Download Argentum luminaria in the Past

Seed Case:
Developing a Darwinian Explanation
During the next several days your research group will develop a Darwinian
explanation for variations in seed traits for three populations of a made-up plant
species. You will do this using data on the plants, as well as data about two
species of insects that feed on the plant’s seeds. As you develop your explanation
you will have opportunities to share your progress with other groups that are
working with the same data. You will also create and present a scientific poster
that will convince others of the soundness of your explanation.
The data for the plant and insect species in this activity are made up. They will,
however, allow you to develop your abilities to “think like an evolutionary
biologist”. One reason to use a made-up case is that biologists usually don’t have
as much and as many different kinds of data available for three species of real
living things. Normally, evolutionary biologists find themselves having to create
explanations with only some of these types of data actually available.
In creating your Darwinian explanation for the variations in the seeds you need
to use Darwin’s model of natural selection. You will want to have in front of you
as your group works. As you develop your Darwinian explanation it is
important to realize that, simply using the “right” words from the Darwin-model
handout will not be enough. You need to use the different types of data that are
provided in this Case, as well as Darwin’s ideas. When finished, your
explanation, in the form of a poster, should convince others that your
explanation is supported by both the data and Darwin’s model.
Keep in mind that every complete Darwinian explanation should include:
A. a description of the trait at some time in the past including the variations in the population
B. a description of the selective advantage of the trait in question (how that trait helps that
individual have a better chance to survive than others of the same species)
C. which variation survives and reproduces better and why.
D. Evidence that the trait is inherited from the parents (genes)
E. How the species changed over many generations, including the variations in the
population in the present.
Case Contents:
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Background on the Plant, Argentum luminaria
Describes the general characteristics of the plant, Argentum luminaria , that is the focus
of the case.
Argentum luminaria in the Past
A description of Argentum luminaria in the past and present.
Background on Insects
Contains a description of two species of insects that live on Argentum. luminaria and feed
on its seeds.
Research Brief 1: Data from a Breeding Experiment in Argentum luminaria
Includes the results of an experiment in which individuals from three different
populations of Argentum. luminaria, growing in different environments, were crossed
with one another to determine if the variation in three seed traits is heritable.
Research Brief 2: A Study of the Growth of Argentum
luminaria in Three Different Habitats
Report of a study on how the variation in the three seed traits is affected by
growing individuals from each of the three Argentum luminaria populations under
different environmental conditions.
Research Brief 3: Seed Variation Data in Argentum luminaria
A description of three seed traits and the variation in them in each of the three different
populations.
Research Brief 4: Insect Populations
A description of the beetle and bug populations.
Research Brief 5: Seed Predation
Is a description of study done to determine the extent to which the two insect species
actually feed on the seeds of Argentum luminariArgentum
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Background on the Plant Species, Argentum luminaria
General Description of Argentum luminaria
The plant Argentum luminaria is a member of the morning glory family. It is has been found on
San Salvador, San Cristobal and Santa Cruz islands in the Galapagos. When full grown the
plant is between 75 and 100 cm high, has heart-shaped simple leaves, and produces about 16
tube shaped flowers that have up to 10 seeds each. See below for drawings of the plant and of
its flower structure. The seeds are somewhat oval shaped and have protective spines with
different numbers and lengths.
There are three different populations of Argentum luminaria that are found on different islands.
The three populations have different plant heights when full grown, different colors of flowers,
and different length of time that it takes to make seeds. They also have different number of
spines, different length of the spines and different thickness of the seed coat. See below for
drawings of the seeds as well as information concerning these three seed characteristics for
each of the three populations.
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Population A
Diagram of the seed
Habitat
length of seed
#of seed spines
length of spines
seed coat thickness
San Salvador
approximately 4 mm
varies from12-21 spines per seed
varies from 2.1-2.3 mm in length
varies from .005mm - .009mm in thickness
Population B
Diagram of the seed
Habitat
length of seed
#of seed spines
length of spines
seed coat thickness
Santa Cruz
approximately 4 mm
varies from 7-8 spines per seed
varies from 1.2 –1.8 mm in length
varies from.075mm - .110mm in thickness
Population C
Diagram of the seed
Habitat
length of seed
#of seed spines
length of spines
seed coat thickness
San Cristobal
approximately 4 mm
varies from 2-5 per seed
varies from .4 - .9 mm in length
varies from.005mm - .009mm in thickness
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Argentum luminaria in the Past
Argentum luminaria arrived in the Galapagos from South America and spread from its
point of arrival. The plant has been described by early naturalists writing in the 1750’s.
Their description of Population C of Argentum luminaria and its seeds is similar to
descriptions of a plant that naturalists described in South America. It seems that
Argentum luminaria similar to Population C came to the Galapagos Islands from South
