Studying the altered timing of physiological events during development: it's about time…or is it?

The investigation of the altered timing of developmental events is key to understanding evolution. Most empirical investigations of event timing are biased towards studying morphological variation. Recent reviews, however, have attempted to marshal the evidence for the importance of altered timing of physiological events, focusing on such timing shifts between species (physiological heterochrony) and within species (physiological heterokairy). Here we update these reviews. We firstly take a comparative developmental physiology approach to explore how recent studies have furthered our understanding of the links between physiological event timing shifts at different levels of biological organisation (i.e. individual, population and species). The alternative strategy of concentrating effort on one model system is then considered, in particular focussing on substantial recent advances in our understanding of fetal haemoglobin expression in humans. We conclude that, while the fetal haemoglobin model may be appropriate as a model for some questions, it can never be the model study system. We also discuss the different quantitative analyses available for investigating event timing alterations. We consider the efficacy of the terms heterochrony and heterokairy.