Laboratory Review ❮ 239stasis, or not evolving. Your goal in this lab is to model how allele frequencies change in a
generation of some imaginary population, and you will do this using a computer model.
Basic Setup
On the AP Biology exam, will you have to open up a spreadsheet file and correctly enter a
formula? No. Will you have to understand how to use the Hardy-Weinberg equilibrium and
how to correctly analyze the data obtained? You bet! The idea to understand is how the fit-
ness of an allele affects its frequency in a population. For example, there are two alleles for a
given gene: A and a. If a population is in Hardy-Weinberg equilibrium (i.e., not evolving),
and the frequency of both alleles is 0.5 (meaning one half of the alleles in this population’s
gene pool is the dominant A form, whereas the other half is in the recessive a form), then it
will remain that way for gazillions of generations. But how could you make that ratio
change? That, my friend, is evolution, and that is the point of this lab. Using tools such as
computer programs and spreadsheets, you can model how a hypothetical gene pool changes
from generation to generation.
Results
Though the bulk of this lab was dedicated to creating your spreadsheet, the real investiga-
tion begins when you get to tweak your non-evolving population. The equations you have
to know for this experiment are p+q=1 and p^2 + 2 pq+q^2 =1. Chapter 12 lists the five
conditions required for the existence of Hardy-Weinberg equilibrium:
- No mutations
- No gene flow
- No genetic drift (large population size)
- No natural selection (so that the traits are neutral; none gives an advantage or
disadvantage) - Random mating
If any of these five conditions does not hold true, then the population will experience
microevolution, and the frequencies of the alleles will be subject to change.
Using your computer model, you can design an experiment to measure the effect of
selection, heterozygote advantage, and genetic drift:
- Selection.Imagine that an individual homozygous recessive for a condition does not sur-
vive to reproduce. Because the aa offspring would not survive to reproduce, this will
cause a shift in allele frequencies to include more A children and fewer a children. - Heterozygote advantage.This is a situation in which being heterozygous for a condition
provides some benefit (e.g., sickle cell allele in malarial regions). In this case, the allele
will still decrease, but not as fast as in the selection example. - Genetic drift.Imagine that 60 percent of your hypothetical population were killed in
some horrific environmental disaster. This would leave the remaining 40 percent to con-
tinue breeding and passing on genes to the next generation. The random nature in which
organisms are eliminated can lead to a shift in the allele frequency and the pandqwill
probably change depending on the genotype of those who are left behind.
There are two questions to ponder as you finish this experiment:- Why is it so difficult to eliminate a recessive allele? It is difficult because the allele remains
in the population, hidden as part of the heterozygous condition, safe from selection,
which can act only against genes that are expressed. So, although the qfor a population
may decline, it will not disappear completely because of the pqindividuals.
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