Heterochrony revisited: the evolution of developmental sequences

The concept of heterochrony is a persistent component of discussions about the way that evolution and development interact. Since the late 1970s heter ochrony has been defined largely as developmental changes in the relationsh ip of size and shape. This approach to heterochrony, here termed growth het erochrony, is limited in the way it can analyse change in the relative timi ng of developmental events in a number of respects. In particular, analytic al techniques do not readily allow the study of changes in developmental ev ents not characterized by size and shape parameters, or of many kinds of ev ents in many taxa. I discuss here an alternative approach to heterochrony, termed sequence heterochrony, in which a developmental trajectory is concep tualized as a series of discrete events. Heterochrony is demonstrated when the sequence position of an event changes relative to other events in that sequence. I summarize several analytical techniques that allow the investig ation of sequence heterochrony in phylogenetic contexts and also quantitati vely. Finally, several examples of how this approach may be used to test hy potheses on the way development evolves are summarized. (C) 2001 The Linnea n Society of London.