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 reflectreality of some kind in nature--reality that is either independentof our understanding or takes the form of a historical entity--thentheir discovery is not easily forwarded by either strictly operationaldiscussions 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 identifyingspecies 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 interestingin 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

tokogeneticrelationships?

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

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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

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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

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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

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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?

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B

species monophyly?• what does paraphyly mean at this level?

• relationships here are tokogenetic, right?

• yes, within species; but between?

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species monophyly?• two representations of the same pattern...

• there are several ways to view this

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A B

species monophyly?

• species delimited by character difference only

• B diverged from an unchangedA; 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 terminalsis primarily ideological

• insistence that “monophyly” and “paraphyly” are notapplicable 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-0542228 (CDB) for financial assistance; Dr. C.D. Bailey for discussion and many helpful comments.