 Philosophical discussion of systematics was a response to a “scientific revolution” in that discipline in the 1960s and 1970s. This revolution saw the discipline transformed from phenetics and evolutionary taxonomy alongside the application of quantitative methods. The successor was the “cladistic” approach due to Willi Hennig (1966), which argues that the sole aim of systematics should be to represent “natural” groups of organisms. Cladists took natural groups to be monophyletic groups (where a monophyletic group includes an ancestor species and all of its descendant species). Thus, a cladistic approach only represents phylogeny, which is the pattern of common descent. Philosopher David L. Hull was an active participant in scientific debates throughout these two revolutions; in fact, he was president of the Society of Systematic Zoology in 1984–1985 and of the Philosophy of Science Association in 1985–1986 (Hull 1965; Hull 1970; Hull 1988; Sober 1988). Additionally, a great deal of conceptual work appeared on the nature of phylogenetic inference. Phylogenies are commonly represented with phylogenetic trees. Discovering phylogenies presents the familiar problem of underdetermination of theory by evidence. For example, if we have four taxa, there are fifteen possible rooted trees. However, only one of those trees corresponds to the actual historical lineage. Additionally, the phylogeny of given taxa cannot be directly observed, and so must be inferred. Thus, the problem of phylogenetic inference is how to justify such historical hypotheses. The original phylogenetic method of choice was parsimony, which says to choose the tree which proposes the fewest evolutionary events. Some phylogenetic systematists argued for this approach using Sir Karl Popper’s falsificationism (Wiley 1975, Farris 1983). In the 1970’s, Joseph Felsenstein demonstrated that lineages with a certain topology were subject to a systematic error in parsimony analysis (Felsenstein 1978, 2004). This is the problem of “long-branch attraction” in which taxa at the end of long branches are mistakenly grouped together by parsimony analysis, instead of with taxa with which they share a more recent common ancestor (Haber 2008). Felsenstein proposed using statistical techniques (in particular maximum likelihood methods) to avoid this problem. Debates between the camps continue (Sober 2004). 