Vocabulary: Rainfall and Bird Beaks Vocabulary  Adaptation – a trait that is beneficial to an organism.  Beak depth – the distance from the top to the bottom of a beak, as shown at right.  Directional selection – a process of natural selection in which phenotypes at one extreme are favored over other phenotypes. o For example, directional selection is occurring if the finches with the largest beaks are surviving and reproducing at higher rates than finches with averagesized or small beaks. 