American Cliff Swallow (Petrochelidon pyrrhonota) Science Article 5
abstract
The study of adaptive behaviors lies at the very heart of behavioral ecology. Although there is certainly no lack of debate about what constitutes an adaptation (2), most researchers in the field probably would not quibble with the statement that adapted traits are those that are the product of natural selection. Evidence for adaptive behavior then can be found only when the criteria for the process of natural selection to operate are met, namely: (i) variation in the trait of interest, (ii) fitness differentials across variants of the trait in question, and (iii) a means by which the trait is transmitted across generations.
Lee Alan Dugatkin, PNAS, January 2, 2001, vol. 98, no. 1