1.2 Taxa and relationships
Until the mid-20th century, inferences about evolutionary relationships between species were generally based upon as wide a range of evidence as could be mustered. Evolutionary systematics is the name given to this eclectic approach, because of its explicit focus on evolutionary conclusions. The disparate nature of the evidence used (ranging from the taxonomic attributes and geographical distribution of living organisms to the stratigraphical distribution of fossils) meant that there was no single underlying method of analysis, and so the conclusions were reached by a variety of lines of reasoning. Consequently, the discipline became notoriously the domain of widely experienced experts, who tended to acquire an unfortunate reputation in the popular imagination as a sort of unassailable priesthood. Frustration with the lack of a consistent method of analysis, and hence with the ultimately subjective nature of evolutionary systematics, led, in the 1950s, to the development of two new approaches to systematics both of which claimed to be more objective: in phenetics, species are clustered according to their overall morphometric similarities; in cladistics, relationships are inferred from the extent to which different species share evolutionarily modified features apparently derived from common ancestors. Yet neither new approach proved to be without its problems, and so all three continue to be practised today. The methods and the advantages and disadvantages of each approach will be discussed in the following sections. Nevertheless, cladistics has emerged in recent years as the most powerful and widely used method of phylogenetic analysis in most instances, and so most emphasis will be given here to this approach.
Before considering the different approaches to phylogenetic analysis, some general points need to be made concerning evolutionary relationships.