America and evolved, forming two new populations on different islands.
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Background on TwoInsects
Biologists who have studied three populations of Argentum luminaria have noted the presence of
two insect species that spend much of their life cycles (from egg to adult) on or near the plants.
What follows is a general description of the life history of each of the insect species. One of the
species is a beetle. The second is a true bug.
Description of Megamorsas mandibulari
Order: Coleoptera (Beetles)
Family: Elateridae (click beetles)
Genus: Megamorsas
Species: mandibulari
Members of this species are almost always found living on Argentum luminaria plants. The
adults mate in early May through mid-June and the female deposits up to 300 eggs along the
larger stems of the plant. Typically the eggs are laid in the newer, softer, growth. The eggs
hatch in 7-10 days depending on the temperature. Once hatched, the beetle larvae burrow into
the stem tissue where they feed. They remain there for up to 30 days, going through larval and
pupa stages. The adults emerge from the pupa stage at the same time that the seeds are fullgrown in Argentum luminari. As is the case with all beetles, these have hard, chewing mouth
parts (See below for diagrams of adult beetle and mouth parts).
Adult Beetle
Description of Stylus elaganti
Order: Hemiptera (True Bugs)
Family: Lygaeidae (seed bugs)
Mouth Parts
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Genus: Stylus
Species: elaganti
Members of the species Stylus elaganti (bugs) are almost always found on Argentum luminaria.
However, they have also been found on closely related species. The adults mate and lay their
eggs in the fall of the year. The female burrows into the soil and lays her eggs on the roots of
Argentum luminaria. Stylus elaganti (bugs) undergoes incomplete metamorphosis. The immature
bugs emerge from the eggs in early spring and continue their development throughout the
spring and summer. In late summer or early fall the adult stage is reached. When they reach
adulthood, they start feeding on seeds. Because they are true bugs, they have piercing and
sucking mouth parts which are soft. See below for diagrams of the adult bug and its mouth
parts.
Seed Bug
Part diagram.
Adult
Mouth
Notice the mouth parts (labeled bk on the diagram) are in the form of a slender segmented beak that
arises from the front part of the head and usually extends along the bottom of the body, sometimes as
far back as the base of the back legs. It is fairly soft.
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Research Brief 1:
Data from a Breeding Experiment on Argentum luminaria
Breeding experiments have been conducted on three populations of Argentum luminaria. These
experiments were done with each of the three populations (A with A, B with B and C with C.) The
offspring were then grown under their normal environmental conditions. There were 100
repeated trials for plants from each population.
Experiment 1:
Title: What happens when each population is bred with itself?
Materials: Plants, soil, water, containers
IV: What population was crossed with what population
__________________________________________________________________________
Levels of IV
A with A
B with B
C with C
__________________________________________________________________________
Repeated
Trials
100
100
100
__________________________________________________________________________
Dependent Variables: seed coat thickness, number of seed spines, and length and thickness of seed
spines
Constants: Type of soil, amount of light, amount of soil
Procedure:
1. Cross Population A with Population A.
2. Cross Population B with Population B.
3. Cross Population C with Population C.
Conclusion:
When each population’s seeds were grown in the environment they were usually grown in, the
offspring were similar to the parents in regard to seed coat thickness, number of seed spines and
length and thickness of seed spines. So, for example, in each population the number of seed spines
was within the range of the parental generation (12 to 21 for Population A; 7-8 for Population B;
and 2-5 for Population C).
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Research Brief 2:
A Study of the Growth of Argentum luminaria in Three Different Habitats
Experiment 2
Title: The effect of a “nonnatural” environment on plant growth.
Materials: Plants, soil, water, containers
IV: Where the seeds were grown
________________________________________________________________________
Levels Pop A Pop A Pop A Pop B Pop B Pop B Pop C Pop C Pop C
Of IV Env A Env B Env C Env A Env B Env C Env A Env B Env C
________________________________________________________________________
Dependent Variables: seed coat thickness, number of seed spines, and length of seed spines
Constants: Type of soil, amount of light, amount of soil
Procedure:
Seeds from experiment 1 were planted in each of the three environmental conditions. Once the
plants grown from these seeds reached maturity they were then crossed with others grown in
the same conditions. When the seeds were then collected from this generation they were
examined for three seed characteristics.
Results
The seeds produced in each of the crossings were like those of the parents and were not
influenced by the environment within which the seeds were grown. In fact, this same
experimental design was used to generate four (4) more generations of seeds and each time the
results were the same. The seeds of offspring plants were like those of the parents that
produced them and the environment in which they were grown did not make any difference.
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Research Brief 3:
Seed Variation Data in Argentum luminaria
For the past 10 years we have collected seeds from each of the three populations of Argentum
luminaria (A, B, C) for the purpose of gathering data on the variation of each of three traits:
1. # of seed coat spines
2. length and width of seed coat spines
3. thickness of the seed coat.
The three populations are found growing in three different conditions:
A from high elevations and dry environments
