Spring 2012
DUE Tues 4/17
Lab #12: NATURAL SELECTION (Fork-n-beans)
SLO 7: Explain natural selection and evolutionary processes in nature
I: NATURAL SELECTION
 Part
Evolution is precisely defined as the change in allele frequency over time. This exercise illustrates the effect of natural
selection on populations of predators and prey. Students, in groups of 4, will represent predators, each with a different
adaptation for capturing prey. Several species of prey are represented by different colored beans.
1. Each team of 4 students will count out exactly 100 dried beans of each color.
2. Thoroughly mix the beans and spread them evenly over your “habitat”.
a. Each team will mark off a 1mx1m “habitat” in the grass.
b. All “prey” are confined to this habitat.
3. Each student will have a different feeding apparatus (fork, spoon, knife or forceps).
4. When everyone is ready, predators will spend 60 seconds capturing prey with their devices and depositing them
into a cup while obeying the following rules:
a. Predators must only use their capture device to capture prey
b. Predators may not scoop prey up with their cup
5. Each predator determines the number of prey captured and records results in Data Sheet: Generation 1.
6. Calculate and fill in the remaining statistics on the data sheet (see example below):
Data Sheet: Generation 1
Prey Type
Pop. Size
Black
100
Pinto
100
Red
100
White
100
Total
% Captured
Forceps
Spoon
Fork
Knife
Total Kills
# Survived
% Survived
% Total Pop.
Example of data collection and analysis for generation # 1:
Prey color
Pinto
Red
Black
White
Total
% Captured
Pop. Size
Forceps
Spoon
Fork
Knife
100
8
14
10
15
100
15
29
20
30
100
22
21
14
20
100
12
18
19
10
400
57 (8+15+22+12)
82 (etc.)
63
75
-----------------14 (57/400) X 100
21 (82/400) X 100
16 (etc.)
19
Total Kills
# Survived
% Survived
47
53(100-47)
53
94
6 (100-94)
6
77
23 (etc.)
23(etc.)
59
41
41
----------123 (53+6+23+41)
-----------
------------------
(53/100) x 100
(6/100) x 100
43
5
19 (etc.)
33
-----------
------------------
(53/123) x 100
(6/123) x 100
% Total Pop.
# Survived = population size – total kills
% Survived = (# survived/population size) X 100
% Total Population = (# survived/total survived) X 100
Biology 1: Riggs
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Spring 2012
7.
8.
9.
The predator with the lowest capture percentage will go “extinct” and will not participate in the next exercise. The
predator with the highest capture percentage will reproduce itself and the “offspring” will participate in the next
exercise.
The surviving prey will also reproduce itself by adding the same number of colored beans to the population as
survived the first hunt.
Repeat the process until there is only one predator remaining.
Data Sheet: Generation 2
Prey Type
Pop. Size
Black
Pinto
Red
White
Total
% Captured
Forceps
Spoon
Fork
Knife
Total Kills
# Survived
% Survived
% Total Pop.
[Note: For population size in generation #2, multiply # survived in generation #1 by two]
Data Sheet: Generation 3
Prey Type
Pop. Size
Black
Pinto
Red
White
Total
% Captured
Forceps
Spoon
Fork
Knife
Total Kills
# Survived
% Survived
% Total Pop.
[Note: For population size in generation #3, multiply # survived in generation #2 by two]
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Pie Chart Analysis of Predator and Prey Populations
Original Predator Population (% of total pop.)
Original Prey Population (% of total pop.)
Predator Type
Prey Type
spoon
white
knife
black
fork
pinto
forceps
red
Using data of generations 1-3 from previous page, fill in the pie charts below:
END OF FIRST GENERATION
END OF FIRST GENERATION
END OF SECOND GENERATION
END OF SECOND GENERATION
END OFTHIRD GENERATION
Biology 1: Riggs
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Spring 2012
A. Explain in your own words the process of natural selection.
B. What conclusions can you draw regarding the effect of natural selection on the predator populations
in this exercise?
