Taxonomists increasingly use the phylogenetic system called cladistics, devised in the 1950S by the German entomologist Willi Hennig. Cladistics defines taxa according to the recent common ancestors of species. If a species has a detailed fossil history its cladistics history – a kind of family tree – can be precisely established. Without the benefit of fossil, taxonomists must employ more indirect means, such as the concept of primitive and derived traits (see box). The traits of a species alter through time by a sequence of mutational changes and selection. Traits from an early phase are called primitive, those from a later phase are derived.
The central principle of Hennig’s system is that “shared derived traits show common ancestry while shared primitive traits do not”, No actual classifications is based on a sigle shared derived trait. In practice, reliable classifications are produced when a range of comparisons, such as outgroup comparison (see diagram), point to the same conclusion.