[Animals (Animalia) in system of organisms. 2. Phylogenetic understanding of animals]

Abstract

The development of systematics in last decade has shown that typological classifications of five-six Kingdoms is not adequate for describing the diversity of organisms. Information from the sequences of small subunit rRNA is not sufficient to reconstruct the position of eukaryotes on the phylogenetic tree due to the effect of long branches. Totally new reconstruction of eukaryotic phylogeny was built upon the analysis of many new molecular markers. Evolution of eukaryotes had two mainstreams. One has been connected with diversification of ancestral biciliate forms (Bikonta). Sister-group of Bikonta (Unikonta) includes some originally uniciliate amoebae and moulds (Amoebozoa), and uniciliate eukaryotes with posterior cilium (Opisthokonta). The taxon Opisthokonta unites Fungi, Nuclearimorpha, Mesomycetozoa, Choanozoa and Metazoa. The latter three groups or only Metazoa are attributes to animals. The following differentiation of the groups used in systematic for the description of diversity of organisms is proposed. (1) Taxon is a group which is defined on the basis of ancestry: taxon includes all species descended from one ancestor. Taxon differs from logic classes of typology at an ontologic level. Taxon arises and exists, and its composition and occupied niches can constantly change; taxon can flourish or, on the contrary, fade up to full disappearance. Thus, the predicative characteristic of taxon, including characters which are considered significant, are not absolute. It is significant only at the moment of consideration. But characters (synapomorphies) are important as the practical tool for discerning taxa at given time period. Taxa unite species into unique classification. This understanding of taxon corresponds to monophyletic group sensu Willi Hennig. (2) Class of organisms is a group which is defined on the basis of characters: class includes all species having the given character. The class is only a logic object. Unlike taxa grouping species into classes may be through different and crossed classifications. Inside the given category of groups it is possible to distinguish: (2.1) Level of the organization (grade) described by the differences on the levels of organization: for example prokaryotic and eukaryotic levels of the organization. Eukaryotes can be divided into unicellular (Protoctista, Protista) and multicelluar (tissue-specific-Histonia) forms. (2.2) Types of the organization distinguishing groups of one level: for example, amoedoid type (Sarcodina), naked (Gymnamoebia), and testate (Testacea) amoebas. (2.3) Taxonomic groups as set-theoretical approximations of taxa. (2.4) Groups of the mixed nature. For example, Haeckel has recognized Protophyta and Protozoa describing the unicellular level of the organization inside plants and animals accordingly. Protozoa in Cavalier-Smith's system (2002, 2004) is also an example of groups of the mixed nature.