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"Sampling Variation in a Natural Population"

By Larry MacPhee



Type of Entry:

  • project

Type of Activity:

  • inquiry lab
  • community outreach/off-site activity

Target Audience:

  • Life Science
  • Biology
  • Advanced/AP Biology
  • Genetics, Biotechnology
  • Environmental studies

Notes to Teacher:

It is important that the concepts of species, population, phenotype, genotype and genetic variation are familiar to the students before this study is begun. It will help if students discuss their ideas for this project with the instructor, since a great deal of independent thinking is required. Students should also be reminded to respect life, and to avoid causing pain, suffering, or death to any living things. In addition, it is important to emphasize that some organisms are dangerous, and that students must exercise caution and good judgment to avoid getting bitten, stung, poked, jabbed, or otherwise injured. As a general policy species that are likely to cause any kind of harm, such as small mammals (which might carry rabies), venomous insects (bees, wasps), toxic plants (poison oak) etc, should be avoided. Required of students: Since this is an out-of-class activity, students should be sure that they understand what is required of them before beginning this project. Additionally, students should not collect more than 10 data points until they have received approval of their research proposal, since this is the point at which the validity and feasibility of the study will be checked. Preparation time: This project is best placed in the curriculum after completion of Mendelian genetics and is a good introductory project for studies of Population and Quantitative genetics. Very little preparation is required for Part I of the project, but it will be necessary to read lots of research proposals in about 3 weeks time. It is important to give students many feedback on their proposals, and, in some cases, to reject their first submissions if there are serious problems. Class time needed: If students are familiar with Mendelian genetics, approximately 30 minutes should be spent explaining the project, suggesting ideas, giving examples and answering questions. Parts I and II of the project are done outside the classroom, but Part III will require at least a day in the computer lab or working with statistical calculators. Another full day will be necessary for student presentations.


Abstract

Students were directed to non-destructively sample the phenotypic variation within a natural population of a single species. Choice of research topics was up to the individual, but was subject to the approval of the instructor based upon evaluation of a formal research proposal with preliminary data. Upon approval, each student collected up to 100 data points, each point representing a measurement of a specified phenotype on a different individual within a population. Data were analyzed using statistical/spreadsheet software in the computer lab. Students were expected to interpret their results and to present them orally in the format of a professional scientific meeting.


Background

What question does this activity help students to answer? It is surprisingly difficult to adequately impress upon students exactly how much variation exists in natural populations. Students will gain a greater appreciation for the diversity of living things by personally collecting data on variation in natural populations of their own choosing.


Project

Materials needed:

Part I: Instruction sheets for students, field guides to aid in the identification of local species, tools for measurement and collection of organisms.

Part II: None

Part III: Access to a computer lab with statistical or spreadsheet software, or calculators with statistical functions.

Activity: See "Population Sampling" Handout


Method of Evaluation/Assessment

Part I: Evaluation based upon answers to the following 10 questions:

  1. Describe the general biology of the particular species which you have selected and indicate the source of your information.

  2. What was your reason for choosing this species?

  3. Describe in detail your method of organism collection or identification.

  4. Explain how you will ensure that all individuals collected or identified are members of the same species and population.

  5. Describe in detail the trait which you will be measuring and why it is biologically relevant.

  6. Describe in detail your method for measuring the trait of interest.

  7. Explain how will you deal with additional sources of variation in the trait such as those caused by age and/or sex of the individual sampled.

  8. In Table 1, Column A, give some preliminary data (10 data points) on the character of interest which indicates that your study is feasible and that there is indeed variation in the chosen characteristic.

  9. In Table 1, Column B, also record a "reference measurement" on each individual which can be used to control for body size variation. Appropriate measures might include total length, weight, or some other measure of size.

  10. Draw an accurate sketch of the organism you will be studying, with particular reference to the chosen trait.

Part II: Evaluation based upon ability to successfully collect remaining data.

Part III: Evaluation based upon quality of analysis and presentation of research.


Extension/Reinforcement/Additional Ideas

Student Seminar Day: Each student will get to see what the other students chose to study, and to see what kind of variation exists in a wide variety of species. Time will be allowed for a question and answer session to learn about these other studies.

Extra credit assignment: Have students collect data on a new population of the same species in a different locale and use a t-test to compare the mean value of the measured trait in the two populations to look for geographic variation.

