The Interaction of Environment and Chronological and Developmental Time

Abstract

As organisms develop from embryos to adults, adaptive phenotypes must emerge to meet the demands of the habitat. This physiological and morphological transformation occurs along a continuum, where the emerging traits are often illustrated as landmarks charted along a predictable chronological timeline. Chronological time is measured according to astronomical phenomena, in hours, days, weeks, months, and years. However, developmental timing is largely driven by innate molecular oscillators that are independent of chronological time and species-specific. Environmental stressors can alter the timing of emergence of developmental phenotypes, creating further discord between developmental time and chronological time. Further, variation in the timing of emergence of developmental phenotypes is the norm, rather than the exception both within and between species. Such examples of environmentally driven variation of developmental timing abound, including alteration in development speed within chronological time and alteration of the sequence of emergence of landmark traits. It follows that if treatment groups (or species) are at different developmental stages, experimental comparisons become increasingly complicated. Within the context of the growing use of developmental model organisms in environmental sciences, the effect of alteration of developmental timing is particularly relevant. This is well illustrated in several examples utilized here to describe how alteration of developmental time can be a maladaptive consequence, or an adaptive compensatory response to environmental stressors, which can be selected for during evolution. Thus, such alterations of developmental timing can be measured and accounted for when designing developmental studies and in the interpretation of resultant data.