Selective Forces on a Population:
An example of Natural Selection with humans, sickle cell disease and malaria.
Sickle cell disease is a recessive genetic disorder where individuals make a ‘crystallized’ form of hemoglobin.
This causes the red blood cells that contain the hemoglobin to take on a sickle shape in the absence of oxygen. These
sickled cells get caught in blood vessels and create immense amounts of pain and even death.
Heterozygote individuals are resistant to malaria though. When the protozoan that causes malaria is deposited
into their blood stream by the mosquito vector, the protozoan is not able to infect the sickled cells that they have. Here is
what we call an ‘Evolutionary Tradeoff’.
In this activity you will be looking at the allele frequencies of the normal hemoglobin (A) and sickled
hemoglobin (a) in a population. Allele frequency refers to how often an allele occurs in a population. Remember,
each individual in a population carries two alleles for every gene. The collection of all the alleles (genes) in a population
is called a GENE POOL Allele frequencies can change in a population over time, depending on the ‘selective forces’
shaping the population. Predation, food availability, and disease are all examples of selective forces. Evolution is
defined as a “change in the allele frequency of a population”.
What do you think will happen to the frequencies of the A (normal hemoglobin) and a
alleles (sickled hemoglobin) as a result of the presence of malaria?
Question:
Write your hypothesis down in your journal.
Activity:
1.
2.
3.
4.
5.
6.
7.
8.
Red beans represent gametes carrying the A allele (normal hemoglobin).
White beans represent gametes carrying the a allele (sickled hemoglobin).
Obtain five containers (plastic cups).
Label them as follows: 1. AA, 2. Aa, 3. aa, 4. Non-surviving alleles, 5. Gene Pool
The gene pool exists in a region of Africa infested with malaria!
Place 75 red and 25 white beans in the GENE POOL container and mix the beans up.
Simulate fertilization by picking out TWO alleles (beans) WITHOUT LOOKING.
For every two beans that are chosen from the gene pool, another person needs to flip a coin
to determine whether that individual is infected with malaria. Heads – Malaria, Tails – No
malaria
9. Use the table below to determine what to do with your beans based on your results:
Genotype
AA
Phenotype
No sickle cell disease.
Malaria susceptibility.
Aa
No sickle cell disease
(faint symptoms).
Malaria resistance.
Sickle Cell disease.
Malaria resistance.
aa
Malaria (HEADS)
DIE!
Put in non-surviving
cup.
LIVE!
Put in Aa cup.
Not Infected (TAILS)
LIVE!
Put in AA cup.
DIE because of sickle
cell disease.
Live for a brief period of
time, usually die because of
sickle cell anemia before
they can reproduce.
Put in aa cup.
LIVE!
Put in Aa cup.
10. Repeat the above steps until the Gene Pool is completely used up (all the beans are gone).
11. At the end of the round, COUNT the number of individual red beans (A alleles) and white
beans (a alleles) in the containers labeled AA and Aa. These individuals survive to
reproduce! Record these numbers in your data table. Then, have them reproduce by adding
one more bean for each one that survived. So, for example, if you had 15 red beans and 10
white beans survive, then you should put 30 red beans and 20 white beans back in the starting
gene pool for the next generation.
12. Because aa individuals do not usually survive to reproduce, move all the SS beans to the
non-surviving alleles container.
13. Now, do the process THREE more times to ensure reliability in our data, starting with the
gene pool from the previous generation. Record the bean number at the end of each round.
Data Table:
Gener
ation
1
Starting Population
Allele A Allele a
Total
Allele
(Red
(White Number percentage
beans) Beans)
of
Starting Startin
Beans
A
a
#
g#
75
25
100
75
25
Ending Population (after malaria)
Allele A
Allele a
Total
Allele
(Red
(White Number
Percentage
Beans)
Beans)
of
surviving
Ending # Ending # Beans
A
a
2
3
4
Gener
ation
1
2
3
4
Sample Data Table
Starting Population
Allele A Allele a
Total
Allele
percentage
(Red
(White Number
beans) Beans)
of
Starting Startin
Beans
A
a
#
g#
75
25
100
75
25
120
26
146
82
18
Ending Population (after malaria)
Allele A
Allele S
Total
Allele
(Red Beans) (White Number
Percentage
Ending #
Beans)
of
surviving
Ending
Beans
#
A
a
60
13
73
82
18