Evolutionary constraints are generally understood as properties of the structural elements of an organism that constrain the pathways that the evolution of the given species may or may not follow. Some biologists, called particularly by their opponents selectionists (functionalists, panselectionalists) are of the opinion that the only constraints that stand in the way of evolution follow from external constraints, i.e. from the laws of mathematics, physics and chemistry. If the existence of a certain structure is not excluded by the laws of the surrounding nonliving nature, then this structure (e.g. flying ears) must be formed in evolution through the effects of the relevant selection pressure. In other words, what is suitable and functional from an evolutionary standpoint and is not prohibited by natural laws will be formed sooner or later.
Other biologists, frequently designated as structuralists, on the other hand, think that certain mutations and thus certain structures can never be formed as, objectively, there exist certain internal constraints, barriers that evolution cannot overcome. In the extreme case, they state that the direction of macroevolutionary development is determined by just these evolutionary constraints, which decide which genetic changes will occur in the particular species. In macroevolution, they attribute a secondary and passive role to selection; according to them, it cannot form new evolutionary forms, but can only constrain the fixation of new forms that are unsuitable from the standpoint of survival of the organism or can hinder this.
According to structuralists, an important category of evolutionary constraints consists in the (evolutionarily) historical internal constraints, specifically ontogenetic constraints.These constraints determine which structures can or cannot be formed in the context of the existing ontogenesis. For example, if a certain pattern on butterfly wings is formed by the diffusion of a morphogen from a single place and if, after a certain time, individual dyes begin to be formed at places with certain concentrations, it is clear that only concentric patterns can be formed on the wings and not patterns of a different type (Beldade & Brakefield 2002). It is apparent that, if a certain structure cannot be formed in ontogenesis, it can also not become fixed during the course of evolution.