Download simulation of natural selection

APES
Due Date:_____________
Name__________________________
A SIMULATION OF NATURAL SELECTION
Introduction:
Evolution is a process that changes the genetic make-up of a population over time. Presumably, those
genetic changes are reflected in the observable characteristics of the population. This simulation 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. If all gene variations allowed every
individual the same capability for survival and reproduction, then the composition of a population would
never change. If a variation of a characteristic increases an individual’s chances of survival or allows it to
have more offspring, then that variation will be naturally selected. Darwin reasoned that the environment
controlled in nature what plant and animal breeders control 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 therefore, more offspring in the next
generation.
Purpose:
We will use three different beans to represent heritable variation in beetle morphology (size and coloration of
carapace). These three beetle populations will be studied in two different habitats.
Procedure:
1. Work with 1 or 2 partners. Count out exactly 10 each of white northern, pinto, and black beans. These
represent the three beetle types that are all equally common initially.
2. Choose two habitats and perform the procedure the same way in each one. Choose from this list:
sidewalk, asphalt, brick, sand, dirt, or grass. The point is that the two substrates must be different colors.
Label your data table with the type of habitat for both Habitat 1 and Habitat 2.
3. Scatter the beans randomly over an area about one square meter. This will be your predator foraging
area. The beans must be scattered, (tossed), not dumped in one 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: pick up what you see
first as quickly as you can. Place the 20 beans in the cup provided. Leave the rest of the beans on the
ground. They have survived the predator and will reproduce. (Their offspring will be represented by
adding beans to adjust the population size back up to 30 (line F in the data table).)
5. Count the number of each bean collected (make sure you have exactly 20) and record the numbers on
line B of your data table.
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 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 – 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 of your beans when you are finished. Repeat the entire procedure in Habitat #2.
Results and Analysis:
We will determine whether evolution has occurred by comparing the frequencies of each bean type at the
beginning and at the end of our predation experiment. We will use a Chi Square statistic (x2) to determine
whether or not final frequencies are significantly different from our initial ones.
O = observed count in a category
E = expected count in a category
Σ = sum
The Chi Square value that you get is located on the attached table. The degrees of freedom (df) is one less
than the number of observable characteristics. We have three observable characteristics (3 bean morphs).
A probability (P value on the chart) of less than 0.05 tells you that there is less than a 5% chance that the
differences between our beginning and ending counts could be due to random variation (chance). This
means that there is a 95% probability that the differences are not due to random factors, but are the result of
our experiment.
Any statistic can test only two hypotheses: the null hypothesis of no difference, and the alternative
hypothesis of significant difference. Note that in science, we can never prove that any hypothesis is true,
since there is always more data to gather. We can only prove hypotheses to be false.
The experimental hypothesis in this lab involves the role of predators and habitats in natural selection. What
hypotheses were tested in this simulation? Remember the purpose of the lab is to determine whether or not
natural selection occurred in this experiment. Your hypothesis, discussion and conclusion should answer
that question. The statistical analysis is ONLY used to determine whether your hypothesis or your null
hypothesis is supported.
Individual Lab Write-up:
Turn in a report that includes the following:
1.
2.
3.
4.
The prelab.
An Observations section describing, but not explaining, the results of your experiment.
A Results section that includes the data sheets with your hypothesis and actual data.
An Analysis section in which you discuss your results. Be sure to state whether or not the Chi Square
supports your hypothesis or the null hypothesis. (In other words, was there a statistically significant
difference the results obtained in your two habitats as determined by the Chi Square test?) This
section should include the answers to these two discussion questions: How and why do your results
differ for the two habitats? (If they are not different, why are they not different?) How can you
explain your results in terms of predator/prey relationships and the hypothesis? (Be sure to discuss
the effects of camouflage.)
5. A well-constructed Conclusion section that is composed of no more than two sentences that includes
a statement that revisits your hypothesis.
Please type your report.
PRELAB QUESTIONS
1. What is the purpose of this lab?
2. What do the beans represent?
3. What does it mean when two values are statistically different?
4. How many trials do you have to complete?
5. What kind of backgrounds do you plan to choose?
6. Do you expect that a change in the environment will cause a change in the composition of the population
of “beetles?” Why or why not?
Χ2 ANALYSIS OF BEETLE CAPTURES
Habitat #1
Characteristics
Black
Expected (P1)
Χ2
Observed (P4)
Pinto
Northern White
Χ2 =
Total
df*=
Habitat #2
Characteristics
Black
Expected (P1)
P=
Χ2
Observed (P4)
Pinto
Northern White
Χ2 =
Total
df*=
df = degrees of freedonm; P = probability
P=
Name________________________________________
Group#_____
HYPOTHESIS:
SUMMARY OF BEETLE CAPTURES
HABITAT #1
Northern
White
10
HABITAT #2
Pinto
Black
Total
10
10
30
Northern
White
10
Pinto
Black
Total
10
10
30
A
P1
B
“Eaten”
20
20
C
Survivors
(A – B)
Offspring
(2C)
P2
(C + D)
“Eaten”
10
10
20
20
30
30
20
20
Survivors
(E – F)
Offspring
(2G)
P3
(G + H)
“Eaten”
10
10
20
20
30
30
20
20
Survivors
(I – J)
Offspring
(2K)
P4
(K + L)
10
10
20
20
30
30
D
E
F
G
H
I
J
K
L
M