The emergence of the new field of evolutionary developmental biology (“evo-devo”) has been fueled by the discovery that many very distantly related organisms utilize highly similar genes and molecules to establish important structural patterns during their development (defining, for example, the borders between the head, trunk, abdomen, and tail). The goal of research in this field is to determine how evolutionary mechanisms (such as mutation and natural selection) act on embryonic development, generating diversity in the body plans of various organisms but also constraining the possible directions of evolutionary change. Of particular interest are the so-called macroevolutionary transitions, in which a major change in morphology occurred, such as the emergence of limbed vertebrates (tetrapods) from fish or the evolution of birds from dinosaurian ancestors.