Development is the process by which single-celled zygotes become multicellular organisms.  Alterations in the developmental program often lead to differences in adult morphology.  When these alterations are beneficial, they become fixed in a population and can drive the evolution of new phyla.  The field of Evolutionary Developmental Biology or “The Evolution of Development” (Evo-Devo) seeks to understand how new groups arise by understanding how the process of development has evolved in different lineages. 

While both “Evo-Devo” and “Developmental Biology” have their roots in traditional Embryology, modern Developmental Biology focuses on understanding the developmental genetic mechanisms of normal development in a small number of model organisms (horizontal arrows in the drawing).   Evo-Devo applies the key findings and techniques of Developmental Biology and genetics to a wider phylogenetic sampling to elucidate the developmental genetic bases of evolutionary change (vertical arrow in the diagram).