Phylogenesis, ontogenesis and evolution

Abstract

Haeckel distinguished between ≪Phylogenie≫ and ≪Ontogenie≫, as opposed to ≪Phylogenese≫ and ≪Ontogenese≫. The former concepts are supposed to deal with the history of phyletic and biontic (i.e. individual) development, respectively, while the latter are concerned with the mechanisms of these processes. Haeckel's terminological distinction can no longer be upheld, ≪phylogeny≫ and ≪phylogenesis≫ are currently used as synonyms, and the same holds for the other pair of words.

If we accept these words to represent the historical aspects, then phylogenesis may be equated with the phylogenetic classification of plants and animals. It is suggested that correct phylogenetic classifications can be obtained only by following the theory of Hennig. According to a personal interpretation of the latter the phylogenetic hierarchy is a dichotomous dendrogram, where the vertical lines represent taxa which pass from the superior taxa in the apex to the species at the base. To pass from the phylum to a species one must thus pass through a succession of subordinate taxa.

The basic notion in von Baer's laws may be interpreted to imply that during development the embryo passes through a succession of stages which corresponds to the subordination of the taxa to which it belongs. In other words, there is a correspondance between phylogenesis and ontogenesis, to the extent that it is warranted to claim that phylogenesis is recapitulated during ontogenesis, even if the recapitulation is different from that envisaged by Haeckel.

Haeckel's concept of ≪Ontogenese≫ is currently covered by the terms ≪developmental biology≫ or ≪epigenesis≫. It is impossible to find a term which corresponds to Haeckel's ≪Phylogenese≫, which ought to represent the mechanism of evolution. Rather, the mechanism of evolution comprises two aspects, the creation of new forms of life and their survival, and we therefore need two theories on the mechanism of evolution, the epigenetic theory accounting for the first of these aspects, and the ecological theory for the second.

The epigenetic theory is briefly discussed, and it is shown that even in this case is von Baer's laws borne out. The ecological theory is based upon the axiom of competitive exclusion, from which it is inferred that the survival of living organisms may be ensured by isolation, that is, by avoiding competition. The outcome of isolation is divergent evolution. If two taxa engage in competition, the dominant taxon will prevail, and the outcome will be progressive evolution.

It is shown that if the phylogenetic hierarchy is constructed in a particular fashion, these predictions of the ecological theory are corroborated.

Key words:

• Phylogenesis
• Autogenesis
• Epigenesis
• Evolution
• Baer's law