The Meaning of Developmental Time: A Metric for Comparative Embryology

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Abstract
The lack of a suitable metric for developmental time has prevented a comparison of developmental rates and tempos between species, thus precluding analyses of heterochronic processes in evolution that do not depend on the use of morphology or size as an index of age. At least two problems exist in using clock time itself as the measure of developmental time: temperature effects within a species, and size effects across species. Consideration of two important distinctions that can be made in the notion of time.sbd.that between time as sequence and time as duration and that between extrinsic and intrinsic time.sbd.suggests that a metric, to be useful, must satisfy seven criteria: (1) it should be independent of morphology; (2) it should be independent of size; (3) it should depend on only one a priori homologous event; (4) it should be unaffected by changes in temperature for any given species; (5) closely related organisms should undergo homologous events at similar developmental ages as measured by the metric; (6) it should increase monotonically with clock time; and (7) it should be defined in a physically measurable way. I propose that physiological time, here defined as the integral of mass-specific metabolic rate over clock time, may be such a metric; this definition is based on an extension of previous concepts of physiological time to reflect ontogenetic changes in mass and metabolic rate. The proposed metric inherently satisfies criteria 1, 3, 6, and 7; satisfaction of the others must be assessed empirically. In spite of problems with strict comparability, literature data from 22 vertebrate species strongly suggest that criterion 2 is also satisfied, although they cast doubt on the satisfaction of criterion 5. No available data bear directly on criterion 4, although circumstantial evidence suggests that it may also be satisfied. A more rigorous assessment of the hypothesis awaits additional data, especially those allowing a correlation between physiological time and morphological development. However, even if this particular proposal for a metric proves unworkable.sbd.which is likely, given its simple formulation and broad scope.sbd.the criteria suggested will serve to judge any future proposal of a metric for developmental time. For the present, one must at least be aware of the hidden assumptions that have entered into much previous work on heterochrony, particularly that of an equivalence between size and age. Without a metric for developmental time, the extent and meaning of evolutionary changes in developmental timing simply cannot be assessed.