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.
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.
50 minutes to measure, quantify, and discuss variation in snail traits. The extension projects described in the Going Further section may last several months.
Teams of 3-4 students.
For each group of 3-4 students you need:
Other supplies you may want on hand for groups to share:
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.
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.
None required although familiarity with observation, measurement, and histograms is helpful (see Human Traits Survey lesson).
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.
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.
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.