Overview
In this lesson, students learn about natural selection, the mechanism that
drives evolution. They begin by viewing short videos about the evolution
of physical and behavioral adaptations in various organisms. This is
followed by a discussion of the natural selection mechanism, which has,
over many generations, given rise to these and other extraordinary
traits. Students then watch a video about peacocks that explores the
interplay between two evolutionary imperatives: the need to survive and
the need to reproduce. Lastly, students pair up to perform experiments
with simulated populations of guppies, another species whose evolution
is driven by the interplay between the forces of predation and sexual
selection.
Objectives
- Explore and begin to understand the meaning of
natural selection
- Explore and begin to understand the meaning of
sexual selection
- Discover the role that genetic variation, adaptation,
and sexual selection play in natural selection
- Learn some experimental techniques and how to
analyze experimental results
- Understand that there is an interplay between different
selection factors and that these factors sometimes have
competing outcomes
Suggested Time
Resources
Before the Lesson
Read the background essay that accompanies each resource to gain
information that will help you facilitate class discussion.
The Lesson
Part I: A Pressure-Filled World
1.
Show the Hummingbird Species in the Transitional Zones video
and discuss the following:
- What is the relationship between the length of the
hummingbirds' beaks and the flowers from which they feed?
- If the size and shape of the flowers available to a
group of hummingbirds were to change dramatically over a short
period of time, would individual hummingbirds have the ability
to change the size and shape of their beaks to adapt?
- Although individual members of a species may look very
much alike, there is often a great deal of variation among them.
What role might this variation play in natural selection and
evolution?
2.
Show the Evolution of Camouflage video
and discuss the following:
- What is the praying mantis's strategy for self-defense?
- The mantis is nearly invisible sitting on a leaf
in the forest, but when the scientist places the insect on his
blue shirt, it becomes very obvious. What does this suggest about
how well this species of mantis would survive in a different
environment -- a desert or a short-grass prairie,
for example?
- Would an individual mantis be able to transform its
appearance if it were placed in another type of environment?
Why or why not?
- If the forest were to dry out and turn to grassland,
or if this species of mantis began to expand its range to an
area with fewer trees, what might happen to the species over
time?
3.
Show the Floral Arrangements video
and discuss the following:
- Some plant species rely on the wind for pollination.
What other pollination strategies do plants use?
- Insects are often critical to a plant species' survival.
What reward do plants usually offer in return for the insects'
services?
- The orchid in the video promises a different kind of payoff.
What is that false promise?
Part II: Competing Pressures
4.
Show the Tale of the Peacock video
and discuss the following:
- From the research discussed in the video, what appears
to be the most important factor in a male peacock's ability to
attract mates and successfully reproduce?
- What happened when the scientists altered the
peacocks' tails by cutting them short?
- Aside from providing the egg, what role do peahens play
in the reproductive success or failure of a would-be mate?
- How might this trait have begun, and how might it have
evolved over millions of years?
5.
Lead students through the introductory screen and the section titled
"Read about Endler's discovery" of the
Sex and the Single Guppy Web activity.
During this time, remind students that scientific research
begins with systematic observations, followed by the formulation
of a hypothesis that explains the observations, and then
experimental testing of the hypothesis. Explain to students
that they are going to follow these same steps in search of
a scientific explanation for the differences in coloration among
different guppy populations.
6.
After reading and discussing the two introductory sections listed
above, ask students to work in pairs and begin their investigation of
guppies and their habitat. One student should use the computer
while the other takes notes on what they both observe.
Recommend that students spend time clicking on the pools
on the "Endler's discovery" screen to learn more
about the conditions in each. Students can also spend time
in the Guppy Gallery to learn more about the guppies, their
predators, and real-life stream habitats. At the end of
the observation period, students should consider the given
hypotheses -- and any they may have come up with on their
own -- and choose the one they think best explains the
differences in guppy coloration. Ask students to record
their chosen hypothesis.
7.
When students are finished with their preliminary observations, tell
them to trade places. Ask students to set up their first experimental
simulation by selecting the distribution of each guppy color type as
well as the number and type of predators that their experimental pool
will have. Recommend that students run the simulation for 10 to
15 guppy generations to yield observable results. Then ask students
to run three or four more simulations and experiment with different
sets of variables to test their hypothesis. Students should record
and/or print out their results.
8.
Following the experiments, have students consider the following
questions and present their answers to the class.
- What hypothesis did you test?
- What conclusions did you reach based on your
simulation results?
- How do sexual selection and predation interact in
this guppy simulation? How might this interaction differ in
a natural environment?
- What other questions did the guppy simulation raise?
How might you test for answers?
Check for Understanding
Have students discuss the following:
- Explain the statement that predation and sexual
selection pressures have competing outcomes. Give examples
from the media you've seen.
- What would likely result from a sudden, dramatic
change in the praying mantis's environment?
- How might the outcome of a more gradual change -- one
that takes place over thousands or millions of years -- differ
from the outcome of a sudden change?
