Natural Selection in 50 Minutes

By Brian Kloepfer
Education Resource Manager

“Only the strong survive.” Even singers croon this often-quoted phrase while performing lyrics that lament the theme. But how accurate is this statement in the world of natural selection?

“Not very” is the answer. However, students often have misconceptions about natural selection, such as only the fittest survive and unwanted traits will disappear from a population. In this activity, students can observe genetic equilibrium within a breeding population over several generations. They will observe the presence and persistence of recessive alleles that are often misconceived as “unwanted” or “undesirable” traits.

Using 2 different colors of beads (found at craft stores), students will randomly select 2 beads at a time to create a genotype. They will record their results and determine the number of individuals in the population that express each phenotype. Do the activity several times in a class period to simulate several generations.

Materials (per group)

• 100 Colored Beads
• 100 Plain Beads
• 3 Small Cups
• Data Sheets (to record genotypes)

Pre-activity

If you have not discussed Hardy-Weinberg, this would be a good starting point. Allow students to calculate the expected genotype and phenotypes of a given population. To tie it into the activity, use an example with 100 individuals in a population, 10 individuals expressing the dominate phenotype and 90 individuals expressing the recessive phenotype. The calculation should look as follows:

p² + 2_pq + q² = 1
90 recessive = b/b = 90 ÷ 100 = 0.9 = q²
q = √0.9 = 0.95
p + q = 1
p = 1 – 0.95 = 0.05
The frequency of B = p = 0.05
The frequency of b = q = 0.95

Next, students can calculate the expected genotypic frequencies for B/B, B/b, and b/b:

B/B = p² = 0.05² = 0.003

B/b = 2_pq = 2 × 0.05 × 0.95 = 0.095

b/b = q² = 0.90

Teacher Tip: When calculating the allelic frequency using the Hardy-Weinberg equation, you must always use the number of organisms that express the recessive trait, which is q.

Activity on natural selection

1. Divide the class into groups of 2 to 3. Each group places all the colored beads into 1 cup and all the plain beads into another cup. The 3^rd cup represents the gene pool for the population.
2. Using the above example, students add both the number of colored beads (representing the dominant allele) that was calculated (5 beads) and the number of plain beads that was calculated (95 beads) into the gene pool cup.
3. Holding a hand over the cup, students shake the cup to mix the beads. Note: This step must be done before each genotype is pulled.
4. Without looking, a student pulls 2 beads from the cup. If the student pulls 2 colored beads, he/she records it as (B/B). If a colored bead and a plain bead are pulled, record it as (B/b); if 2 plain beads are pulled, record it as (b/b). Place the beads back in the gene pool cup.
5. Repeat this process 100 times to create the genotypes for a population of 100 individuals. Students can then count the number of individuals expressing each phenotype. The theoretical should show only 10 individuals expressing the dominate phenotype and 90 expressing the recessive phenotype.
6. Each group can share their results, and calculate the class average to determine if their expected genotypes and actual genotypes are similar.

Alternatives

Modify this activity to show the change in genotype and phenotypes of the population due to natural selection pressures. Create a scenario where suddenly the recessive phenotype becomes easy prey for an exotic introduced to the population. Of the 90 individuals in the population that express this phenotype, only 2/3 of them survive to breed. Have students recalculate the allelic frequency of a population with 86% recessive phenotype (60/70) and 14% dominant phenotype (10/70). Rerun the activity for several generations and observe the shift in allelic frequencies.

See our teaching aids

Carolina offers a complete kit, the Natural Selection BioKit®, that explores the popular peppered moth phenomenon, which affected the population dynamics of light- and dark-winged moths during and following a period of intense air pollution. This kit contains detailed background, student activity and data sheets, materials to do all the activities, plus punch-out peppered moths and “environment” sheet to illustrate natural selection to students. The kit has enough material for 6 student groups working in teams of 5 students each and all material is reusable.