Project - Snail Variations

Summary
Variation in a population is the raw material on which natural selection works. How do scientists measure and quantify variation in traits? We use garden snails as a model organism in order to describe and measure several different traits. Groups are given a small population of snails and must devise an objective way to measure a trait of their choosing (length, mass, speed, color intensity, stripes, withdrawal reflex reaction time, number of pennies it can carry, etc.). There are many ways to extend this activity. For instance, scientific protocols may be traded between groups, hypotheses may be made concerning what individuals will survive better in different environments, and snails may be tagged and released into one or more environments and the populations monitored over time. A long term open-ended project such as this provides a natural extension and assessment opportunity for both evolution and ecology concepts.

Objectives

Can make observations in an objective, quantifiable manner.
Can select and use tools to collect data.
Can use tables and graphs to represent data and identify patterns in data.
Can describe the role of variation in a population in natural selection and evolution.

Vocabulary

Traits
Variation
Population
Natural selection
Evolution
Objective
Subjective
Quantify
Histogram

Attachment Size
Proj_Snail_Variations.doc 47 KB

Snail Variations - Logistics

Time
50 minutes to measure, quantify, and discuss variation in snail traits. The extension projects described in the Going Further section may last several months.

Grouping
Teams of 3-4 students.

Materials
For each group of 3-4 students you need:

  • 1 plastic shoebox
  • wet paper towels
  • vegetables
  • 4-6 snails
  • 2 hand lenses
  • 1-2 clear rulers
  • 1 stop watch
  • Optional: 2 bottles of nail polish in different colors

Other supplies you may want on hand for groups to share:

  • scale or balance
  • Petri dishes
  • pennies or washers for students to measure how much a snail can carry
  • masking tape
  • paint color samples from a local hardware store in all shades of beige, brown and black
  • different textured surfaces (copy paper, mirrors, construction paper, overheads, sand paper, aluminum foil etc.)
  • pH paper

Setting
Initial measurement of snail traits can be done in the classroom. Extension projects should be done in a schoolyard, garden, creek, park or other local outdoor area that has a resident snail population – ideally, this is the location where the snails were collected.

Snail Variations - Background

Teacher Background
Natural selection and evolution are core ideas in biology and, in fact, all of science. Natural selection can briefly be described the process by which those individuals whose traits best fit their environment are most likely to survive, reproduce, and pass their genes on to the next generation. One of the critical “raw ingredients” of natural selection is variation in a population. All natural populations (groups of organisms of the same species) vary in their traits based on the interplay between genetics and environmental factors.

This activity uses the common garden snail (Helix aspersa) to measure variations in a population. These animals are garden pests found throughout North America and are readily captured from around most neighborhoods in California. I generally pay my neighbor’s kids 5¢ a snail and end up with upwards of 40 snails in less than an hour.

Snails are incredibly easy to keep in the classroom. They can survive in the classroom almost indefinitely with regular feeding and cleaning. Keep snails in a plastic shoebox or glass terrarium. Keep the terrarium covered securely while letting in air for them to breathe. Snails are strong and can easily push off a plastic lid, so secure the lid with rubber bands if necessary. Stock their habitat with several wet paper towels and vegetables from the grocery store (lettuce, carrots, apples, etc.). Twice a week, clean out their habitat by throwing away the old paper towels and food and giving them new wet paper towels and food. If you are keeping the snails longer than a week, place pieces of chalk in each container since they need calcium for shell growth and repair.

At the end of your project, snails may be released if they were collected locally. It is often interesting to “tag” the snails before you release them with a dot of nail polish on their shells. Thus, individuals may be tracked over time. If you choose not to release these pests back into your neighbors’ gardens, they may be frozen then thrown away. The adventurous can try cooking and eating them. That’s right! The garden snails found in North America are the same species that is used in escargot. In the going further section, there are resources for how to make escargot – although beware… this may be traumatic to some of your students.

Student Prerequisites
None required although familiarity with observation, measurement, and histograms is helpful (see Human Traits Survey lesson).

Getting Ready

  1. Prepare snail habitats and capture snails.
  2. Set shared materials in a central location for groups to access.

 

Snail Variations - Lesson Plan

Lesson Plan

  1. Discuss any ground rules (like do not hurt any snails) then jump right in! Pass out the snails, hand lenses, rulers, and stop watches. Ask groups to spend a few minutes observing the features and behavior of the snails.
  2. When students have had enough time to study the snails, have them close the lids. Ask students what they noticed. In particular, focus on how individual snails differ from one another. Discuss both the physical and behavioral traits of the snails.
  3. Ask students how these different physical and behavioral characteristics could be measured. Discuss the difference between subjective (bigger, faster, smarter) and objective (4.5 cm, travels 8 cm/min, figures out a maze in 2 min) measurements.
  4. Challenge students to pick a snail trait to measure. They should write down a procedure for their test and record the result for each snail that they were given. Everyone in the group must agree on the procedure such that the results would be the same, no matter who conducted the test. That means that your procedure should describe exactly what to do as if you were describing how to conduct the test over the phone to a friend. For instance, if you want to measure “size”, do you measure weight or length or width or height? If you measure length, what do you do when the snail is hiding inside its shell? Do you count antennae or not? Do you use centimeters or inches?
  5. Give students time to choose a trait, agree on a procedure and record their data. Circulate among the groups to help students that are struggling. Groups that finish early should be challenged to design a second procedure – possibly with the requirement that if they already tested a physical trait, that their second test should be of a behavioral trait.
  6. When all groups have finished, have them close the lids again. Discuss different ways to graphically present the results – pie charts, histograms, line graphs, etc. Tell students that they will be given 5 minutes to prepare a presentation for the rest of the class. Their presentation should include:
    • a description of their procedure
    • a table of results for their population of snails
    • a graphical presentation of their results

