Defining and redefining monophyly: Haeckel, Hennig, Ashlock, Nelson and the proliferation of definitions

Abstract

The various existing definitions of monophyly have resulted in confusion within the systematics community. The divergence in terminology started with the work of Willi Hennig who attempted to introduce a precise definition of phylogenetic relationship in 1950, a term that he had synonymised with monophyly by 1953, thereby creating a new definition. In 1965, Hennig introduced paraphyly to distinguish his version of monophyly from groups based on symplesiomorphies or stem groups. In attempting to resolve the confusion, Ashlock synonymised Hennig’s monophyly as holophyly, resulting in another new term. Ashlock, Mayr and others defended Haeckel’s original use of monophyly, by including holophyly and paraphyly. The result was an unresolved 21-year debate on monophyly and its various uses. A review of the history of monophyly and the origins of its various definitions has resulted in two new terms to distinguish the different versions of monophyly currently in use: diamonophyly, which group definitions based a notion of ancestor-descendant relationships, and synmonophyly, which groups definition based on kinship relationships. The terms ‘reciprocal monophyly’ and ‘oligophyly’ are discussed as being diamonophyletic.