In selection, individuals with both extreme forms of a trait are at a selective advantage.

disruptive/diversifying selection  

Explanation:

Disruptive (also known as diversifying) selection is a type of natural selection where extreme phenotypic values are more favored rather than intermediate phenotypes. The disruptive selection favors the survivance and reproduction of organisms with extreme phenotypic traits, and in turn impairs the reproduction of organisms with intermediate traits, thereby changing the allele frequencies in the population in order to favor extreme values. An example of disruptive selection is the beak size of Galapagos finches, where extreme phenotypes of this trait have shown to be adaptive according to the type of food found in each island.