Species Concepts: what is the problem & why is it still here? P.J. Alexander, New Mexico State University

Introduction my focus has changed a bit since the abstract I’ll be talking about: the role of operationality in species controversies ideological disagreements lead us astray criticism of the phylogenetic species concept

Ludwig Wittgenstein

Goldstein & Desalle, 2000: “If, as most would argue, our species delimitations are to reflect reality of some kind in nature--reality that is either independent of our understanding or takes the form of a historical entity--then their discovery is not easily forwarded by either strictly operational discussions or by the generation of new vocabularies.”

Concepts vs. criteria stated most explicitly by de Queiroz, 1998 species concept: what species are (in some ± non-empirical/non-operational sense) species criterion: operational method for identifying species

Where de Queiroz went with it... disagreement is primarily in operational criteria agreement conceptually that species are lineages (1998)

Where de Queiroz went with it... “Operational” criterion from species as lineages: all previously suggested methods for identifying species sufficient, but none necessary separate lineages are species whether identifiable, divergent, reproductively isolated, etc., or not (2005)

Where de Queiroz went wrong... what kind of conceptual agreement is it to say that species are lineages? one bad term for another... does screening off operational disagreement help us? what kind of “operationality” does de Queiroz give us?

What are species concepts for? they focus research in speciation? (Wiens, 2004) they state common or necessary attributes of species? (Dobzhansky, 1935) they justify species criteria? (Nixon & Wheeler 1990) or are they too vague to do anything? (de Queiroz, 1998)

Since this is the WHS meeting... species concepts unrelated to criteria are not interesting in systematics BSC & PSC contain both conceptual and criterial aspects

BSC... concept: groups of actually or potentially interbreeding organisms criterion: direct breeding data generally unavailable or unusable (Darwin, Dobzhansky, Mayr, etc.) instead we have a “morphological yardstick” (Mayr, 1942)

Criticism of BSC poor ability of the morphological criterion to predict characteristics implied by the conceptual aspect ➤ “potential” interbreeding? poor applicability in asexual or hybridizing taxa

PSC... concept: largest groups of tokogenetically-related organisms terminals in phylogeny; identified prior to phyl. analysis criterion: direct data on tokogenetic relationships may be possible (e.g., microsatellites), but generally not used instead we have diagnosability (Nixon & Wheeler 1990)

PSC... continuum from tokogenetic to phylogenetic relations so what connects disjunct populations? potential tokogenetic relationships? from Christin Slaughter

PSC... asexual & hybridizing taxa? same problems as for BSC! there is no reticulate/divergent boundary in asexuals how much hybridization is too much?

Criticism of PSC poor ability of the morphological criterion to predict characteristics implied by the conceptual aspect ➤ “potential” tokogeny? poor applicability in asexual or hybridizing taxa BSC ⇒ PSC: a change in emphasis; the same problems! but I haven’t talked about phylogenetic terminals yet!

species = phylogenetic terminals? hybrids diagnosable groups of populations ➤ always appropriate terminals? from Flora of North America, 1993

species = phylogenetic terminals? priority do we need to identify terminals prior to phylogenetics? we can be misled by interpreting tokogeny as phylogeny but under what circumstances?

species = terminals? when are we misled? example using population aggregation analysis (PAA) Doyle, 1995 from Davis & Nixon, 1992

species = terminals? when are we misled? same data in a phylogeny; root added are we misled? the two diagnosable groups are still diagnosable groups (but not clades) there is no spurious resolution out A B

species = terminals? when are we misled? ah, but what if we had a dataset that gave a single fully-resolved tree? well, with this information we can’t do better out

species = terminals? when are we misled? if we add population information... ➤ it is perfectly obvious when we aren’t looking at divergent relationships between populations! if we can delimit species, we can also identify spurious resolution both require the same kind of grouping information out

species = terminals? when are we misled? identifying/diagnosing evidence of reticulation is important ➤ not insistence that terminals must be species, nor application of any particular species definition out

species monophyly? I’m not advocating a topological species criterion species need not be characterized by apomorphies in each species ➤ and thus need not appear as ‘monophyletic’ clades (Nixon & Wheeler, 1990) does this mean species can be paraphyletic? out A B

species monophyly? what does paraphyly mean at this level? relationships here are tokogenetic, right? yes, within species; but between ? out A B

species monophyly? two representations of the same pattern... there are several ways to view this out A B A B

species monophyly? species delimited by character difference only B diverged from an unchanged A; A is paraphyletic species delimited by inferred nodes A has the same characters as ancestral C; A is not paraphyletic A B A A B C

species monophyly? which is implied by the PSC? A B A A B C

Conclusions purely ideological disagreement is irrelevant; different approaches are important when they yield different results change from BSC to PSC is largely a change in focus ➤ conceptual & criterial aspects are largely unchanged ➤ problems faced are the same ➤ except that species are terminals under PSC insistence that species are the only appropriate terminals is primarily ideological insistence that “monophyly” and “paraphyly” are not applicable to species is primarily ideological instead, our focus should be on finding solutions to problems common across approaches, like hybridization & asexual taxa

Acknowledgements: NMSU Dept of Biology and NSF EF (CDB) for financial assistance; Dr. C.D. Bailey for discussion and many helpful comments.