5 Steps to a 5 AP Biology, 2014-2015 Edition

144 ❯ STEP 4. Review the Knowledge You Need to Score High

When Evolution Is Not Occurring: Hardy-Weinberg Equilibrium

Evolutionary change is constantly happening in humans and other species; this seems sen-

sible because evolution is the change in allele frequencies over time. It makes sense that

these frequencies are highly variable and subject to change as the environment changes.

However, biologists use a theoretical concept called the Hardy-Weinberg equilibriumto

describe those special cases where a population is in stasis, or not evolving.

Only if the following conditions are met can a population be in Hardy-Weinberg

equilibrium:

Hardy-Weinberg Conditions

1.No mutations

2.No gene flow

3.No genetic drift (and for this, the population must be large)

4.No natural selection (so that the traits are neutral; none gives an advantage or

disadvantage)

5.Random mating

Notice items 1–4 in this list are the four modes of evolution, which makes sense—if we are

trying to establish the conditions under which evolution does notoccur, we must keep these

processes of evolution from occurring! The fifth condition, random mating, is included

because if individuals mated nonrandomly (e.g., if individuals mated with others that

looked like them), the allele frequencies could change in a certain direction, and we would

no longer be in equilibrium.

Determining Whether a Population Is in

Hardy-Weinberg Equilibrium

Unfortunately for you, there is an equation associated with the Hardy-Weinberg equilib-

rium that the test writers love to put on the exam. Don’t let it scare you!

p+q= 1

This equation is used to determine if a population is in Hardy-Weinberg equilibrium.

The symbol pis the frequency of allele 1 (often thedominant allele), and qis the fre-

quency of allele 2 (often the recessive allele). Remember that the frequency of two alleles

alwaysadds up to 1 if the population is in Hardy-Weinberg equilibrium.For example, if

60 percent of the alleles for a given trait are dominant (p), then p=0.6, and q(the recessive

allele)= 1 −0.6, or 0.4 (40 percent).

There is a second equation that goes along with this theory: p^2 + 2 pq+q^2 =1, where

p^2 andq^2 represent the frequency of the two homozygous conditions (AA and aa). The

frequency of the heterozygotes is pqplusqpor 2pq(Aa and aA). Since prepresents the

dominant allele, it makes sense that p^2 represents the homozygous dominant condition.

By the same logic, q^2 represents the homozygous recessive condition.

Let’s say that you are told that a population of acacia trees is 16 percent short (which

is a, recessive) and 84 percent tall (which is A, dominant). What are the frequencies of

KEY IDEA

KEY IDEA

CT teacher:

“Knowing how

to do Hardy-

Weinberg prob-

lems is worth

2 points to

you... easy

points.”

BIG IDEA 4.C.3
Allelic variation in
a population can be
modeled by the
Hardy-Weinberg
equation.