Natural Selection and Allelic Frequency
Problem: How does natural selection affect allelic frequency?
Hypothesis: If we use everyday objects, we will be able to simulate how natural selection works, and see
how it affects the frequency of certain alleles in a population.
Data/ Observations:
Data Table:
Allele G
Allele g
Generation
Number
Percentage
Frequency
Number
Percentage
Frequency
Start
50
50
0.50
50
50
0.50
1
34
68%
0.68
16
32%
0.32
2
55
81%
0.81
13
19%
0.19
3
82
85%
0.85
14
15%
0.15
4
86
90%
0.90
10
10%
0.10
5
91
93%
0.93
7
7%
0.07
Graph on separate page.
Analysis:
1. Did either allele disappear? Why or why not?
No, neither allele disappeared. The numbers for allele G increased, and the numbers for allele g
decreased, but it did not decrease to the point that it disappeared. Were we to perform more trials, the
allele g would have disappeared at some point. Also, it was hard for that allele to disappear because
there were very few left, and it was hard to get the two white alleles to throw away. So it would have
taken more time to get that.
2. What does your graph show about allelic frequencies and natural selection?
My graph shows that in allelic frequencies, it depends totally on chance. If it is a recessive allele, it is
harder for the organism to get both recessive alleles to get that trait, so natural selection will not do
away with that organism. My graph shows that in allelic frequencies, when an allele is beneficial, it will
continue to increase, but if it is harmful, the allele will decrease. However, it is difficult for the allele to
completely disappear, although it is easier to be a carrier for it.
3. What would happen to the allelic frequencies if the number of eagles declined?
If the number of eagles declined, the allelic frequencies would go up because there would not be as
many predators to feed on the rabbits who the alleles belong to. If it was in an environment in which
eagles feed on gray rabbits more often then white rabbits, then the allelic frequencies for gray would
rise, due to natural selection and the fact that the number of gray rabbits would rise because they
weren't killed by eagles. So, the gray rabbits would live longer, be able to reproduce, and the gray rabbit
population would grow. If eagles fed on white rabbits, the allelic frequencies for white fur would grow,
while the alleles for gray would decline.
Conclusion:
In this lab, we simulated allelic frequencies and natural selection using brown and white beans
and two different cups. First, we took 50 white beans and 50 brown beans, and mixed them together in
a cup. Then, without looking, we took out two beans from the cup labeled "Surviving" until there were
no more beans in that cup and we had 50 pairs of beans on the table. We sorted the different pairs: any
pair with a brown bean would go in one row (brown is dominant, so it could be heterozygous brown or
homozygous brown) and the other row was homozygous white, because white was recessive. The row
with all white beans was trashed into the "Dead" cup to represent the organisms that had not survived,
and then we counted the number of brown beans and white beans left in the other row, and recorded
that in our data table. Then we found the percentage of brown beans to white beans using the equation
given to us below:
Number of Brown/White Alleles
(100)
Total Number of Alleles
For the second generation, we used the percentages to see how many beans we would put in the cup. If
the percent of brown beans was 70% and the percent of white beans was 30%, we would take 70 brown
beans and 30 white beans. We repeated this for 5 generations. My hypothesis was correct; we were
able to simulate how natural selection works, and we were able to see how it affected allelic frequencies
using everyday items.
This lab relates to what we are learning in class because we are learning about natural selection,
and with this lab, we were able to see that it all comes back to chance, and any undesired trait would
decrease because the animal with that trait would die, and, unable to reproduce, would not be able to
spread that trait to others. In this experiment, I have learned that it is by chance whether the organism
will have the undesired trait or not. If the organism does get the undesired trait, he will die earlier, and
will not be able to reproduce, so by natural selection, the trait will gradually go out of the population.
Although this lab was time-consuming and took a lot of math and counting, I was able to learn from it
and learn more about natural selection. Now I can relate our textbook material back to this hands-on
lab.