In the previous chapters, we have evaluated evidence for equilibrium and nonequilibrium conditions, and for the significance of interspecific competition, using ecological and macroecological/biogeographical, as well as some experimental studies. Here we look at two closely related species using everything that is known about them, including phylogeny based on DNA and morphology (including ultrastructure), life cycles, morphology and ecological evidence, in order to arrive at an understanding of whether and to what degree competition can be implicated in the evolution of the species' characteristics, i.e., we use an autecological approach.

Hengeveld and Walter in a number of papers (e.g., 1999), advocated a radically new approach to ecology (see pp. 10–11). They distinguish two coexisting but mutually exclusive paradigms, the demographic and the autecological paradigm. Autecology, the study of the ecology of a single species, its phylogenetic history and adaptations, is an old concept, new is the radical view that these two approaches are incompatible. According to Hengeveld and Walter, the demographic approach is based on unrealistic premises and cannot answer many of the questions posed by the way species are distributed and how common they are.

The examples selected are two species of the Aspidogastrea. I chose this group for various reasons. Firstly, I am very familiar with it, having done a lot of work on its phylogeny based on molecular, morphological, and ultrastructural data, as well as on the taxonomy, development, and ecology of several species.