LAB – Evolution Via Selection
AP Biology
Variation is inherent in all populations, and it is required for selection to occur. Variety results
from changes to the DNA nucleotide sequence, as from mutations. In the wild, when resources
are plentiful, populations tend to over-reproduce overwhelming the resource and creating
competition within a population. In other ways environments may change and variants may have
an advantage over others. For whatever reason, when some individuals in a population
experience greater success, particularly in reproduction (‘fitness) selection will occur. Natural
selection is the primary mechanism of evolution.
One classic example of evolution, (from Darwin, naturally) are the finches of the Galapagos
Islands. On the various islands of the archipelago, these birds exploit different food resources
such as seeds, insects, flowers, and leaves. This is reflected in beak design – each population has
a beak that reflects their preferred food supply – an adaptation.
Of course Darwin, and many before him, took note of the selective breeding practices that
produce domesticated crops and animals. Like in natural selection, variation is exploited to
produce a generation with a far greater frequency of a desired trait. As a result we now have a
tremendous variety of dog breeds and a host of cruciferous vegetables.
A. RECOGNIZING VARIATION
Obtain a large sample of a population of organisms, represented by shells, seed pods,
fruit, whatever is available in lab.
1. Brainstorm a descriptive list of phenotypes (traits) for which the individuals in the
population are clearly varied.
2. Choose a quantifiable trait from you list. Carefully and accurately measure more than
50 members of the population for that trait. Record in a table.
3. Repeat for a second population of the same species. Record.
4. Describe the population mathematically.
• Graph each population as a histogram with 7-10 bars. (Do so by identifying the
high and low values, finding the range, and dividing by the number of bars you
will use. Round; use this value to find the range for each bar.)
• Identify the mean, median and mode.
• Use the mean to calculate the standard deviation (SD). Use the mean and SD to
find the standard error (SE).
• Summarize your results in a table
5. Determine if the difference between the means of the populations is significant via a
bar graph of the means and SE of each population.
B. NATURAL SELECTION INQUIRY
1. Pre-lab
• Discuss model objective; write a testable question
• Identify & describe the variation in beaks at your table
• Establish and agree upon the feeding rules
• Hypothesize the feeding success of each phenotype, or choose the null (no
difference) hypothesis
• Agree upon a reproductive constant that reflects any differential success of the
beaks
• Design data table on white board
• Have the teacher approve your methods before starting
2. Generate Data
• Run multiple trials per generation; average the results per beak type
• Apply reproductive constant to reflect differential beak success
• Repeat for 2 more generations
• Change a condition of the model
• Run the model for 3 more generations
3. Results
• Determine the number of each beak type, and total population size, per
generation
• Calculate frequency of each beak for each generation
• Graph changes in frequency for all beak types
Did the fitness vary between beaks? Did the fitness of any beak change over time?
Did the population change over time? Make claims from the results, and cite evidence
and reasoning in support.
C. ARTIFICIAL SELECTION INQUIRY
1. As a class, agree upon a trait of Wisconsin Fast Plants to quantify.
2. Survey a segment of the population for the trait. Graph as a histogram. Calculate
mode, median, mean, SD, and SE.
3. Select for the top 10-25% of the population for this trait; remove the remainder to
another light bank.
4. Allow the plants to grow to maturity, cross-pollinate, and set seed.
5. Plant the F1 seed; survey for the same trait at the same time in the life cycle. Calculate
mode, median, mean, SD and SE.
Does the F1 demonstrate artificial selection? Justify, based on your graphs and statistics.
Selection: QUESTIONS for DISCUSSION
1. Explain why an individual organism cannot be said to evolve.
2. What is the immediate effect of both overpopulation and environmental
change on a population? …The long term effect(s)?
3. Which of the following traits in the human population is the better example
of polymorphism: blood group (A, B, AB, o), height, number of fingers?
Justify.
4. In what sense is natural selection an editing process as opposed to a creative
process?
5. What are other phrases used synonymously with natural selection?
Explain each.
6. Which kind of population is more varied: wild or domesticated?
Justify your choice.
7. Regarding the peppered moths of England, what adaptation do the lighter
moths employ today? What type of selection is this?
8. The black-bellied seed cracker population of West Africa relies on two food
sources: large, hard seeds, easily cracked by large billed birds, and smaller,
softer seeds, eaten by variants with small beaks. What type of selection and
population distribution graph (histogram) results?
Move to demo/lecture?
D. (REPRODUCTIVE POTENTIAL
1. Take a small sample of your fruit (pods are fruit!) and count the seeds. Determine the
mean. Round down to a whole number.
2. Assume that all fruits contain this average number of seeds and, for the first
generation only, all seeds germinate into plants.
3. Further assume that all plants produce 80 fruit, each with the average number of
seeds. This value is the total seeds produced.
4. Assume 0.1% of the total seeds produced by the previous generation germinate and
produce the next generation.
5. Continue with the model for 5 generations.
6. Graph any column of data from your table. Graph it twice – use a linear scale on one
Y axis, and an exponential scale on the other. Distinguish between them using color
or pattern.
What is the pattern of the line(s) produced, in light of their Y axis scale? What is
the inevitable result of this kind of population growth? )
Generation
P
F1
F2
F3
Avg seeds/plants
12
115
1104
10598
# fruit (pods)/plant
960
9200
88320
847840
# seeds
11,520
110,400
1,059,840
10,174,080
ARTIFICIAL SELECTION INQUIRY
1.
As a class, agree upon a trait of Drosophila melanogaster to quantify from the website
http://sciencecourseware.org/vcise/drosophila/
2.
Survey the population for the trait and record the data.
3.
Select for the trait agreed upon; remove the remainder.
4.
Do three total generations and record the numbers for each generation.
5.
Repeat steps 2-4 3 times and calculate SD and SE. Then Graph the averages and add SE
bars.
Does the 3rd generation demonstrate artificial selection? Justify, based on your graphs
and statistics.
If you are done early proceed to this website and learn about
artificial selection in dogs
http://www.wolf2woof.com/EVOLUTIO/SELECTIO/ARTIFICI.HTM
TOOL
(beak)
Start
RATIO
Start
FREQ
__
X
#
SEEDS
EATEN
Gen1
RATIO
Gen1
FREQ
__
X
#
SEEDS
EATEN
Gen 2
RATIO
Gen 2
FREQ
__
X
#
SEEDS
EATEN
Gen 3
RATIO
Gen 3
FREQ
X
#
SEEDS
EATEN
Final
RATIO
Demonstrating Variation:
From famers selective breeding, etc
1. Have available a variety of populations: shells (lisa!), pears, Catalpa pods, flowers, lima
beans, peanuts, etc.
2. Discuss at least three ways the population demonstrates variation qualitatively
3. Choose one of the three that can be quantified. Record as table & graph.
Over-reproduction/Biotic potential:
1. Calculate the biotic potential of your population:
Final
FREQ
Pear
5-10 seeds/pear
hundreds
Catalpa pod
10+ seeds/pod
hundreds
beans, pea pods
5-8 per pod
30-50/plant
Sunflower
75-200 seeds/flower
15 + per plant
Mollusca
1-80 million*
Peanuts
1-3 nuts/shell*
* why ?
25-50 nuts/plant