
this number is simply the single copy of the recessive allele that each
heterozygote possesses
25/76 = 33% is equal to the new q value
if q is equal to 33%, the new p is equal to 67%
renew the gene pool to 100 alleles reflecting the new p and q values
draw again
Sample problem
• A liver disease is caused by a
recessive allele. One person in
one hundred possesses the
condition.
• What are the values of p and q
in this population?
What is the percentage of carriers
in the population?
Sample problem
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Tall is dominant to short. A group of
individuals left to colonize an island probably
wishing to get away from the rat race. Two
short women and a short man join three
heterozygous tall men on the raft.
a.
What are the p and q values for the
pioneering group?
b.
After several years of inhabiting the
island, its population grows to 2500 people.
If H-W conditions
were in effect, how
many homozygous tall people would you
find in the population?
c.
A decree is handed down from the
governing body which rules that all short
people have to leave the island tomorrow
because it is getting too crowded. What are
the new p and q values of the population?
B. Mutations
• 1. occur very infrequently
• 2. in a large population, the
values of p and q are not
shifted rapidly due to
mutation
• 3. this is the source of
evolutionary change as it
produces the raw material on
which natural selection
operates
• 4. new alleles arise as
mutations
C. Migration
• 1. another name for migration is gene flow-either immigration or
emigration
• 2. usually the numbers of individuals who move out or into a population do
not change the p and q
• 3. the inertia of a large population mass buffers changes brought about by
gene flow
• 4. obviously migration has a homogenizing effect on two separate
populations if enough occurs Time_Fall_1993.jpg
• 5. populations evolving under different local conditions will look very
different
D. Population size
• 1. population size can produce
a sampling error
• 2. coin flip
• 3. any population that is over
10,000 individuals is relatively
free from sampling error
• 4. if a population becomes very
small, the p and q values of the
population can change by
chance alone
• 5. chance change in p and q
values due to small population
size is called genetic drift.
Sample calculations
• a. allele “a” makes up 1% of the gene pool or q = .01
• b. in a population of 1,000,000 people the gene pool is
2,000,000
• c. there would be 20,000 recessive alleles in the pool
• d. in a population of 100 individuals the gene pool is 200
alleles
• e. there are two recessive alleles in this pool
• f. you can see how it is easier to lose the two alleles than the
20,000 alleles
Example of genetic drift
5. Founder effect
6. Bottleneck effect
Example of bottleneck effect
Another analogy
E. Nonrandom mating-assortative mating
1. inbreeding
• a. Inbreeding
depression
• b. Increase in
homozygosity
2. Sexual selection
IV. Different types of selection
• A. Directional
selection
B. Disruptive selection
C. Stabilizing selection
D. Clinal variation
• 1. Bergmann’s rule
• 2. Allen’s Rule
Another example of clinal variation
E. Frequency dependent selection-scale eating
cichlids
F. Heterosis or hybrid vigor
Sickle cell vs. malaria