Return of the grades: towards objectivity in evolutionary classification.

Evolutionary classification, i.e. a biological classification that recognizes paraphyletic groups as formal taxa, is often regarded as highly subjective and therefore unscientific. We argue that clades with evolutionary key innovations are real biological units and that, as a logical consequence, paraphyletic grades are equally real; if a clade with evolutionary key innovations is nested within an older clade, the remainder of the more inclusive clade forms a paraphyletic grade. Therefore, we regard an evolutionary classification, which recognizes grades and gives formal names to them, as a desirable supplement to the purely phylogenetic classifications, which are dominant today. To increase the objectivity of evolutionary classifications, an approach called “patrocladistic classification” was proposed. We adopted this approach using the approximate number of apomorphies separating two clades along the phylogenetic tree as the patristic distance. Based on a cluster analysis of all angiosperm families, we outline an evolutionary classification of the angiosperms, which includes three subclasses (one of them paraphyletic), 12 superorders (four of them paraphyletic) and ~ 74 orders (12 of them paraphyletic).We suggest that well characterized monophyletic taxa can be reproduced by both phylogenetic and evolutionary approaches and used as a cladistic backbone of any classification. For the remaining groups, we advocate a peaceful coexistence of phylogenetic and evolutionary classifications, admitting both narrowly defined clades and broadly defined paraphyletic grades as valid taxa.