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—createsboundaries between a succession of quantitative intervals—steps.Thresholds can be modified by an altered time of appearanceof structures and functions (heterochrony), especially duringearly ontogeny, to form an operational basis for the prolongationof juvenile characters and adaptability into later ontogeny.Whereas such prolongation enables juvenilization in phylogeny,analogous principles may operate on a much shorter time scaleto produce the r-selection-like altricial and the K-selection-likeprecocial trends in ontogeny. The inherited capacity to adjustconstantly to the environment (heterochrony) selects for structural,biochemical and behavioral improvements. The tendency is towardthe 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 becomeless stable and/or the community less competitive. Juvenilization,capable of turning gerontomorphosis into paedomorphosis in evolution,and heterochronous shifts of character anlagen, capable of turninga precocial trend into an altricial trend in ontogeny are bothpart of the same biological process which operates during earlyontogeny.