Concept of species structuralist
Essentialism can, in a certain sense, be considered to be a certain, rather extreme form of structuralism. The structuralist concept of species is based on the idea that the phenotype of organisms is determined primarily by almost deterministic processes following from the properties of their structural elements and also the character of the mechanisms governing ontogenesis. In contrast, the action of external factors is assigned a secondary role, either in the basically deterministic processes of selection or in random historical processes of the type of drift and speciation. According to structuralists, there is only a limited number of ways in which a functional organism can be formed from biomolecules through existing ontogenetic processes. According to them, each species is a specific manifestation of one of these means. Thus ontogenic constraints play a decisive role in determining the phenotype of an organism and, consequently, the evolution of a new species. The Vavilov model of homologous series (Vavilov 1967), one of the oldest complete structuralist concepts of the character of species, was proposed in the 1920’s. The Russian geneticist N.I. Vavilov noticed that a certain combination of phenotype traits occurs in various genera of plants, permitting the individual species to be distinguished. On the basis of study of the morphology of the members of a certain genus, he was thus capable of predicting the existence of so-far unknown species in a different genus and a great many of these predicted species were, in fact, discovered (Fig. XX.3).
It is probable that developmental limitations can be of substantial importance in the evolution of viruses. For example, in these cases, the morphology of the virus capsid is actually determined to a major degree by the laws of crystallography rather than by the character of selection pressures acting on viruses. In more complex organisms, developmental limitations can very substantially predetermine the evolution (and thus also the phenotype diversity) of the individual traits (for example the possible shapes of mollusc shells (Raup 1962; Raup 1966)). The structuralist model could also explain the diversity of the patterns on the wings of butterflies and on the body cover in general. Another area where the structuralist explanation could be important consists in the nonadaptive variability in the individual traits between species within a single genus. As an enormous number of traits contribute to the phenotype of more complicated organisms, the total number of possible combinations of these traits is infinitely large. Thus, it is not very probable that structuralist laws would determine which species would be formed in evolution and which not. This factor tends to rather have a limiting and defining role; of the enormous number of phenotypes that would correspond well to the requirements of the environment, it eliminates a certain percentage of those that, in actual fact, cannot be formed for internal reasons. However, I am of the opinion that this will tend to be a small percentage, but cannot, of course, be certain of this. Which of the species amongst the almost infinite number of other possibilities are actually formed tends to be determined rather by chance, selection and evolutionary drives.