B from high elevations and moist environments
C from low elevations and moist environments
During each year for which data were gathered, measurements were taken on each of the three
traits. (So, for example, for trait #1, the number of seed coat spines, each of the collected seeds
was examined and the number of spines counted.) Measurements were also taken for traits #2
and #3. One conclusion was that there is much variation in seed characteristics both within
each population and across all three populations. This variation for the combined 10 years, is
shown in the Tables on the following three pages.
.
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Variation of Traits in Population A
Range of Variation
#of seed spines
length of spines
seed coat thickness
12-21
2.1-2.3 mm
.005mm - .009mm
Distribution of Variation
(Note seed numbers given above are in 100s of seeds.)
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Variation of Traits in Population B
Range of Variation
#of seed spines
length of spines
seed coat thickness
7-8
1.2 –1.8 mm
.075mm - .110mm
Distribution of Variation
(Note seed numbers given above are in 100s of seeds.)
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Variation of Traits in Population C
Range of Variation
#of seed spines
length of spines
seed coat thickness
Distribution of Variation
2-5
.4 - .9 mm
.005mm - .009mm
(Note seed numbers given above are in 100s of seeds.)
Variation of Seed Coat Thickness Across Populations A, B, and C
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Seed Coat Thickness
Population A
.005 mm
.006 mm
.007 mm
.008 mm
.009 mm
658
721
779
412
344
.075 mm
.080 mm
.085 mm
.090 mm
.095 mm
.100 mm
.105 mm
.110 mm
Population B
Population C
635
694
715
588
423
522
592
684
777
692
790
201
210
Note: The numbers in the above table represent distributions of seeds of the varying seed coat
thicknesses.
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Research Brief 4:
Studies were conducted on how many insects there were of the two insect
populations in each of the areas within which populations A and B of Argentum
luminaria are found. The two species of insects were, a beetle, Megamorsas
mandibulari, and a true bug, Stylus elaganti. Insect sampling in the area in which
Population A is found yielded high populations of beetles and low populations of
bugs. The reverse was found in the area in which Population B is found. There
were low numbers of beetles and high numbers of bugs
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Research Brief 5: Seed Predation
Title: The Effect of Insect Predation on Seed Damage
Materials:
6 cages
2000 seeds of Population A
2000 seeds of Population B
2000 seeds of Population C
30 beetles
30 bugs
IV: Type of Seed and Type of Insect
______________________________________________________________________________
Level
Pop A
Pop A
Pop B
Pop B
Pop C
Pop C
Of IV
Beetle
Bug
Beetle
Bug
Beetle
Bug
______________________________________________________________________________Repeat
ed
Trials
1000
1000
1000
1000
1000
1000
DV: Number of seeds that were eaten and damaged by the insect
Constants: same amount of time, same type of cages
Procedure:
Two thousand (2000) seeds from each of three populations, A, B, and C of the species Argentum
luminaria were collected at the end of the 1996 growing season. The seeds were then placed in cages
with two insect species (Megamorsas mandibulari, a beetle; and Stylus eleganti, a bug) that are known to
feed on the seeds. Specifically, the seeds from each population were divided into two sets of 1,000 seeds
each. One of the sets was made available for 24 hours to 10 individuals of Megamorsus mandibulari and
the second set of 1,000 seeds was made available, also for 24 hours, to 10 individuals of Stylus eleganti.
At the end of the 24 hour period each of the six sets of seeds were examined for insect damage. The
number of seeds that had been damaged to the point that it was believed that they would not start
growing, if planted, were noted and are shown in the table that follows.
Damaged Seeds from Population
Population
A
Population
B
Population
C
Megamornus (beetle)
27
258
386
Stylus (Bug)
401
31
397
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Things to Consider as You Make Your Scientific Poster
The main purpose of the poster is to convey your Darwinian explanation to readers. To do this
effectively your group's poster should contain both data and the Darwinian model itself. The
following suggestions should help you prepare your poster.
What to Include:
1.
2.
3.
4.
5.
6.
7.
8.
Includes a description of the trait at some point in the past
Includes an explanation of the advantage of the variations of the trait
Includes data to support the explanation of the advantage of the variation of the trait.
Includes a discussion that shows that the trait’s variations are inherited (passed on by
genes.)
Includes data to show that the trait is inherited (passed on by genes.)
Talks about the variation in the trait in the past and at the present time.
Includes data to support the idea of variation in the trait in the past and at the present time.
Includes a description of the trait in the present and tells how the population changed over
time
Visual Considerations
 Numbers can be used to guide the reader’s eye through the poster.
 Main headings should be readable from six feet away.
 Supporting text follows the main headings and should be readable from three feet away.
 Numbered or bulleted lists show a series of points.
 Capitals and lower case are easier to read than all capitals.
 Illustrations and tables usually contain the main content of a successful poster, showing
rather than merely telling a “story”
 Each figure or table should illustrate no more than one or two major points
 Each figure or table should have a heading of one or two lines
Names________________________
______________________________
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______________________________
______________________________
Hour______
Group # _______
Teacher Rubric for Seed Case Poster
Poster Content
Rate the poster for the following components of a Darwinian explanation:

