- The effect of evolutionary constraints is another mechanism that can cause an evolutionary trend. Evolutionary constraints can, to a substantial degree, determine the character of the variability that will appear in a certain species and thus also the character and potential direction of anagenetic changes that can occur in a particular phylogenetic line as a result of the action of natural or species selection. As the relevant ontogenetic mechanisms of the daughter species are inherited from the parent species, the relevant evolutionary constraints will also be inherited and the same trend will appear in all the species of a particular phylogenetic line. The existence or absence of evolutionary constraints is considered by some authors to be the main difference that permits mutual differentiation of two, at first glance similar, evolutionary phenomena, convergence and parallelism. In both cases, the originally phenotype dissimilar species of organisms become more similar through the action of similar selection pressures. If the anagenesis of both species occurred with the participation of the same evolutionary constraints, the evolutionary trajectory of this change was similar. This phenomenon is termed parallelism. Parallelism is manifested in the existence of the same evolutionary trend in both lines. If evolutionary constraints were not active in the formation of a similar phenotype, because, for example, unrelated organisms with different ontogenetic mechanisms were involved, anagenesis occurred in each line through a different evolutionary pathway. This phenomenon is termed convergence.
Especially in the past, evolutionary constraints were considered to be the main driving force for most evolutionary trends. The actual phenomenon of the tendency of the members of a certain phylogenetic line to change during evolution in a certain manner independent of the selection pressures acting on it from without and thus manifested as an evolutionary trend, is called orthogenesis. Some proponents of the orthogenetic concept of evolution assumed that the source of the relevant internal tendencies of an organism is nonmaterial in nature, for example that this could consist in a manifestation of the internal tendency of living creatures to gradually improve. Henry Fairfield Osborn (1857–1935) called the tendency of organisms to improve aristogenesis, while Pierre Teilhard de Chardin (1881–1955) spoke of moving towards the Omega point in the same context. These, in normal terminology, finalistic and, in some cases, directly theistic concepts were more likely to attract the attention of both humanitarian scientists and the general public. Consequently, at the present time, we have a tendency to automatically connect orthogenesis with some of these idealistically oriented concepts. However, in actual fact, most orthogenetic concepts were materialistically oriented and assumed that evolutionary trends arose through the action of normal physical or chemical processes occurring during orthogenesis In the 19th century, Galton already presented a very illustrative mechanical model of orthogenesis (Fig. XXVI.10). The model was based on comparison of the movement of a bead and an irregular polyhedron over a flat surface through the action of external forces. The bead moves strictly in dependence on the direction and intensity of the forces acting at the particular moment. The evolution of organisms through natural selection would occur in this manner if there were no evolutionary constraints. In contrast, for a polyhedron, the direction and rate of movement will be determined both by the direction and intensity of the external forces and also the shape of the body. The polyhedron will not react at all to the action of forces in a certain direction because, in order for it to roll in a certain direction, it must first lift up its centre of gravity, while it will react readily to forces acting in a different direction. A polyhedron is a model for an organism with evolutionary constraints. Such an organism reacts selectively to selection pressures acting in various directions. It need not react at all to a selection pressure acting in a certain direction, while it will react very readily to other selection pressures. In a certain direction, it can even evolve as a consequence of the action of random processes (mutagenesis, drift), i.e. in the absence of natural selection.
Classical Neodarwinism has a substantial tendency to doubt the importance of evolutionary (and other) constraints in evolutionary processes, i.e. also in the occurrence of evolutionary trends. However, a large portion of evolutionary biologists did not adopt this attitude for substantive reasons, but rather because of the general tendency of Neodarwinists to prefer evolutionary mechanisms including natural selection – i.e. a mechanism that biologists understand and are able to model, in contrast to the complicated and divergent processes of ontogenesis.