Saltatory Processes and Altricial to Precocial Forms in the Ontogeny of Fishes

SYNOPSIS. Development is not a gradual but a saltatory process. A combination of qualitative changes in form and function—thresholds—creates boundaries between a succession of quantitative intervals—steps. Thresholds can be modified by an altered time of appearance of structures and functions (heterochrony), especially during early ontogeny, to form an operational basis for the prolongation of juvenile characters and adaptability into later ontogeny. Whereas such prolongation enables juvenilization in phylogeny, analogous principles may operate on a much shorter time scale to produce the r-selection-like altricial and the K-selection-like precocial trends in ontogeny. The inherited capacity to adjust constantly to the environment (heterochrony) selects for structural, biochemical and behavioral improvements. The tendency is toward the precocial but the way back to altricial forms is left open. The heterochronous adaptations in early ontogeny can “reverse” the vulnerable specialization, should the environment become less stable and/or the community less competitive. Juvenilization, capable of turning gerontomorphosis into paedomorphosis in evolution, and heterochronous shifts of character anlagen, capable of turning a precocial trend into an altricial trend in ontogeny are both part of the same biological process which operates during early ontogeny.