Abstract

The relationships between cladistics and evolutionary theory are complex since cladistics' very beginning. This complexity is exemplified by cladistics' most extreme positions: first, Hennig's (1950, 1966) process cladistics, which is based on a rich set of evolutionary hypotheses, notably on speciation processes; second, Nelson and Platnick's (1981) pattern cladistics, for whom "evolution is an explanatory theory for systematic patterns that are observed on independent theoretical premises" (Brower 2000: 145), which prevents us from basing cladistics on any kind of evolutionary claim. Most cladists find their way between the two, depending on their own aims, methods and scientific background. In order to criticize one or the other position, one must first know what s/he is talking about. While the debate between process and pattern cladistics is very lively, it is almost never based on an accurate description of what kind of evolutionary hypotheses are effectively used in cladistics, at which step of the investigation, and for which purpose. Now, a quick review of cladistic literature since the 50s shows that the ways cladists refer to evolutionary hypotheses are as diverse as they are numerous. My aim is to provide a typology of the roles that evolutionary theory can play in cladistics, based on an analysis of which roles systematists had it actually play in cladistics' history. While evaluating the relevance of each of these roles is my ultimate goal, this particular talk is only a first step, and hence only descriptive. I shall describe and discuss three possible roles evolutionary theory can play in cladistics: First, evolutionary theory can be a founding theory for cladistics. In this case, evolutionary biology provides hypotheses which allow to found and therefore to justify the main principles of the cladistic method. Second, evolutionary theory can be an auxiliary theory for cladistics. In that case, evolutionary biology provides a certain amount of evolutionary hypotheses which can help formulate such or such assumption about cladistic relationships, in a more or less explicit and crucial way. Third, evolutionary theory can be a background theory for cladistics, i.e. a theory which is not directly used by it, but which nonetheless justifies its general aim and therefore its use. There are at least three ways in which evolutionary theory can be said to be a background theory for cladistics, which I shall detail