Evolutionary Biology and Understanding Adaptation
Evolutionary biology works by one cardinal rule: to understand an adaptation, one has to understand its evolved function. The analysis of adaptations is more than just-so stories, because according to evolutionary theory there are only two fundamental kinds of functions that explain adaptations. Adaptations can arise through natural selection for survival advantages, or sexual selection for reproductive advantage. Basically, that's it.
from "The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature"
Quoted on Sun Feb 17th, 2013