Bees may pick up pollen of one flowering species on a certain place on their bodies. If this area does not come into contact with the receptive structures of the flowers of another plant species, the pollen is not transferred. This is an example of:

Mechanical isolation

Explanation:

Mechanical Isolation is an evolutionary mechanism that results from barriers or structures that prevent the mating of different species. Evolution has resulted of the difference in the location, size and shape of the male and female genitalia, which further prevents mating in different species and sometimes, closely related species.

This type of isolation is mostly found to be common in flowering plants. For example, flowering plants that doesn't have the correct shape or size for a pollinator will definitely not be able to receive the transfer of pollen and hence fertilization will not occur.