Apostatic selection is frequency-dependent selection by predators. It applies when prey are two or more forms (morphs) which look quite different. This is a kind of polymorphic species, where the predator learns to select the more common form. Then there is an advantage in looking different, and this advantage keeps the frequency above the mutation rate. The second form, or 'morph', becomes better known by predators as its number increases. So the population tends to have a kind of balance between the frequency of the two morphs.