 However, at this time, cladistics, which appeared in the 1950s, was starting to change scientists' approach to biological classification. Unlike previous methods, cladistics attempts to consider all the characteristics of an organism, rather than those subjectively chosen as most important. As a result, it gives less significance to unique or bizarre characteristics than to those that are shared, since only the latter can demonstrate relationships. Cladistics also emphasises the concept of a monophyletic group, in other words one that consists only of a common ancestor and all its descendants – for example it regards the traditional term "reptile" as useless, since mammals and birds are descendants of different groups of "reptiles", but are not considered "reptiles". The concepts of crown groups and stem groups, first presented in English in 1979, are consequences of this approach. A crown group is a monophyletic group of living organisms, and a stem group is a non-monophyletic set of organisms that do not have all the shared features of the crown group but have enough to distinguish them clearly from close relatives of other crown groups – in very simple terms, they are "evolutionary aunts" of the organisms in the crown group. Phyla are crown groups, and the fact that some of their characteristics are considered defining features is simply a consequence of the fact that their ancestors survived while closely related lineages became extinct.
