concept that should already be familiar to students. The instructor
can then relate probabilities to HWE.
• The instructor displays the classroom population data and asks
students what the probability is of pulling a red bead out of the
community gamete bucket. The instructor can demonstrate
probability by having multiple students randomly select 10
beads and count the number of each color.
• With only two bead colors, students should be able to determine the yellow bead frequency given the red bead frequency.
• Once students understand the relationship between color frequencies in the gamete pool and probability, the instructor then
demonstrates probability rules by pulling various bead combinations ( i.e., two reds, two yellows, and a red and yellow in
any order). This last part of the activity provides the framework
Building upon student knowledge of probability, HWE can
then be introduced simply as the probability of each possible gamete combination ( i.e., Red-Red, Yellow-Yellow, Red-Yellow).
• The expected genotype and allele frequencies within a population
are now generated from the class gamete pool using the HW
• The calculated values are then compared to “actual” values,
determined by each student randomly pulling two new gametes
(beads) from the gamete bucket to make a developing zygote.
• The genotypes of these developing zygotes are recorded, then
the class calculates the genotype and allele frequencies from this
• These frequencies, generated from the “actual” population, are
then compared to the counts calculated for the “expected” pop-
ulation (completed earlier) using a chi-square test.
Random gamete selection, counting genotypes, and calculating
allele frequencies are core tasks of part 2. Thus, the end of part 1 is
an opportune time to clarify any conceptual or mathematical challenges associated with these tasks.
Activity Part 2: Comparing Actual
The following materials are required (Figure 1):
• Bag of red and yellow marbles (100 of each color)
• Organizing tray (a 24-well ice-cube tray works well; wells
• Sets of “event cards” (or a die), corresponding to the populations being used ( http://evo-ed.org/pages/resources.html#abt_resources); each set of event cards is randomly shuffled
• A data sheet and student worksheet to record genotype and
allele count and frequency data and student worksheet (http://
In part 2, students work in groups of two to four to simulate
changes to the allele frequencies in different populations of
M. arenaria. Students track two alleles (resistant and sensitive) of
the gene responsible for producing the voltage-gated sodium chan-
nel in its neuron cell membranes (Bricelj et al., 2005). The resistant
(R) allele is the result of a mutation from adenine (A) to cytosine (C),
We framed part 2 using data from Connell et al. (2007) that
characterized five populations on the eastern seaboard of North
America (Figure 3). Each population has its own set of event cards.
These cards describe events that may disrupt or change a popula-
tion, leading to drift and natural selection. Event cards vary by pop-
ulation, as different clam populations are subjected to different
environmental stressors. For example, Havre-Aubert and Lawrence-
town have no history of dinoflagellate blooms (Figure 3); therefore,
the event cards associated with those populations contain only drift
events, whereas Lepreau Basin, Essex, and Orleans have a docu-
mented history of dinoflagellate blooms and therefore contain drift
and selection events.
The instructor can start the activity with some background
on the different populations of clams and a review of the learning outcomes from part 1 (see resources at http://evo-ed.org/
pages/ resources.html#abt_resources). The activity then proceeds
• Each student group is assigned one of the five populations and
its associated event cards. Their task is to manage their population by following the instructions given by the event cards and
modifying their population, as needed.
• All populations begin with the same allele frequency (p = 0.5,
q = 0.5); each group should have one bag of alleles with 100
marbles of each color to represent the gamete pool of their
• The organizing tray (Figure 1) is used to count out 24 individuals
randomly created from the gamete pool (two marbles per well).
• Students work in groups, following the instructions on the worksheet ( http://evo-ed.org/pages/resources.html#abt_resources) and
drawing event cards, which can result in allele changes in their
• After each event card, students calculate and record their new
allele frequencies from the standing population because these
individuals are going to contribute gametes to the next generation.
• The students then reset their gamete-pool bag to reflect the new
• They should continue to draw a card and record the new allele
frequencies for at least four to six generations.
• Students then graph changes in allele frequencies across generations and describe these changes in relation to the events their
population experienced (for examples of student results, see
• In addition to generating a summative figure, groups might
also test for a statistical measurement of evolution in their