Download Calculation of allele frequencies of breeding

Lab 14 – Microevolution
Objectives


Compare how natural selection and genetic drift alter allele frequencies in a population
Calculate allele frequencies using Hardy-Weinberg equilibrium
Introduction
A population consists of individuals of the same species living in the same area. Allele
frequencies refer to the percentage of certain alleles in that population. When the allele
frequencies shift over time, microevolution occurred in that population. Five main reasons why
allele frequencies change include natural selection, genetic drift (chance events), nonrandom
mating, gene flow, and mutations.
Charles Darwin and Alfred Russel Wallace are well-known for outlining the ideas behind
natural selection. Four major ideas include:
 There is inheritable variation in the offspring.
 Organisms produce a surplus of offspring.
 Organisms compete for limited resources.
 Organisms with inherited characters that best fit them with the environment are more
likely to leave more offspring.
Variation of traits exists in a population. This is evident as you look around at your
classmates. Are you the same height as your lab partner? Does the person across the lab have
the same eye color, hair color, or body build as you? Variation is easy for us to see in humans,
but variations exist in all organisms. To us, all mosquitoes look the same, but upon closer
observation, we can see many variations in the population of mosquitoes. These variations arise
from random mutations in DNA. Some mutations go unnoticed; others create dramatic changes
in an organism; and still others produce only slight changes. In any population individuals will
produce more offspring than can be supported by available resources. More individuals than
resources lead to competition for the limited resources. The individuals that survive are those
individuals within a population that have the best fit to that environment. The survivors
reproduce and leave more offspring, passing along variations that were beneficial in the
environment to their offspring. This increases the frequency of that allele. It is important to note
that random chance produced the mutation that caused the variation, but the selection of that
allele can be directional, stabilizing, or disruptive.
Genetic drift means random, chance events that affect changes in allele frequencies. For
instance, if a volcano eruption killed off the majority of a certain plant population, the remaining
survivor population would have different allele frequencies from the original population.
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Lab 14 – Microevolution
We will use the Hardy-Weinberg equilibrium to calculate allele frequencies. The HardyWeinberg equilibrium assumes that allele frequencies remain in equilibrium if five conditions are
met. These five conditions include:
 all the reproducing individuals have similar survival and reproductive rates;
 the population is infinitely large such that chance events don’t affect the equilibrium;
 the remaining prey organisms will mate at random;
 no migration occurs within the population;
 no mutations occur.
In nature, these conditions rarely exist, but these conditions serve as a hypothetical
starting point to gauge if microevolution is occurring. When a population changes in allele
frequencies over time, then microevolution occurred.
In this lab, beans will represent insects. Imagine that the beans are individuals in the
same population, that is, they are the same species that could mate and produce offspring. Each
of our insects will have one of the following genotypes: BB = dark phenotype, Bb = dark
phenotype, bb = light phenotype. Since the only alleles the insects can have are B or b, we will
say “p” represents the dominant allele frequency, and “q” represents the recessive allele
frequency. The sum of their frequencies must equal 1, so p + q = 1. The Hardy-Weinberg
formula is
p2 + 2pq + q2 = 1
This states that p2 represents the homozygous dominant genotype (BB) frequency, 2pq
represents the heterozygous genotype (Bb) frequency, and q2 represents the homozygous
recessive genotype (bb) frequency.
Our starting insect population will be 50 individuals. We will start our simulation with
the allele frequencies of p= 0.7 and q = 0.3. Remember, if a population maintains these allele
frequencies over time, then microevolution did not occur. If a population changes these allele
frequencies over time, then microevolution did occur.
Calculate the number of individuals (out of 50) that have the appropriate genotypes:
Type of individual
BB
Bb
bb
Percentage of individuals
Number to start out of 50 total
individuals
p2 =
2pq =
q2 =
You now have the appropriate number of insects (beans) of each genotype to start the simulation.
