Table 1 Outline of the sections of the Darwinian Snails modules in both versions

Prelude

1. Snail shells

Introduces students to the study system and to histograms as a way of graphing data to visualize the distribution of trait variation in a population

1. A model of evolution by natural selection

2. Evolution by natural selection

Students play the role of a crab preying on snails. They discover that thinner-shelled snails are easier to eat and go through several rounds of selection and reproduction to see a shift in the trait distribution in the population, thus visualizing natural selection in action

2. The requirements for evolution by natural selection

3. Darwin’s theory of evolution by natural selection

3. Requirements for natural selection

Students turn off three conditions for evolution by natural selection in turn to see the effect of each: variability; heritability; and differential survival. In turning off differential survival, students also see genetic drift in the small population. Mutations are disabled in this section

4. The source of variation among individuals

5. What makes populations evolve?

4. The source of variation

The students can now see the effect of mutations in the population. Starting with a snail population with reduced trait variation, students “turn on” mutations and examine parent/offspring combinations and histograms to see whether mutations are directional in the presences and absence of crab predation

6. Challenge: evolution by natural selection in flat periwinkles

5. Testing for natural selection

6. Extension: experimenting with snails

Data from Seeley (1986) are shown and students are asked to interpret the data

Students are asked to devise their own experiments to determine whether the conditions for evolution by natural selection are met in a snail population