describes the trait in the past
10
_____

talks about variation in trait past
uses data to support
data labeled on the poster
10
5
5
_____
_____
_____

explains advantage of the trait
uses data to support
data labeled on the poster
10
5
5
_____
_____
_____

shows that the trait is inherited (passed on by genes)
uses data to support
data labeled on the poster
10
5
5
_____
_____
_____

talks about variation in the trait in the present
uses data to support
data labeled on the poster
10
5
5
_____
_____
_____

describes the trait in the present and tells how the
population changed over time
10
_____
TOTAL
100
_____
Comments:
(Please give a general description of the strengths and weaknesses of the poster in the above
categories.)
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Poster Defense

all group members participated and answered questions

group members answered questions in a way that
demonstrated understanding of the explanation
Comments:
(Please explain the scores you gave above)
5
_____
5
_____
TOTAL
_____
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Name of Student Being Evaluated:
____________________________
Name of Evaluator:
____________________________
Hour______
Group # _______
Student Rubric for Seed Case Poster
Poster Content
Rate the poster for the following components of a Darwinian explanation:

describes the trait in the past
10
_____

talks about variation in trait past
uses data to support
data labeled on the poster
10
5
5
_____
_____
_____

explains advantage of the trait
uses data to support
data labeled on the poster
10
5
5
_____
_____
_____

shows that the trait is inherited (passed on by genes)
uses data to support
data labeled on the poster
10
5
5
_____
_____
_____

talks about variation in the trait in the present
uses data to support
data labeled on the poster
10
5
5
_____
_____
_____

describes the trait in the present and tells how the
population changed over time
10
_____
100
_____
TOTAL
Comments:
(Please give a general description of the strengths and weaknesses of the poster in the above
categories.)
Comments:
21
Poster Defense

participated and answered questions
10
_____

answered questions in a way that
demonstrated understanding of the explanation
10
_____
20
_____
TOTAL
Comments:
(Please explain the scores you gave above)