Description forms the foundation for systematic biology. It provides the basis for taxonomic recognition, from which phylogeny, classification, and higher levels of evolutionary interpretation are ultimately constructed. For the purposes of taxonomic recognition, biologists have long distinguished between a general, broadly inclusive description, and one that is limited to characteristics that show how a taxon differs from others similar to it. This distinction is embodied in Linnaeus' concepts of the descriptio and the differentia specifica (Linnaeus 1758; see also Svenson 1945). The essential characters of a taxon are embodied in the modern concept of the diagnosis. Although the diagnosis has long been a component of systematic accounts in which new taxa are proposed or existing taxa revised, interpretation of the content and concept of a diagnosis varies widely among recent authors. Herein we argue that a diagnosis must be differential and that it must include, at least implicitly, comparison to similar taxa.
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