Species Concepts: what is the problem & why is it still here?

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

2. 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

3. Ludwig Wittgenstein

4. 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.”

5. 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

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

7. 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)

8. 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?

9. 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)

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

11. 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)

12. 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

13. 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)

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

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

16. 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!

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

18. 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?

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

20. o u t 1 - 1 1 - 2 1 - 3 1 - 4 1 - 5 2 - 1 2 - 2 2 - 3 2 - 4 2 - 5 • 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 A B

21. o u t 4 1 2 3 5 7 9 10 6 8 • 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

22. o u t 1 - 4 1 - 1 1 - 2 1 - 3 1 - 5 2 - 2 2 - 4 2 - 5 2 - 1 2 - 3 • 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

23. o u t 1 - 4 1 - 1 1 - 2 1 - 3 1 - 5 2 - 2 2 - 4 2 - 5 2 - 1 2 - 3 • 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

24. o u t 1 - 1 1 - 2 1 - 3 1 - 4 1 - 5 2 - 1 2 - 2 2 - 3 2 - 4 2 - 5 • 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? A B

25. o u t 1 - 1 1 - 2 1 - 3 1 - 4 1 - 5 2 - 1 2 - 2 2 - 3 2 - 4 2 - 5 • species monophyly? • what does paraphyly mean at this level? • relationships here are tokogenetic, • right? • yes, within species; • but between? A B

26. o u t 1 - 1 1 - 2 1 - 3 1 - 4 1 - 5 2 - 1 2 - 2 2 - 3 2 - 4 2 - 5 • species monophyly? • two representations of the same pattern... • there are several ways to view this A B A B

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

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

29. 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

30. Acknowledgements: NMSU Dept of Biology and NSF EF-0542228 (CDB) for financial assistance; Dr. C.D. Bailey for discussion and many helpful comments.