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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)
- 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:
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 Argentumluminaria 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 ArgentumluminariArgentum
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.
Population A
Diagram of the seed
HabitatSan Salvador
length of seedapproximately 4 mm
#of seed spinesvaries from12-21 spines per seed
length of spinesvaries from 2.1-2.3 mm in length
seed coat thicknessvaries from .005mm - .009mm in thickness
Population B
Diagram of the seed
HabitatSanta Cruz
length of seedapproximately 4 mm
#of seed spinesvaries from 7-8 spines per seed
length of spinesvaries from 1.2 –1.8 mm in length
seed coat thicknessvaries from.075mm - .110mm in thickness
Population C
Diagram of the seed
HabitatSan Cristobal
length of seedapproximately 4 mm
#of seed spinesvaries from 2-5 per seed
length of spinesvaries from .4 - .9 mm in length
seed coat thicknessvaries from.005mm - .009mm in thickness
Argentum luminaria in the Past
Argentum luminaria arrived in the Galapagosfrom 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.
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 full-grown 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 BeetleMouth Parts
Description of Stylus elaganti
Order: Hemiptera (True Bugs)
Family: Lygaeidae (seed bugs)
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 Argentumluminaria.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.
Adult Seed BugMouth Part diagram.
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.
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 IVA with AB with BC with C
______
Repeated
Trials100100100
______
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).
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.
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.
.
Variation of Traits in Population A
Range of Variation
#of seed spines12-21
length of spines2.1-2.3 mm
seed coat thickness.005mm - .009mm
Distribution of Variation
(Note seed numbers given above are in 100s of seeds.)
Variation of Traits in Population B
Range of Variation
#of seed spines7-8
length of spines1.2 –1.8 mm
seed coat thickness.075mm - .110mm
Distribution of Variation
(Note seed numbers given above are in 100s of seeds.)
Variation of Traits in Population C
Range of Variation
#of seed spines2-5
length of spines.4 - .9 mm
seed coat thickness.005mm - .009mm
Distribution of Variation
(Note seed numbers given above are in 100s of seeds.)
Variation of Seed Coat Thickness Across Populations A, B, and C
Seed Coat Thickness / Population A / Population B / Population C.005 mm / 658 / 635
.006 mm / 721 / 694
.007 mm / 779 / 715
.008 mm / 412 / 588
.009 mm / 344 / 423
.075 mm / 522
.080 mm / 592
.085 mm / 684
.090 mm / 777
.095 mm / 692
.100 mm / 790
.105 mm / 201
.110 mm / 210
Note: The numbers in the above table represent distributions of seeds of the varying seed coat thicknesses.
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
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 IVBeetle Bug Beetle Bug Beetle Bug
______Repeated
Trials1000 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
PopulationA / Population
B / Population
C
Megamornus(beetle) / 27 / 258 / 386
Stylus(Bug) / 401 / 31 / 397
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:
- 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______
______
______
______
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 past10_____
- talks about variation in trait past10_____
uses data to support 5_____
data labeled on the poster 5_____
- explains advantage of the trait10_____
uses data to support 5_____
data labeled on the poster 5_____
- shows that the trait is inherited (passed on by genes)10_____
uses data to support 5_____