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Lab 14 – Microevolution
14.1 Natural selection and Genetic Drift
1.
2.
3.
4.
5.
6.
7.
8.
9.
Calculate your starting population genotypes as described on the previous page.
Count the appropriate number of beans. The dark beans with no scars are the
homozygous dominant (BB). The spotted beans are heterozygous (Bb). The light colored
beans are homozygous recessive (bb).
Distribute the starting population evenly over the carpet. Select one person to be the
timer and another person to be the predator.
Follow the following procedures for the predator.
a. Predators can only pick up one insect at a time. Each insect must be picked up with
forefinger and thumb to simulate a bird beak and placed into a small hole in a cup.
b. Predators can only hunt for 30 seconds. In this time the predator will pick up as many
insects (beans) as possible.
c. Your Instructor may demonstrate the procedure of hunting.
d. Do not try to pick up certain colors of prey selectively. A hungry bird would not be so
selective.
After the 30 seconds of hunting, count the remaining survivor insects. These survivors
represent the breeding individuals of the next generation. Calculate the next generation’s
genotypes by completing the worksheet.
Place the appropriate number of beans to begin the next generation on the carpet and
repeat above procedures.
You will perform three generations in this manner.
Suddenly, a horrific wind killed the majority of individuals in the population. Only 8
brown individuals and 2 spotted individuals survived. Using the appropriate worksheet,
calculate how many individuals you need to begin the next generation.
Distribute the individuals in their environment again and hunt as before. Repeat the
repopulation and hunting for three additional generations.
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Lab 14 – Microevolution
Example Worksheet
Prey generation
1
Habitat
Brown
Number of insects surviving predation
Dark (BB)
12
Dark (Bb)
13
Light (bb)
2
Total
27
Calculation of allele frequencies of breeding-surviving insects
(2 x BB parents)
24
+ (1 x Bb parents)
+
13
= (Total # of B alleles)
=
37
(2 x bb parents)
4
+ (1 x Bb parents)
+
13
= (Total # of b alleles)
=
17
(# of B alleles)
37
+ (# of b alleles)
+
17
=
=
(Total # of alleles)
54
Calculating p and q
# of B alleles ( 37
)/ total # of alleles (
54
) = p(
.69
)
# of b alleles ( 17
)/ total # of alleles (
54
) = q(
.31
)
p .69
=1
+ q .31
Calculations of genotypic frequencies in next generation
p2 (BB) (.69)2 + 2pq (Bb) 2 (.69)(.31) + q2 (bb) (.31)2 = 1
p2 (.69)2 x 50 = Dark (BB) 24
2pq 2(.69)(.31) x 50 = Dark (Bb)
21
q2 (.31)2 x 50 = Light (bb)
5
Total is 50 individuals ready to be placed back on carpet.
Make up a new population of insects in these proportions and start cycle over
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Lab 14 – Microevolution
Worksheet
Prey generation
1
Habitat
Number of insects surviving predation
Dark (BB)
Dark (Bb)
Light (bb)
Total
Calculation of allele frequencies of breeding-surviving insects
(2 x BB parents)
+ (1 x Bb parents)
+
= (Total # of B alleles)
=
(2 x bb parents)
+ (1 x Bb parents)
+
= (Total # of b alleles)
=
(# of B alleles)
+ (# of b alleles)
+
=
=
(Total # of alleles)
Calculating p and q
# of B alleles (
)/ total # of alleles (
) = p(
)
# of b alleles (
)/ total # of alleles (
) = q(
)
p
+q
=1
Calculations of genotypic frequencies in next generation
p2 (BB) _______+ 2pq (Bb) _______+ q2 (bb) _______ = 1
p2 (_______) x 50 = Dark (BB)
2pq (_______) x 50 = Dark (Bb)
q2 (_______) x 50 = Light (bb)
Total is 50 individuals ready to be placed back on carpet.
