Medflies and Malathion
There are 50 medflies on a farm. 2 medflies are heterozygous for the malathion resistance gene
(Mm), while the other 48 are homozygous recessive (mm) and cannot live in the presence of melathion.
Fill in the “before malation” column below.
Before Malathion
After Malathion
# homozygous dominant
# heterozygous
# homozygous recessive
Copies of M
% of M
Copies of m
% of m
The farmer sprays the farm with malathion, killing all flies without the resistance phenotype. The
remaining two flies have children, bringing the farm’s fly population back to a total of 50 flies. Show a
Punnett square for the cross of these two flies.
Assume that the two remaining flies after the malathion application have 12 offspring form each
box in your Punnett square. Consider all 50 flies now on the farm and fill in the “after melathion” column.
Graph the % of M and % of m in the population below. Use one color for M and another for m and include
a key.
100
75
50
25
0
before melathion
after melathion
1. What type of heritable variation did the flies have?
2. What was the selective pressure?
3. How did natural selection change the frequency of the M allele in this population?
4. Is there a difference in evolutionary fitness between heterozygous flies and homozygous dominant
flies? Why or why not?
Natural Selection Comic!
Panel 1: Draw three organisms (one homozygous dominant, one heterozygous, and one
homozygous recessive) of the same species and label their genotypes. Your drawings should show their
phenotypes. Also, draw the food that these organisms eat. Beneath your drawing, write what type of
organisms they are and the food they eat. Also explain their phenotypes in words. *The dominant
phenotype should have higher evolutionary fitness than the recessive phenotype. Vocabulary: genetic
variation, phenotype
Panel 2: Show how the phenotypes of the organisms affect their ability to compete for food. Give a
written description of why there is competition for food and why this phenotype affects whether or not
they can get food. Vocabulary: selective pressure, limited resources, competition
Panel 3: Draw only the organism(s) with the highest evolutionary fitness. In writing, explain why this
organism was best able to compete for the food. Vocabulary: evolutionary fitness
Panel 4: Show the next generation of this species with a Punnett square (the offspring of the surviving
organism). In writing, explain how the population is different from the original population. Be sure to
include how the genes in the population are different from the genes present in the first panel.
Vocabulary: natural selection, microevolution