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:.

Mechanical incompatibility.

Explanation:

Mechanical incompatibility: occurs when species cannot mate because their reproductive structures are incompatible – In animals with internal fertilization, male and female sexual organs may not fit together – For example, snails of species whose shells have left-handed spirals may be unable to copulate successfully.

Mechanical incompatibility of male and female genitalia is common in animals with internal fertilization. However, our knowledge regarding the precise mechanisms is limited.