 One assumption that species pluralists make is that there are three (perhaps more) types of lineages that biologists call ‘species’. Hence when biologists refer with the word ‘species’ they refer to different types of lineages. As we saw earlier, three types of species lineages are brought to the fore by three different species concepts: (1) The Biological Species Concept and related concepts highlight those lineages bound by the process of interbreeding. (2) The Phylogenetic Species Concepts highlight those lineages of organisms that share a common and unique ancestor. (3) The Ecological Species Concepts highlight lineages of organisms that are exposed to common sets of stabilizing selection. According to species pluralism, the Tree of Life is segmented by different processes into different types of species lineages. Barker, however, questions the pluralist assumption that there are three distinct types of lineages that fall under the heading ‘species’. Barker argues that pluralists have not sufficiently established the plausibility of that assumption. For example, he argues that while pluralists assume that interbreeding and ecological species are different types of lineages, many interbreeding species and ecological species lack explanatorily or predictively significant differences. So, the pluralist’s assumption that there are distinct types of species lineages is problematic. 