Evolution by natural selection and predator induced phenotypic plasticity are mechanisms that have been proposed to describe the effect of crab predation on snail shell thickness. Biologist Robin Seeley hypothesized that the periwinkle populations of New England evolved by natural selection as the European green crab was introduced to the area (Seeley, 1986). The three requirements for evolution by natural selection are that there is variation among a population, the variation is partly hereditary, and certain variants are able to survive to reproduce at higher rates than others. Once these three conditions are met, it is believed that the snail population will evolve toward thicker shells. A simulation was used to model the distribution of shell thickness in two different environments that were either crab infested or crab free. The objectives of the experiment are as follows:
1) To determine if the snails from the crab infested environment have thicker shells, on average, than the snails from the crab free environment.
2) To determine if the snail populations meet all three requirements for evolution by natural selection.
3) To determine if the snails from the two environments differ because one or both has evolved by natural selection, or if they differ because snails can smell crabs and grow thicker shells when they need them.
The first experiment showed that on average, the snails from the crab infested environment had thicker shells than the snails from the…