In Ms. Havlik's idealized simulated
population (Population One), students all began with heterozygous genotypes (Aa). That
imaginary population reached the Hardy-Weinberg equilibrium in one generation and did not
shift significantly in subsequent generations.
Then the simulation began to explore what would happen to allele frequencies in two
subsequent populations, where evolutionary forces were in play.
In the second round of mating (Population Two), the homozygous
recessive genotype (aa) was at a disadvantage and died. In order to keep the size of the
population constant, each pair of students was instructed to continue mating their
populations until they produced two and only two living offspring. This resulted in a
shift of genotype frequencies within the population and, therefore, evolution of the
population by natural selection.
In the next round (Population Three), the simulation demonstrated what
would happen if the heterozygous genotype (Aa) was at an advantage and the homozygous
dominant genotype (AA) had only a 50-50 chance of survival. Again, in this situation the
frequency of genotypes shifted from the equilibrium state to reflect the effect of natural
selection on genotypes (expressed as phenotypes) in the population.
This situation represented balanced polymorphism -- a heterozygous genotype confers an
advantage over the homozygous condition. This allows a deleterious recessive allele to
remain in a population-because the heteozygous genotype has an advantage and contains one
Ms. Havlik then helped students connect their simulation to a
real-world application based on an earlier reading. Homozygous recessive genotype is
responsible for sickle cell anemia in individuals, but balanced polymorphism
(heterozygosity) protects against malaria, an often fatal disease present in areas of
Africa. The homozygous dominant does not protect against malaria, so therefore
heterozygosity is favored. The classroom simulation also demonstrated the advantages of
balanced polymorphism (heterozygosity) and showed that evolution occurs at the level of
populations, not individuals.
For a variation on this lab, see the University of Michigan Web site Establishing Hardy-Weinberg