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NATURAL SELECTION IN POPULATIONS
DR WEINER
BIO H LAB
Background
Evolutionary changes occur in populations rather than individuals. Changes in a species
can be traced by measuring changes in the frequency of alleles within a population. If there is no
selection on a given trait in a population then the population is in Hardy-Weinberg equilibrium
and the allele frequencies for that particular trait will remain unchanged over the generations. If,
however, that trait is being acted upon by natural selection, the frequency of the alleles will
change over the generations and the population is no longer in Hardy-Weinberg equilibrium.
If a population is in Hardy-Weinberg equilibrium for a given trait, one can calculate the
genotypic frequencies for that trait. The mathematical equation for Hardy-Weinberg equilibrium
is:
p2 +2pq + q2 = 1 and p + q = 1
p2 = homozygous dominant 2pq = heterozygous and q2 = homozygous recessive
where p represents the dominant allele and q represents the recessive allele.
Purpose
In this investigation, you will determine the allele frequencies for bead color in two populations,
one that is in Hardy-Weinberg equilibrium and one in which one of the alleles is selected against.
Materials
Box of beads
2 beakers
Procedure
Part A: A population in Hardy Weinberg equilibrium
1. Obtain a box of red and white beads. Count out 100 red (R) and 100 white (r) beads and
place them in the beaker. These represent your P generation, the parents.
2. Determine the frequency of the R allele and the r allele for the P generation and record them
in the table.
3. WITH YOUR EYES CLOSED, pick out two beads, one at a time. KEEP THEM IN THEIR
PAIRS.
4. Repeat step 2 until you have 30 pairs of beads.
5. THESE 30 PAIRS REPRESENT YOUR F1 GENERATION. Record the number of RR, Rr,
and rr offspring produced in the table on the blackboard. WHEN ALL GROUPS HAVE
RECORDED THEIR DATA INTO THE TABLE AND THE CLASS DATA HAS BEEN
COMPILED, RECORD THE CLASS DATA INTO THE TABLE BY F1 GENERATION..
6. The F1 generation will now become the parents to produce the F2 generation. Place the 30
pairs of beads into another beaker.
7. WITH YOUR EYES CLOSED, pick out two beads, one at a time. Keep them I theirs pairs.
8. Repeat step 6 until you have 20 pairs of beads.
9. THESE 20 PAIRS REPRESENT YOUR F2 GENERATION. Record the number of RR, Rr,
and rr offspring produced in the table on the blackboard. WHEN ALL GROUPS HAVE
RECORDED THEIR DATA INTO THE TABLE AND THE CLASS DATA HAS BEEN
COMPILED, RECORD THE CLASS DATA INTO THE TABLE BY F2 GENERATION.
10. Calculate the frequency for the R allele and the r allele for both the F1 and F2 generation by
using the following formula:
Allele frequency = number of alleles in the population/total alleles in the population
Part B: A Population Under Natural Selection
1. Return the 200 beads that you counted to the original beaker.
2. WITH YOUR EYES CLOSED, pick out two beads. Keep them as a pair.
3. Repeat step 2 until you have 30 pairs of beads. HOWEVER, EVERY TIME YOU
PRODUCE AN rr OFFSPRING THAT ORGANISM DIES AND CANNOT REPRODUCE.
YOU MUST PICK AGAIN UNTIL AN rr OFFSRING IS NOT PRODUCED.
4. Record the number of RR Rr rr offspring produced THAT CAN REPRODUCE AGAIN, in
the table on the blackboard. WHEN ALL GROUPS HAVE RECORDED THEIR DATA
INTO THE DATA AND THE CLSS DATA HAS BEEN COMIPLED, RECORD THE
DATA INTO THE TABLE. THIS REPRESENTS YOUR F1 GENERATION
5. The F1 generation will now become the parents to produce the F2 generation. Place the 30
pairs of beads into another beaker.
6. WITH YOUR EYES CLOSED, PICK OUT TWO BEADS. Keep them in their pairs.
7. Repeat step 6 until you have 20 pairs of beads. H OWEVER, EVERY TIME YOU
PRODUCE AN rr OFFSPRING, THAT OFFSPRING DIES. YOU MUST PICK AGAIN
UNTIL AN rr OFFSPRING IS NOT PRODUCED.
8.These 20 pairs represent your F2 generation. Record the number of RR, Rr, and rr offspring
produced in the table on the blackboard. WHEN ALL GROUPS HAVE RECORDED THEIR
DATA INTO THE TABLE AND THE CLASS DATA HAS BEEN COMPILED, RECORD
THE CLASS DATA INTO THE TABLE BY F2 GENERATION
9. Calculate the frequency of RR Rr and rr for the P generation, F1 generation and F2 generation
in the table.
OBSERVATIONS PART A HARDY WEINBERG EQUILIBRIUM
TABLE 1 LAB GROUP DATA
RR
Rr
rr
RR
Rr
rr
F1 GENERATION
F2 GENERATION
TABLE 2 CLASS DATA
F1 GENERATION
F2 GENERATION
CALCULATE ALLELE FREQUENCIES SHOW WORK:
TABLE 3 ALLELE FREQUENCIES
ALLELE FREQUENCIES
R ALLELE
r ALLELE
P GENERATION
F1 GENERATION
F2 GENERATION
OBSERVATIONS PART B NATURAL SELECTION
TABLE 4 LAB GROUP DATA
RR
Rr
rr
RR
Rr
rr
F1 GENERATION
F2 GENERATION
TABLE 5 CLASS DATA
F1 GENERATION
F2 GENERATION
CALCULATE ALLELE FREQUENCIES SHOW WORK:
TABLE 6 ALLELE FREQUENCIES
ALLELE FREQUENCIES
R ALLELE
r ALLELE
P GENERATION
F1 GENERATION
F2 GENERATION
Conclusions
1. What happened to the allele frequencies for R and r from P to F1 to F2 in Part A?
2. What happened to the allele frequencies for R and r from P to F1 to F2 in Part B?
3. Explain why the allele frequencies in Part A were the way they were.
4. Explain why the allele frequencies were the way they were in Part B.
Applying the Hardy Weinberg Law.
Sample question:
Suppose that in a population of frogs, a dominant allele is for red spotted skin and a recessive
allele is for brown spotted skin. Nine out of every 100 frogs is brown spotted. How many of the
100 frogs are heterozygous?
Solution:
p = dominant allele q = recessive allele. 9/100 frogs are recessive = q2 = 0.09
q= square root of 0.09 = .3
Since p + q = 1, p = 1 – 0.3 = 0.7
Put p and q in to Hardy-Weinberg formula
p2 +2pq + q2 = 1 and p + q = 1
2
2
p = (0.7)
2pq = 2 x .3 x .7 = 0.42
q2 = (0.3)2
PROBLEMS:
1. The ability to taste PTC paper is a dominant trait. Nontasters are recessive. A population in
South Plainfield High School has 325 out of 1237 people being nontasters.
a. What is the frequency of q allele. Explain
b. What is the frequency of the p allele. Explain.
c. What is the frequency of tasters in the populations. Explain.
d. What is the frequency of heterozygotes in the population. Explain.
SHOW ALL WORK OR YOU GET NO CREDIT
2. Albinism is a recessive disorder. A population has 2731 individual are normal out of 2811
total individuals.
a. What is the frequency of q allele. Explain
b. What is the frequency of the p allele. Explain.
c. What is the frequency of normal individuals in the populations. Explain.
d. What is the frequency of heterozygotes in the population. Explain.
SHOW ALL WORK OR YOU GET NO CREDIT