Some closely related species of snails cannot mate because their shells spiral in opposite directions, making it impossible for their reproductive organs to be aligned. The type of evidence is.

The type of evidence is Mechanical incompatibility.

Explanation:

Copulation is often impossible between different animal species because of the incompatible shape and size of the genitalia.It is called mechanical incompatibility.

This isolation prevents species that diverge from interbreeding thus they lo longer able to mate with each other.

Here morphological difference  takes place which ultimately leads to behavioral isolation and thus no mating takes place.

Here snail can not mate because  their morphological structures are incompatible.