Table 1. Constraint classification of Richardson and Chipman (2003).

Constraint Category	Type of Constraint	Definition
Generative (limited production of variant phenotypes during ontogeny)	Historical	Evolutionary history of organism limits developmental pathways because of current form or patterning mechanism available
	Developmental buffering	Genetic changes occur without phenotypic effect because of feedback loops and buffering in developmental pathways
	Genetic	Bias in appearance of certain genotypes due to nature of genetic system, probability of mutation, or recombination
	Physical	Physical properties of egg or embryo, extra-embryonic membranes and environment, limit or bias evolutionary change during development
Selective (limited survival of variant phenotypes during ontogeny or phylogeny)	Horizontal	Some developmental pathways influence many different developmental processes; changes in pathways likely deleterious because of pleiotropy
	Vertical	Changes in early developmental stages may have multiple effects on later stages, and are thus constrained
	Structural	Organisms limited in form because exceeding certain physical boundaries is selected against (i.e., limitations caused by physical laws)
	Functional	Functional integration of different systems limit morphologic variability