Make up a new population of insects in these proportions and start cycle over
5
Lab 14 – Microevolution
Worksheet
Prey generation
2
Habitat
Number of insects surviving predation
Dark (BB)
Dark (Bb)
Light (bb)
Total
Calculation of allele frequencies of breeding-surviving insects
(2 x BB parents)
+ (1 x Bb parents)
+
= (Total # of B alleles)
=
(2 x bb parents)
+ (1 x Bb parents)
+
= (Total # of b alleles)
=
(# of B alleles)
+ (# of b alleles)
+
=
=
(Total # of alleles)
Calculating p and q
# of B alleles (
)/ total # of alleles (
) = p(
)
# of b alleles (
)/ total # of alleles (
) = q(
)
p
+q
=1
Calculations of genotypic frequencies in next generation
p2 (BB) _______+ 2pq (Bb) _______+ q2 (bb) _______ = 1
p2 (_______) x 50 = Dark (BB)
2pq (_______) x 50 = Dark (Bb)
q2 (_______) x 50 = Light (bb)
Total is 50 individuals ready to be placed back on carpet.
Make up a new population of insects in these proportions and start cycle over
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Lab 14 – Microevolution
Worksheet
Prey generation
3
Habitat
Number of insects surviving predation
Dark (BB)
Dark (Bb)
Light (bb)
Total
Calculation of allele frequencies of breeding-surviving insects
(2 x BB parents)
+ (1 x Bb parents)
+
= (Total # of B alleles)
=
(2 x bb parents)
+ (1 x Bb parents)
+
= (Total # of b alleles)
=
(# of B alleles)
+ (# of b alleles)
+
=
=
(Total # of alleles)
Calculating p and q
# of B alleles (
)/ total # of alleles (
) = p(
)
# of b alleles (
)/ total # of alleles (
) = q(
)
p
+q
=1
CAUTION: STRONG WIND ADVISORY. INSECTS: TAKE COVER OR
FACE THE CONSEQUENCES.
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Lab 14 – Microevolution
Worksheet
Prey generation
4
Habitat
Number of insects surviving STRONG WIND
Dark (BB)
8
Dark (Bb)
2
Light (bb)
0
Total
10
Calculation of allele frequencies of breeding-surviving insects
(2 x BB parents)
+ (1 x Bb parents)
+
= (Total # of B alleles)
=
(2 x bb parents)
+ (1 x Bb parents)
+
= (Total # of b alleles)
=
(# of B alleles)
+ (# of b alleles)
+
=
=
(Total # of alleles)
Calculating p and q
# of B alleles (
)/ total # of alleles (
) = p(
)
# of b alleles (
)/ total # of alleles (
) = q(
)
p
+q
=1
Calculations of genotypic frequencies in next generation
p2 (BB) _______+ 2pq (Bb) _______+ q2 (bb) _______ = 1
p2 (_______) x 50 = Dark (BB)
2pq (_______) x 50 = Dark (Bb)
q2 (_______) x 50 = Light (bb)
Total is 50 individuals ready to be placed back on carpet.
Make up a new population of insects in these proportions and start cycle over
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Lab 14 – Microevolution
Worksheet
Prey generation
5
Habitat
Number of insects surviving predation
Dark (BB)
Dark (Bb)
Light (bb)
Total
Calculation of allele frequencies of breeding-surviving insects
(2 x BB parents)
+ (1 x Bb parents)
+
= (Total # of B alleles)
=
(2 x bb parents)
+ (1 x Bb parents)
+
= (Total # of b alleles)
=
(# of B alleles)
+ (# of b alleles)
+
=
=
(Total # of alleles)
Calculating p and q
# of B alleles (
)/ total # of alleles (
) = p(
)
# of b alleles (
)/ total # of alleles (
) = q(
)
p
+q
=1
Calculations of genotypic frequencies in next generation
p2 (BB) _______+ 2pq (Bb) _______+ q2 (bb) _______ = 1
p2 (_______) x 50 = Dark (BB)
2pq (_______) x 50 = Dark (Bb)
q2 (_______) x 50 = Light (bb)
Total is 50 individuals ready to be placed back on carpet.
