Characters and the Rationale Behind the New Classification

Abstract

Classifi cation of a group of organisms typically begins “at the bottom” with an examination of the variation in characters of species—the genus-species level. A variety of methods have been used to determine species of ciliates from the interbreeding criterion of the biological species to a variety of features related to the morphology and ecology of a species, including life history traits, behavior, and size and shape of a variety of structures revealed by observation of living or stained cells. Genetic approaches are becoming increasingly more popular, especially molecular genetic ones. These have included the use of isoenzymes, randomly amplifi ed polymorphic DNA (RAPD), and restriction fragment length polymorphisms (RFLP). The current cutting edge approaches are the sequencing of genes, such as small and large subunit rRNA, histone, actin, heat shock proteins, tubulins, and translation factors. Most recently, the mitochondrial cytochrome c oxidase 1 (cox1) gene has been chosen by some as a species “ barcode ”.

Above the genus-level, establishing groups is more problematic, but should always rely on the establishment of monophyly using synapomor-phies or shared-derived characters. These characters can be ultrastructural features of the somatic and oral kinetids, patterns of morphogenesis, and gene sequences. Taxonomy ultimately uses nomenclature and its rules to establish priority, ensure consistency, and maintain stability.