Visual and clear example of the idea that "animal doesn't need to understand the evolutionary function of its actions"
Use human behavior, they're human, do they understand why they like salty foods or why thiols smell bad to them. These behaviors still work even when you have no idea why you are doing them.

Using humans circumvents the problem of them not understanding animal behavior, which they already have to do to grasp your explanation for animal behavior. Using any other animal requires them to understand that animals mind which an entry level student will not, however as humans they can assess their own knowledge directly.