An integrative approach to heterochrony: the distinction between interspecific and intraspecific phenomena

While a framework and terminology for heterochrony has been referenced widely in the literature and appears to be accepted by nearly all workers in the field we have found it to be a confusing and incomplete model that has led to varying degrees of misunderstanding about heterochrony among evolutionary biologists. Much of the confusion exists because the model is explicitly limited to phylogenetic patterns (interspecific comparisons), but has been used for intraspecific comparisons. Because heterochrony may underlie all morphological variation and possibly is the developmental phenomenon producing all morphological change it is important that descriptions of heterochronic patterns and processes be clear and precise over all levels of analysis. To this end we discuss and clarify the previous model for heterochrony, reject some of the terminology and suggest alternatives, and then expand the model to include a new nomenclature for intraspecific heterochronic phenomena. Our modifications are essential to maintain the critical conceptual distinction between intervs. intraspecific heterochronic patterns and processes in evolutionary biology.