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Selective Herbivory
How selective are the larvae of the cabbage white butterfly on the plants
that they eat? These larvae prefer members of the Brassica family, which
includes cabbage, broccoli, Brussels sprouts, kale, mustard, radish,
and numerous others. One species — Brassica rapa — includes
turnips, Chinese cabbage, pak choi, broccoli raab, and the Fast Plants
that were selectively bred by Bottle Biology creator, Paul Williams.
All of these are varieties of the same species, so they can be considered
to be members of the same population when grown together.
Selective Herbivory
tests whether or not cabbage white butterfly larvae feed selectively
within a population of Brassica rapa. A population of
lettuce has been sown with the other seeds to make additional comparisons.
If larvae do feed preferentially, they could be considered agents of
natural selection. Such agents would be expected to cause the population
to evolve over many generations.
Materials Needed
Instructions
- If you are
using the same system for “Assessing
Variation,” you’ll begin this study about 10 days after
sprouting.
- To begin your study, gently gather
numerous larvae from the Brassica nursery. You’ll need at least
three for each system.
- Introduce equal numbers of larvae
to each system, trying to spread them around and start them at the
base of the plants’ stems.
- At least once each day for the duration
of the study period (3 – 5
days), use your “Selective Herbivory” Data Sheet
to make observations. You’ll be recording where the larvae
are and what is happening to the plant populations.
- At the end
of the study period, assess which plants were selectively
fed upon, if any.
Activity Questions
Before the study period begins
- What traits vary among the plants in your
Field Population System?
- Among the traits that vary for Brassica
rapa, which do you think might affect feeding behavior of the butterfly
larvae? Why?
- How will you be able to tell which variants the larvae
select?
- What has to happen in order for selective herbivory to have
an impact on the next generation of plants?
After the study period ends
- What occurred during the study period to the Fast Plants? The
turnips? The lettuce?
- Was there a pattern to feeding behavior over
time (i.e., an order to feeding choices)?
- Which of the two varieties
of Brassica rapa, if either, did the larvae select preferentially?
- What implications does this have for the Brassica population in
the next generation?
- Do you think that this is an example of natural
selection? Why or why not?
- How might this result in evolution within
the population?
- If selective herbivory was observed, which trait(s)
do you think might have affected feeding choices? Why?
- One trait
of Fast Plants that was discussed in the video during the Science
Studio was hairiness of plants. Is there
a way that
the Fast Plants in your Field Population System might
become hairier over time? Explain how this might occur.
- Is there a
way that the larvae of the cabbage white butterfly might become more
or less selective over time?
Explain how
this might occur.
SHARE YOUR RESULTS: Selective Herbivory
TRACK OUR PROGRESS: Selective Herbivory
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