The variety of perspectives in Species could be read as a demonstration of the need for a variety of species concepts. The one contributor that directly opposes this is philosopher David Hull, who starts off with a "take no prisoners" argument for realism and against plural species concepts, but then tails off weakly, pretty much admitting that some kind of pluralism is unavoidable. Kevin de Queiroz argues for a unified species concept, but one simplified down to "lineage segment", with the different species definitions that have been proposed then contingent properties of lineages. And John Dupré argues against the idea of a monistic species concept and for a pragmatic approach that recognises the needs of taxonomy (where he puts the case for "taxonomic conservatism").
Getting more involved with actual biology, David Nanney looks at the complications of taxonomy and phylogeny in ciliated protozoa (Tetrahymena). And Kim Sterelny argues for a view of species as ecological mosaics, as complex systems with emergent properties that are themselves a late evolutionary innovation.
Three of the papers rethink the idea of species as natural kinds. In the longest and most technical paper in the volume, Richard Boyd introduces the concept of homeostatic property cluster kinds and the notion of accommodation between conceptual classification and causal structure. He then applies these ideas to biological species, concluding that they are "natural kinds, defined by real essences" — at least once "kind" is saved from positivist hangovers. Robert Wilson uses this framework, and a comparative case study of neural cells, to argue for a realist view of natural kinds somewhere in between the "extremism" of the individuality thesis and species pluralism. In a similar vein, Griffiths argues that natural kinds can have historical essences and reconsiders the individuality thesis as presented by Ghiselin and Hull.
There are two papers in the area of cognitive psychology and folk biology. Scott Atran presents evidence that both Itzaj Maya and residents of Michigan have a preference for taxonomic inference at the level of the generic species: there is an innate tendency to essentialism (non-overlapping kinds) in folk-biology. And Frank Keil and Daniel Richardson argue that recognition of species is a special kind of categorisation.
Finally two papers propose the abolition of species entirely. Marc Ereshefsky argues that the Linnaean system and its rules of nomenclature need to be updated in the light of modern biology. And Brent Mishler argues for "getting rid of species" and replacing them with a general rank-free taxonomic system.