C. What conclusions can you draw regarding the effect of natural selection on the prey populations in
this exercise?
D. What do you predict would happen to both predator and prey populations if the habitat for this
exercise was changed? Give an example.
E. Relate the concept of NATURAL SELECTION to the process of EVOLUTION.
F. Is natural selection the only way evolution occurs? Explain.

Part II: ONLINE DISCUSSION
Post your QUESTIONS and COMMENTS about evolution in the discussion board in MyCR. Do not limit your
questions to things you don’t understand. Let’s engage in a meaningful discussion of evolution! Be sure to cite
your sources when brining new information to the class.
Jot down some of your notes here:
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Spring 2012
 Part III: RADIOMETRIC DATING
Objective:
You will use half-life properties of isotopes to determine the age of different "rocks" and "fossils" made out of bags
of beads. Through this simulation, you will gain an understanding of how scientists are able to use isotopes such as
U-235 and Pb-207 to determine the age of ancient minerals.
Materials:
1 plastic bag “fossil” filled with bead “atoms”
1 bead key
Calculator
Instructions:
Today you will use your knowledge of ratioactive decay and half-life properties to figure out the age of eight
different "fossils" at different stations around the room. Each bag represents one single fossil and the beads inside
the bag represent some of the millions of atoms that make up the fossil. As scientists, your job is to count the
number of parent and daughter isotope atoms in each bag, and from this data, determine how many half-lives the
fossil has gone through. You will record this data in the chart on the back of this page, then use your data and the
graph to determine the age of the rock.
To calculate the number of half-lives each isotope has gone through, use the following formula:
Number of parent isotopes / (Number of parents isotopes + Number of daughter isotopes)
Note: If you add the total number of parent isotopes to the total number of daughter isotopes, you will get the total
number of parent isotopes present in the original sample when the fossil was initially formed!
This ratio gives you the percentage of parent isotope atoms left after radioactive decay. You may then use the
attached graph of predicted decay rates to determine the number of half-lives the isotope has gone through based on
this percentage. For instance, if your sample contains 15 parent isotopes and 45 daughter isotopes, you will take 15
divided by 60 = 0.25 (Remember, this number is the percent of parent isotopes that have NOT decayed yet). Then
examine the graph of decay and see that when 25% of the parent isotope atoms are left, the isotope has gone through
two half-lives. Finally, to figure out the age of the fossil, take the number of half-lives, two in this case, and multiply
it by the length of the half-life (704 million years for Uranium 235).
Number of half-lives x length of half-life = age of sample
Half-life data
Carbon 14 has a half-life of 5700 years
Uranium 235 has a half-life of 704 million years
Potassium-40 found in your body at all times; half-life = 1.3 billion years
Uranium 238 has a half-life of 4.5 billion years
Thorium 232 has a half-life of 14 billion years
Rubidium-87; half-life = 49 billion years
Follow-up Questions:
1. Rank the fossils from oldest to youngest.
2.
Which two were very close in age?
3.
In this activity, which "fossil" came from the time just after the formation of the earth? Do you think any
real fossils could come from that time?
4.
Why do you think there were lots of beads of other colors in the bag besides the ones you were counting?
5.
Do you think scientists can use more than one type of isotope to date the same rock or fossil?
6.
If you wanted to date a sample that you estimated to be about 1 million years old, which isotope would you
use to date it, Uranium 235 or Thorium 232? Why?
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Data Table
"Fossil"
Number
Fossil Half-life
# parent
isotopes
# daughter
isotopes
Total isotopes
% of parents
remaining
Number of
half-lives
Age of "fossil"
Determine from
worksheet
Count
beads…
Count
beads…
Add parents
+ daughters
(# parents/
total) x 100
(Determine
from graph!)
(Half-life x #
of Half-lives)
1
(U-235)
2
(U-235)
3
(U-238)
4
(U-238)
5
(C-14)
6
(C-14)
7
(Th-232)
8
(Th-232)
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