Make up a new population of insects in these proportions and start cycle over
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Lab 14 – Microevolution
Worksheet
Prey generation
6
Habitat
Number of insects surviving predation
Dark (BB)
Dark (Bb)
Light (bb)
Total
Calculation of allele frequencies of breeding-surviving insects
(2 x BB parents)
+ (1 x Bb parents)
+
= (Total # of B alleles)
=
(2 x bb parents)
+ (1 x Bb parents)
+
= (Total # of b alleles)
=
(# of B alleles)
+ (# of b alleles)
+
=
=
(Total # of alleles)
Calculating p and q
# of B alleles (
)/ total # of alleles (
) = p(
)
# of b alleles (
)/ total # of alleles (
) = q(
)
p
+q
=1
Calculations of genotypic frequencies in next generation
p2 (BB) _______+ 2pq (Bb) _______+ q2 (bb) _______ = 1
p2 (_______) x 50 = Dark (BB)
2pq (_______) x 50 = Dark (Bb)
q2 (_______) x 50 = Light (bb)
Total is 50 individuals ready to be placed back on carpet.
Make up a new population of insects in these proportions and start cycle over
10
Lab 14 – Microevolution
Worksheet
Prey generation
7
Habitat
Number of insects surviving predation
Dark (BB)
Dark (Bb)
Light (bb)
Total
Calculation of allele frequencies of breeding-surviving insects
(2 x BB parents)
+ (1 x Bb parents)
+
= (Total # of B alleles)
=
(2 x bb parents)
+ (1 x Bb parents)
+
= (Total # of b alleles)
=
(# of B alleles)
+ (# of b alleles)
+
=
=
(Total # of alleles)
Calculating ending p and q for our experiment
# of B alleles (
)/ total # of alleles (
) = p(
)
# of b alleles (
)/ total # of alleles (
) = q(
)
p
+q
=1
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Lab 14 – Microevolution
14.1 QUESTIONS
1.
The population of insects began with a p (dominant allele frequency) of 0.7 and a q
(recessive allele frequency) of 0.3.
a. What was the ending p?
b. What was the ending q?
c. Based on the premise of the Hardy-Weinberg equilibrium, discuss if microevolution
occurred in the population.
2.
Of the five conditions that could cause allele frequencies to change in a population,
natural selection is a major player. The following questions refer to events that occurred
prior to the strong wind.
a. From the experiment, discuss how the insects had different traits.
b. From the experiment, which insect traits had the best advantage and were passed on
to future generations?
c. How is reproduction that occurred in this experiment different from how reproduction
occurs in the natural environment?
d. If there had been competition for resources in this experiment, how might your results
have been different?
e. Beneficial traits are passed on. How can DNA change so that a beneficial trait may
surface and then be passed on to help future generations survive?
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Lab 14 – Microevolution
3.
To relate natural selection to a real life example, let’s consider antibiotic resistance.
Imagine that in a population of a certain bacterial species, a mutation in DNA of a few
individuals rendered them resistant to ampicillin. You expose the antibiotic ampillicin to
this population. Using ideas of natural selection learned from your insect experiment,
discuss why future generations of this population will be resistant to ampillicin.
4.
Of the five conditions that could cause allele frequencies to change in a population,
genetic drift is another cause. Genetic drift means chance changes. The following
questions refer to events that occurred after the strong wind.
a. A chance event (like the strong wind) could change the allele frequency in a
population drastically. Would such a drastic change occur more in a small population
or in an infinitely large population?
b. From the experiment, discuss why you never saw the white insects again or saw very
few of them.
c. In a sexually reproducing population, explain how it is possible to get a white insect,
even if all the white insects were wiped out.
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