Evolutionary Grade - Grades in Systematics

Grades in Systematics

The traditional Linnaean way of defining taxa is through the use of anatomical traits. When the actual phylogenetic relationship is unknown, well defined groups sometimes turn out to be defined by traits that are primitive rather than derived. In Linnaean systematics, evolutionary grades are accepted in higher taxonomic ranks, though generally avoided at family level and below. In phylogenetic nomenclature evolutionary grades (or any other form of paraphyly) are not accepted.

Where information about phylogenetic relationships is available, organisms are preferentially grouped into clades. Where data is lacking, or groups of uncertain relationship are to be compared, the cladistic method is limited and grade provides a useful tool for comparing organisms. This is particularly common in palaeontology, where fossils are often fragmentary and difficult to interpret. Thus, traditional palaeontological works are often using evolutionary grades as formal or informal taxa, including examples such as Labyrinthodonts, Anapsids, Synapsids, Dinosaurs, Ammonites, Eurypterid, Lobopodes and many of the more well known taxa of human evolution. Organizing organisms into grades rather than strict clades can also be very useful to understand the evolutionary sequence behind major diversification of both animals and plants.

Evolutionary grades, being united by gross morphological traits, are often eminently recognizable in the field. While taxonomy seeks to eliminate paraphyletic taxa, such grades are sometimes kept as formal or informal groups on the basis of their usefulness for laymen and field researchers. In bacteriology, the renaming of species or groups that turn out to be evolutionary grades is kept to a minimum to avoid misunderstanding, which in the case of pathogens could have fatal consequences. When referring to a group of organisms, the term "grade" is usually enclosed in quotation marks to denote its status as a paraphyletic term.