Download Imagine a happy rabbit population living on the bank of a river

Academy Biology 2011/12
ALLELE FREQUENCIES IN POPULATIONS
Learning Goals:
‣ Distinguish between alleles and genes
‣ Determine allele frequencies in a population
‣ State that forces are needed to change allele frequencies in populations
‣ Define evolution as a change in allele frequency in populations
Understanding the Concept of Allele Frequency in a Population
Imagine a population of mice living in a desert. The fur color is determined by one gene. The allele ‘A’ for
white fur is dominant over the allele ‘a’ for brown fur.
1. What phenotype will a mouse have with AA genotype? ______________________
2. What phenotype will a mouse have with Aa genotype? ______________________
3. What phenotype will a mouse have with aa genotype? _______________________
Figure 1 shows all the individuals in the mouse population including their genotype.
Shade in the mice that have brown fur in order to show the correct phenotype.
Aa
aa
Aa
AA
aa
aa
Aa
Aa
aa
AA
Figure 1
4. How often does allele A occur in the population (or how many “A”s do you count)? ___________
5. How often does allele a occur in the population? ___________
ALLELE FREQUENCY means how often an allele occurs in a population. We can determine the allele
frequency for the mouse population above by counting how often A occurs and divide the result by the total
number of alleles (10 diploid individuals, 2 alleles per individual ➮ 20 alleles). For a percentage multiply
that number by 100.
6. What is the allele frequency for A?(or, how often “A” occurs divided by 20 multiplied by 100)
_______________
7. What is the allele frequency for a? ________________
The population of mice lives in a desert area with brown colored sand. A population of eagles moves into
the area and starts eating mice. Figure 2 shows the mice populations after 10 generations in the presence of
eagles feeding on the mice. Shade in the mice that have brown fur in order to show the correct
phenotype.
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Academy Biology 2011/12
aa
aa
aa
Aa
Aa
aa
aa
Aa
aa
aa
Aa
aa
aa
Aa
Figure 2
There are now 14 individuals in the population of mice (total 28 alleles)
8. What is the allele frequency for A?(hint: see equation from Q#6) _______________
9. What is the allele frequency for a? ________________
10. Have the allele frequencies for A changed? ___________
A change of allele frequencies in the mice population has occurred due to the eagles moving in. The
population has adapted to the presence of the eagles, the population has evolved, and therefore we can state
that evolution is the change in allele frequency in a population. Anytime you need to change something
you need to apply a force, in order to change allele frequencies we need a force as well.
12. State the force in the mice example that caused a change in allele frequencies.
13.In this scenario, how was sexual reproduction advantageous over asexual reproduction?
Another Example - Bunnies!
Imagine a happy bunny population living on the bank of a river. The population varies in ear length.
AA or
Aa
aa
Allele A is
dominant
and codes
for long ears
Allele a is
recessive
and codes
for short
ears
LONG EARS mean more heat loss (advantageous in hot climates, helps organism to stay cool)
SHORT EARS mean less heat loss (advantageous in cold climates, preserves energy)
Imagine you are counting all the alleles in the population and you find that the alleles are distributed as
follows: 40% A and 60% a. Suddenly an earthquake occurs, the river changes course and the rabbit
population is split. Bunnies don't swim well and the populations are effectively isolated. The allele
frequencies are the same in each new population (40% A and 60% a). The earthquake has disrupted the
bunnies’ food source. In search of food the population south of the river migrates to the warmer south and
the population north of the river migrates to the colder north. The rabbits reproduce and many generations
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Academy Biology 2011/12
(=time) pass. After many years have passed you again count the allele frequencies for long and short ears in
the southern and northern bunny populations.
13. How do you predict the allele frequencies will have changed for A and a for the north and the
south population? Record your answer in Table 1, then complete the remaining columns.
Table 1: Prediction and Justifications for Change in Allele Frequencies
POPULATION Predicted change in
allele frequency
North
Predicted phenotypes of
bunnies
Reason(s) for prediction(s)
A:
a:
South
A:
a:
14. State the force for change in this example. ____________________________
15. Imagine you found the following allele frequencies:
Southern population: A: 85% a:15% -> more long eared bunnies
Northern population: A:17% a:83% -> more short eared bunnies
Has evolution occurred? ______________
Give reasons for your answer:
14. The allele frequency for A and a before the earthquake had not changed over time (stayed 40%
A: 60% a). How can you explain this observation?
15. Imagine the original bunny population did not have variation in ear length before the earthquake
(all rabbits had long ears AA). Do you predict to get a difference in ear lengths between the
northern and southern populations after the earthquake and migration? What makes you think
so?
17. “Individuals do not adapt (change) but populations do.” Explain this fact using the bunny
population that migrated to the north as an example.
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