A Simulation of Natural Selection
Name____________________________
Introduction
Evolution is a process that changes the genetic makeup of a population over time. Presumably, those genetic changes
are reflected in changes in the phenotypic makeup (the observable characteristics) of the population. This exercise will
demonstrate the effect of natural selection on the frequencies of three populations of “beetles.” Natural selection, as
formulated by Charles Darwin in “Origin of Species (1859)”, is the most important cause of evolution.
An individual’s ability to reproduce depends on its ability to survive. It all gene variations conferred on every individual
the same capacity to survive and reproduce, then the composition of a population would never change. If a variation of
a characteristic increases an individual’s ability to survive or allows it to have more offspring, that variation will be
naturally selected. Darwin reasoned that the environment controlled in nature what plant and animal breeders
controlled artificially. Given an overproduction of offspring, natural variation within a species and limited resources, the
environment would ‘select’ for those individuals whose traits would enable a higher chance of survival and hence more
offspring in the next generation.
Purpose
In this activity we will use three different beans to represent heritable variations in beetle morphology (size and
coloration of carapace). These three beetle morphs will be studied in two different habitats.
Outline of Procedure
1. Working with a partner, count out exactly 10 each of lima, pinto, and kidney beans. These represent each beetle
type; the three types are equally common initially.
2. You will choose 2 habitats and perform the procedure the same way in each one. You will have 2 habitats: dark
marble and light marble. The point is that the two substrates must be of different colors. Label your data table
with the type of habitat for both Habitat #1 and Habitat #2.
3. Scatter your beans randomly over an area about one square meter. This will be your predator foraging area.
The beans must be scattered (toss), not dumped in a small pile.
4. One person will be the designated predator for the habitat. (Switch roles for the second habitat). The predator
will ‘eat’ (pick up) exactly 20 beans. Remember to think like a predator, i.e. pick up what you see first as quickly
as you can. Place the 20 beans picked up and put to the side of your ecosystem. Leave the rest of the beans on
the ground. They have survived the predator and will live to reproduce (their offspring will be represented by
adding beans to adjust the population size back up to 30, line F).
5. Count the number of each bean collected (make sure to have exactly 20) and record the numbers on line B.
6. Subtract the number of each kind eaten (line B) from the number you started with (line A), to obtain the number
of survivors (line C)
7. Assume that each survivor has two different offspring. Record those values in line D. These are the numbers of
each bean that need to be scattered with the survivors to bring the population back up to exactly 30. Count out
and scatter the required number of beans into the same area as your P1 survivors. Now complete line E by
adding lines C and D. These are your P2 or second generation populations.
8. Repeat steps 4 through 7 two more times and complete the table for Habitat #1. Remember, the offspring
values tell you how many beans of each type need to be scattered into your predator foraging area.
9. Pick up all your beans when you are finished. Repeat the entire procedure in Habitat #2.
Summary of Beetle Captures
Habitat #1
Lima
Pinto
A
P1
B
“Eaten”
C
“Survivors” (A- B)
D
“Offspring” (2C)
E
P2 (C + D)
F
“Eaten”
G
“Survivors (E-F)
H
“Offspring” (2G)
I
P3 (G+H)
J
“Eaten”
K
“Survivors” (I-J)
L
“Offspring” (2K)
M
P4 (K + L)
Kidney
Total
Habitat #2
Lima
Pinto
Kidney
Analysis
1. How did your results differ for the two habitats? Why was there a difference, if any?
2. What were some limiting factors that your group encountered during this lab?
3. How did activity replicate natural selection?
4. What variables were missing to determine what was truly the ‘successful’ beetle population?
Total