Biol 112
Homework #2: Sex and the Single Guppy
Due Monday 2/29
Page 1 of 6
Name
Introduction
How does evolution occur? Can we observe evolution occurring in natural populations? Charles
Darwin theorized that the primary mechanism for evolution is the process of natural selection, by
which certain individuals, having heritable traits that make then well-suited to their environment, will
be more successful at survival and reproduction than other individuals. These successful individuals
will then pass on the traits that made them successful to their offspring in the next generation. Over
time, the population will be largely made up of descendants of these successful individuals, and the
beneficial traits will be common.
Darwin himself never observed the process of natural selection in action. In fact, he imagined that
natural selection was so slow that we could not observe it over the course of our lifetimes. This notion
changed in the 1970’s when researchers Peter and Rosemary Grant began studying populations of
finches on the Galàpagos Islands. Through more than forty years of study, the Grants and other
researchers measured changes in the average beak shape and size of finch populations. These changes
occurred in response to environmental variation from year to year, with dramatically wet or dry years
leading to the most obvious periods of selection. The Grants’ finch study has been joined by several
other field studies to demonstrate how variation in local environments can lead to changes in the
population gene pool (evolution) through the process of natural selection.
In this exercise, you will conduct simulated field studies on populations of small tropical fish (guppies)
to demonstrate natural selection in the wild. You will also explore how certain traits that may be good
for survival may not be beneficial when it comes to sexual reproduction. You will consider how the
forces of natural selection and sexual selection can interact to influence evolution within populations.
Procedure:
1. Either by following the link posted in the Links section of your Pipeline course page, or by
typing in the url below, go to the following web site:
http://www.pbs.org/wgbh/evolution/educators/lessons/lesson4/index.html
2. You should now be looking at a web page entitled “Online Lessons for Students: Learning
Evolution. Lesson 4: How Does Evolution Work”? This site is part of an online series on
evolution developed by the Public Broadcasting System (PBS).
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Biol 112
Homework #2: Sex and the Single Guppy
Due Monday 2/29
Page 2 of 6
Questions 
Using either the links provided in pink on this page, your textbook, or class notes, define the
following terms:

Population:

Natural Selection:

Sexual Selection:
3. Click on Activity 2: Flashy Fish.
This activity is based on the research of John Endler, Professor of Ecology, Evolution, and Marine
Biology at the University of California at Santa Barbara (UCSB). For years, Endler has studied several
populations of guppies in the mountain streams of Trinidad.
4. Read the introductory paragraph under Activity 2: Flashy Fish.
As you go through this activity, follow the procedures listed on these assignment pages,
rather than those listed on the website. This procedure is adapted from that provided online.
5. Click on Sex and the Single Guppy (Flash) under the picture icon in the Procedures section.
Note: You will need Adobe Shockwave Player to successfully run this simulation. If you are
working the CAL lab (EBS 215), all computers should be equipped with Shockwave. If you
are working at your home computer, you can download Shockwave from
http://www.adobe.com/products/shockwaveplayer/.
6. Read the text and click on “check out the guppy gallery” at the bottom to see what male
guppies look like. Click on each male guppy pictured.
Questions 
•
•
What species are these guppies (what is/are their scientific names)?
What does information gained from knowing the scientific name tell you about the amount of
color variation found within a guppy species?
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Biol 112
Homework #2: Sex and the Single Guppy
Due Monday 2/29
Page 3 of 6
7. Click on “Predators” and “Habitats”, in the lower right corner, to see the guppies’ predators
and the habitats in which guppies live.
Question 
 Give the common and scientific names of three guppy predators.

What types of habitats are most likely to have predators?

Which habitats would likely have the fewest predators? Why?
8. Click on “Back to Simulation.”
9. Click on “I’m ready to find out.”
10. Read the text, and click on the pools to investigate the guppy stream more closely.
To return to the text page, simply click the X in the orange framed pool box.
Question 
 As you have seen, male guppies exhibit a wide variety of color and spot patterns. What do you
think causes this color variation? Click on “What causes guppy variation?” for a list of
potential hypotheses. You are to either come up with your own hypothesis, or select one from
those listed.
Your hypothesis:
11. After you have reviewed chosen your hypothesis, you will proceed with the simulation by
setting up an experiment to determine some influences on guppy coloration. Click on “To
start, choose how you want to set up your simulation pool.”
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Biol 112
Homework #2: Sex and the Single Guppy
Page 4 of 6
Due Monday 2/29
12. Read this page, and select your guppy types and predator species/numbers. (Note: Rivulus is
the least effective guppy predator). Record your initial settings here:
Guppy color types: __________________________________
Predator species and numbers: _________________________
•
What do you predict will happen during your simulation?
13. Run your initial experiment/simulation for at least seven generations and record your results
here:
Number of guppies:
Number of generations:
Number of weeks:
Male color types:
Brightest male guppies: _____%
Bright male guppies:
_____%
Drab male guppies:
_____%
Drabbest male guppies:
_____%
14. Click on “Results” to see an analysis of your results.
Questions 
 Do these results support your hypothesis? Explain why or why not.
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Biol 112

Homework #2: Sex and the Single Guppy
Due Monday 2/29
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If your initial hypothesis was not supported by your data, you may want to choose another
hypothesis to answer the question, “What causes color variation?”. If so, enter your new
hypothesis here:
Your new hypothesis (if applicable):
15. Click on “Run Another Simulation,” and select new settings. Do this even if your
hypothesis was supported by the results of your first run. Enter your new settings here:
Guppy color types: __________________________________
Predator species and numbers: __________________________
•
Your prediction of what will happen in this simulation:
16. Run your new experiment/simulation for at least seven generations and record your results
here:
Number of guppies:
Number of generations:
Number of weeks:
Male color types:
Brightest male guppies: _____%
Bright male guppies:
_____%
Drab male guppies:
_____%
Drabbest male guppies:
_____%
17. Click on “Results” to see an analysis of your results.
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Biol 112
Homework #2: Sex and the Single Guppy
Due Monday 2/29
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Question 
 Do these results support your hypothesis? Explain why or why not.
18. After you have completed your second simulation and reviewed the results, Click on “Read
Summary.” Read this summary carefully, and if you desire, run more simulations putting to
test the knowledge you have just learned. Use the information gained in the Summary to
answer the discussion questions below:
 Discussion Questions:

Based on the results of your experiments/simulations, would you say that guppy coloration is
heritable (that is, is it genetically determined)? Why or why not?

What role does color play in guppy survival?

What role does color play in guppy reproduction?

Indicate 3 different explanations for why brightly colored males may be more likely to mate:
1.
2.
3.

How are the forces of predation and sexual selection at odds in these guppy populations?

Male guppies are often brightly colored, while females are always drab. How would you explain
this difference between males and females within this species?
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