 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 . <unk> 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 . 
