Heterochrony, disparity, and macroevolution

The concept of heterochrony has long had a central place in evolutionary theory. During their long history, heterochrony and several associated concepts such as paedomorphosis and neoteny have often been contentious and they continue to be criticized. Despite these criticisms, we review many examples showing that heterochrony and its associated concepts are increasingly cited and used in many areas of evolutionary study. Furthermore, major strides are being made in our understanding of the underlying genetic and developmental mechanisms of heterochrony, and in the methods used to describe heterochronic changes. A general theme of this accumulating research is that some of the simplistic notions of heterochrony, such as terminal addition, simple rate genes, and "pure" heterochronic categories are invalid. However, this research also shows that a more sophisticated view of the hierarchical nature of heterochrony provides many useful insights and improves our understanding of how ontogenetic changes are translated into phylogenetic changes