Population Sampling (100 pts )

Name ____________ Per. ___

Your assignment is to collect data on variation in a natural population by measuring the same biological characteristic in 100 different individuals of a species. You may not use humans or domestic animals/plants for this assignment, and you must take your measurements without damaging the organisms you measure. Choose a trait which you think might be heritable (passed down from parents to offspring). Make sure that the trait you choose is variable. (For example, don't count the number of legs on ants, because all insects have six legs and you will not find any variation.) You should attempt to measure individuals of the same age so that the measurement is independent of body size or age. Here are some frequently asked questions. See the next page for all the details of the assignment.

CAUTION: Some animals bite or sting. Some plants are thorny or irritating. Be careful!

  1. How can I tell whether the organisms are members of the same species?

    A species is usually defined as a group of organisms with the ability to successfully interbreed. Since that is a difficult thing to check, we will define a species as a group of organisms which greatly resemble each other in their physical characteristics. For example, all daisies are members of the same species but all flowers are not. There are at least 3 species of roly-polys found locally so careful observation will be necessary in order to properly classify them. Remember that in some species, males and females differ greatly in appearance. Members of a species also sometimes undergo dramatic changes as they age. Use a "field guide" to help with identification of your species. See the nature section of a library or bookstore.

  2. How can I tell whether organisms are of (approximately) the same age?

    Ideally, one would follow marked organisms from birth to death. Since this is not possible unless you use something with a very short lifespan, such as fruit flies, we will use size as an approximation of age. Make sure that the largest and smallest organisms in your sample differ in size by less than 10%.

  3. Where am I going to find 100 organisms of the same species?

    Even though we live in the city, there are lots of places to find living things. Try looking for weedy plants or insects in a vacant lot. Go to a park and examine the trees or walk through the tall grass. Turn over a rock or a log. Visit the beach and look at the organisms of the tide pools and the intertidal zone. Go outside after the rain and the sidewalk will have lots of snails and worms. Examine the edge of a pond or stream. Think small-it is easier to find 100 small things than 100 large ones.

  4. Why can't I just measure 100 leaves on one tree, or a hundred blades of grass?

    That would not represent a sample of 100 individuals - all the leaves on one tree were produced by instructions from the same set of genes so your actual sample size is only one. Grass blades are not individual plants-many blades make up a single plant and identifying separate individuals is very difficult.

  5. What kind of things could I measure?

    There will be points for originality. Find some characteristic that varies greatly within a species. You could measure the color of flower petals, or the number of seeds in a pod, grasshopper jump height, the speed of ants on a scent trail, or the number of bands on snail shells. Look closely at life! Be creative!

  6. How do I record my data?

    You must express you data in numeric form in a table. If you use non-numeric characteristics (example: color) you must show me the color scale you used to convert your data into numbers. Metric units are preferred. Measure the trait of interest and also a reference characteristic such as total length or weight.

Part I: Research Proposal

You must submit a written "research proposal" which will be evaluated by the instructor before you conduct the full study. If the proposal is approved, you may proceed. If it is rejected, you must resubmit the proposal after addressing all suggested changes. A response to each of the following questions should appear in your proposal: (1) Describe the general biology of the particular species which you have selected and indicate the source of your information. (2) What was your reason for choosing this species? (3) Describe in detail your method of organism collection or identification. (4) Explain how you will ensure that all individuals collected or identified are members of the same species and population. (5) Describe in detail the trait which you will be measuring and why it is biologically relevant. (6) Describe in detail your method for measuring the trait of interest. (7) Explain how will you deal with additional sources of variation in the trait such as those caused by age and/or sex of the individual sampled. (8) In Table 1, Column A, give some preliminary data (10 data points) on the character of interest which indicates that your study is feasible and that there is indeed variation in the chosen characteristic. (9) In Table 1, Column B, also record a "reference measurement" on each individual which can be used to control for body size variation. Appropriate measures might include total length, weight, or some other measure of size. (10) Draw an accurate sketch of the organism you will be studying, with particular reference to the chosen trait. Use the table and figure boxes below as a sample of the way to present your data for the research proposal.

Table 1:  Population sample data.                          
              A                 B
1
2
3
4
5
6
7
8
9
10

Figure 1: Sketch of study organism

  
  
  
  
Part II: Population sample (Due 3 weeks after Proposal Approval)

Read all comments on the research proposal. Make suggested changes or defend your methods to the instructor. Complete the data collection process within three weeks after the proposal is approved.

Part III: Data Analysis (To be completed in the computer lab after sample is completed.)

In the Macintosh computer lab, you will enter your population data into a ClarisWorks spreadsheet for analysis. You will create a frequency histogram of your measured data, and you will calculate the mean, median, mode and standard deviation for your data set. You will create an X-Y plot of your trait against the reference trait to adjust for body size differences among individuals. Finally, you will present and discuss the results of your study with relation to the biology of the organism in an oral presentation.


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