Snail Variations - Going Further

Assessment

  1. Students’ data and graphs can be collected and graded.
  2. Written protocols for trait measurements can be passed between groups so that students get practice and feedback on writing a scientific protocol. You may wish to do this before having each group graphically represent their data. This extension generates considerable discussion on the causes of experimental error and measurement inconsistencies. It also allows the full characterization of the population of snails on a wide range of traits.

Going Further

  1. Snails may be “tagged” with spots of nail polish then released into one or more environments (for instance, a school yard versus a vacant lot). Choose your environments carefully such that those areas actually can support a number of snails (a parking lot is probably not the best choice). Students can make hypotheses about which snails with what combination of traits will survive better in which environments. Changes in the populations’ traits may be monitored over time and may be correlated with the students initial hypotheses. In this way, students can ask very open ended questions about natural selection in the real world with living organisms. In the end, students may discover more about habitat choice and survival than natural selection per se, still it is an incredibly rich and varied exercise that the students thoroughly enjoy.
  2. As described previously, garden snails are escargot and are quite tasty if prepared properly. There are several steps to preparing your snails for the table:
    1. Feed your snails just cornmeal (3 tablespoons for a dozen snails) for approximately 4 days.
    2. Fast your snails in clean habitats with just wet towels for 2 more days.
    3. Just before cooking, rinse the snails in cool water from the tap.
    4. Plunge the snails into boiling water, shells and all. Boil uncovered for 2-3 minutes. A lot of foam will develop so watch carefully.
    5. Drain the snails and rinse with cold water. Using a toothpick, carefully pry each snail from its shell.
    6. The tightly coiled gall section of the snail that lies deepest in its shell should be cut off and discarded.
    7. The final cleaning step is to rinse the snails in water with a splash of vinegar until the water no longer turns cloudy.
    8. To cook the snails, boil 2 cups of snails in a broth made from 3 cups of beef broth, 1/2 cup white wine, 1 small chopped onion, 1 bay leaf, 1/2 teaspoon thyme, 1/2 teaspoon parsley, and salt and pepper to taste. Simmer for 1 hour.
    9. For the traditional preparation of escargot in garlic butter, melt 1 stick of butter (8 tablespoons) then add 1 1/2 teaspoons of chopped garlic, 1 tablespoon chopped green onion, and 2 tablespoons chopped fresh parsley. Puree the melted butter and seasonings in a food processor. Add salt and pepper to taste. Drizzle the garlic butter evenly over cooked escargot in an oven proof dish like a ramekin.
    10. Bake at 400 degrees for 7 –10 minutes.
  3. Investigate trait variations in plants. Grow plants in the classroom and compare trits such as plant height, color, time to flower opening, hairiness, and more. See the Raising Plants project for more details.

Snail Variations - Sources and Standards

Sources
This lesson was adapted from a lesson by Karen Kalamuck of the Exploratorium Teachers Institute.

For information on snails and snail care, see this website from the Lawrence Hall of Science.

The escargot recipe is taken from Gourmet Magazine, March 2001. A copy of this can be found at Epicurious.com.

Standards
Grade 7
Evolution
3. Biological evolution accounts for the diversity of species developed through gradual processes over many generations. As a basis for understanding this concept:
  a.    Students know both genetic variation and environmental factors are causes of evolution and diversity of organisms.

Investigation and Experimentation
7. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:
a.     Select and use appropriate tools and technology (including calculators, computers, balances, spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.
e.     Communicate the steps and results from an investigation in written reports and oral presentations.

Grade 9-12
Evolution
7. The frequency of an allele in a gene pool of a population depends on many factors and may be stable or unstable over time. As a basis for understanding this concept:
a.     Students know why natural selection acts on the phenotype rather than the genotype of an organism.

8. Evolution is the result of genetic changes that occur in constantly changing environments. As a basis for understanding this concept:
a.     Students know how natural selection determines the differential survival of groups of organisms.
b.     Students know a great diversity of species increases the chance that at least some organisms survive major changes in the environment.

Investigation and Experimentation
1.  Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other four strands, students should develop their own questions and perform investigations. Students will:
a.     Select and use appropriate tools and technology (such as computer-linked probes, spreadsheets, and graphing calculators) to perform tests, collect data, analyze relationships, and display data.
b.     Identify and communicate sources of unavoidable experimental error.
c.     Identify possible reasons for inconsistent results, such as sources of error or uncontrolled conditions.