A functional explanation defines or elaborates why a behavior evolved in the way that it did. Within a small population a gene that spreads by pure accident is called a genetic drift. At times one dominant male, such as an african lion, has a large number of offspring and spreads all of his genes. The larger the population or community the less impact or powerful the genetic drift is. A functional explanation can pinpoint the advantage.

Another example would be a species that can change his color to match his surroundings. The functional explanationt to this would be the species can change colors to hide from his predators.