In Drosophila, successful mating is frequently dependent upon an elaborate courtship "dance" performed by the male. Over 200 species of these fruit flies occur on the Hawaiian Islands. However, mating between many of these species is avoided, as the females only respond to the particular "dance" of conspecific males. This is an example of:

In Drosophila, successful mating is frequently dependent upon an elaborate courtship "dance" performed by the male. Over 200 species of these fruit flies occur on the Hawaiian Islands. However, mating between many of these species is avoided, as the females only respond to the particular "dance" of conspecific males. This is an example of:  

sexual selection

Explanation:

The sexual selection in this case of the flies, causes that the males by means of the dance are echoed to copulate by the female, the conspecific males  have evolved with this form of selection that prevents the coupling of the female with other species and which allows  the conspecific males to ensure their paternity.

The cell membrane is selectively permeable, meaning it only lets certain things in and out of the cell. If you think about our building analogy, this can be a very important thing. You wouldn't want just anyone running in and out of the White House, for example. It's important to have guards who determine who's allowed in and who's not. A cell is a living thing and needs just the right balance of nutrients and water, called homeostasis. The selective permeability of the membrane allows the cell to stay in homeosis.