Phylogeny

PHYLOGENY. - Within the framework of the theory of evolution, it denotes the historical order according to which the various groups of living beings would have gradually changed and evolved. All animal and plant species, present and extinct, could be gathered into an enormous genealogical or phylogenetic tree, in which common trunks, corresponding to more ancient ancestors, repeatedly divide into secondary branches, which indicate the more recent forms.

The construction of a phylogenetic tree is based on documents deduced from the most diverse disciplines (paleontological, anatomical, embryological, biogeographical, physiological, genetic data), and thus represents a complete synthesis of our zoological and botanical knowledge. It is, however, above all paleontological research (Gaudry) that makes it possible to recognize the succession of variations in related species belonging to successive geological strata and hence to different eras; one can therefore establish, in a quite objective manner, the probable evolutionary filiations of species in chronological series, and recognize the extent of progressive adaptations.

Comparative anatomy (Gegenbaur) and embryological studies also help to clarify phylogenetic affinities. According to a conception of Fritz Müller and Ernst Haeckel (fundamental biogenetic law), the development of the individual (ontogeny) would represent a brief recapitulation of phylogeny, and therefore from the examination of successive embryonic stages one could trace back to the evolutionary history of the species. For example, the fact that all higher metazoans pass early through the embryonic stage of the gastrula would indicate their descent from the poriferans and coelenterates, which in the adult age present an overall appearance similar to that of this developmental stage (gastraea theory, Haeckel); and the fact that in the embryo of higher vertebrates, with pulmonary respiration, the branchial apparatus is sketched out, destined to remain without function, would indicate their descent from lower vertebrates, in which this apparatus is functional.

A succession of species derived from the same original stock by direct filiation is called a phylum. It has often been observed (Bergson, Rosa, Cuénot) how every major phylum tends to divide one or more times by dichotomy. Thus in the animal kingdom a primitive trunk, comprising the coelenterates, divides into the two great branches of the protostomes and the deuterostomes (Grobben). From the main trunks of the genealogical tree of living beings, however, the various systematic groups with more pronounced differentiation generally detach as lateral branches, which exhaust or attenuate their further possibilities of evolution, specializing to a very high degree. Cases have also been brought to light of a phylum that continues its own evolution in a given direction, with apparent independence from any tendencies of progressive adaptation (Eimer's orthogenesis), so that the characters involved in this gradual transformation can sometimes even exceed the favorable dimensions and become manifestly harmful and exaggerated devices (phenomena of hypertrophy), which preludes the extinction of the species. Different phyla, born from the same stock but differentiated from one another in a divergent sense, can evolve successively in the form of parallel lines, in each of which common characteristics can then appear with very remarkable regularity (morphological parallelisms of Colosi; homomorphies of Novikoff). In other phyla, unrelated to one another but secondarily adapted to the same environment, a strong morphological resemblance can be reached, through a phenomenon of adaptation and convergent evolution.

In the current state of our knowledge, the phylogeny of both the major and the minor groups of living beings remains still, for the most part, at the stage of an attempt at morphological synthesis. And the controversy between the supporters of a monophyletic or polyphyletic nature of evolution is not yet resolved: the former admit that all living beings derive from a single ancestral stock, the latter from several stocks. Modern systematics, however, is built on a phylogenetic basis, and would aspire to group animal or plant species according to evolutionary kinship, and to establish a hierarchy and a succession of the various groups thus obtained, reflecting the historical order of their genealogical derivation should experience prove the theory of evolution true. Thus one would tend toward a rational "natural system," to be contrasted with the arbitrary "artificial systems" characteristic of the period preceding the theory of evolution (v.).

Enrico Vannini