 Biologists offer many different species concepts, which disagree on how species are individuated; indeed the different species concepts will disagree about the extensions of species and about the number of species. For example, according to the interbreeding species concept (e.g. Mayr 1969), species are groups of interbreeding natural populations, that are reproductively isolated from other groups. Alternatively, according to the phylogenetic approach to species (e.g. Cracraft 1983), species are classified according to common ancestral descent. These two approaches carve species differently. Some phylogenetic species fail to be interbreeding species. For example, organisms that reproduce asexually may nevertheless have a common descent. They will be grouped together as species by the phylogenetic approach, but not by the interbreeding approach (see Ereshefsky 1998 for discussion). Philosophers question whether the multiple divisions used by biologists reflect an ontological pluralism in the world or, alternatively, whether there is a privileged conception of species that captures ontological reality. Furthermore, some philosophers argue that species are not to be considered as natural kinds at all, insisting instead that species are individuals.