Consideration of the ways in which ontogenetic development may be modified to give morphological novelty provides a conceptual framework that can greatly assist in formulating and testing hypotheses of patterns and constraints in evolution. Previous attempts to identify distinct modes of ontogenetic modification have been inconsistent or ambiguous in definition, and incomprehensive in description of interspecific morphological differences. This has resulted in a situation whereby almost all morphological evolution is attributed to heterochrony, and the remainder is commonly either assigned to vague or potentially overly inclusive alternative classes, or overlooked altogether.
The present paper recognizes six distinct modes of ontogenetic change, each a unique modification to morphological development: (1) rate modification, (2) timing modification, (3) heterotopy, (4) heterotypy, (5) heterometry, and (6) allometric repatterning. Heterochrony, modeled in terms of shape/time/size ontogenetic parameters, relates to parallelism between ontogenetic and phylogenetic shape change and results from a rate or timing modification to the ancestral trajectory of ontogenetic shape change. Loss of a particular morphological feature may be described in terms of timing modification (extreme postdisplacement) or heterometry, depending on the temporal development of the feature in the ancestor. Testing hypotheses of the operation of each mode entails examining the morphological development of the ancestor and descendant by using trajectory-based studies of ontogenetically dynamic features and non-trajectory-based studies of ontogenetically static features.
The modes identified here unite cases based on commonalities of observed modification to the process of morphological development at the structural scale. They may be heterogeneous or partially overlapping with regard to changes to genetic and cellular processes guiding development, which therefore require separate treatment and terminology. Consideration of the modes outlined here will provide a balanced framework within which questions of evolutionary change and constraint within phylogenetic lineages can be addressed more meaningfully.