3.1 Environmental Variation

If the extra-leg deformity in your frogs is caused by worms, the underlying model of frog development works like this:

A flow chart, moving from left-to-right. At left is a cartoon tadpole, with a label underneath reading Has tadpole become infected with worms before it turns into frog? A pair of diverging arrows point up-and-right and down-and-right The up-and-right arrow leads to a cartoon tadpole with an orange dot on it, over a label reading Yes. An arrow points right to a cartoon frog with five legs, over a label reading 5 legs. The down-and-right arrow leads to a cartoon tadpole without an orange dot on it, over a label reading No. An arrow points right to a cartoon frog with four legs, over a label reading 4 legs.

When the variation in leg number is environmental, individuals that get infected with worms as tadpoles grow five legs. All other individuals grow four legs.

The difference between ponds in the frequency of five legs is due to the fact that one of the ponds harbors more worms.