More on paraphyletic species

Harvey E. Ballard, Jr. hballard at STUDENTS.WISC.EDU
Mon Jun 16 17:28:08 CDT 1997


I wrote previously:
>>My problem with accepting only monophyletic species involves three
>>points. (1)
>A >number of papers have recently suggested that an enormous number of
>>paraphyl
>etic species probably exist, in keeping with evolutionary hypotheses of
>>certain
> speciation models.

Tom DiBenedetto wrote:
>In systematics, a species is a taxon; it is something we identify and
>name according
>to our standards. To say that an enormous number of paraphyletic
>species exists is merely to say that we have decided to identify and
>name a number of groups of organisms which are paraphyletic, but meet
>some criterea (your personal crtierea) for being considered a
>species.
>Yours are not the only criterea. The problem, which I have tried to
>point out before, is the use of a biological species concept in a
>historical framework. In systematics, "species" is a rank applied to
>a taxon, and a taxon is a group of organisms identified through an
>analysis of their characters. A biological species concept uses very
>different criterea for recognition, and is not conceptually embedded
>in a historical context (a hierarchy of branching lineages). That a
>species recognized under a biological concept is artificial when
>viewed in a historical context is not surprisisng. But it is not a
>characteristic of nature we are discussing here, but a confusion of
>different menings of the same word.

In my study of the Viola canadensis complex, and in the published
literature that I was referring to that included multiple populations of
widespread species and geographically restricted species, populations of
the widespread species were phenotypically indistinguishable.  Assignment
of these populations to the same species made no reference to a biological
species concept.  In the Viola canadensis complex, the other species (the
narrow endemics) differ from V. canadensis by a number of floral and
vegetative features as well as a distinct preference for rock as opposed to
soil substrate.  So the problem here is not one of a biological species
concept in conflict with a historical context but one of phenotypically
"the same" set of populations that are genetically divergent
and--considering the embedded position of the endemics--paraphyletic.  We
talk of diagnosability of species and imply (although we might not avail
ourselves of) a genetic basis--hopefully an evolutionary--basis underlying
such differentiation that permits us to assign sets of organisms (e.g.,
individuals or populations) to one or another species.  But monophyly (or
paraphyly) in phylogenetic terms with certain forms of data, such as
molecular data, doesn't necessarily equate to the same condition in the
observable phenotype of populations within each of the two or more related
sets or organisms some of us are willing to call species.  Interpreting
paraphyly with, say, molecular data where monophyly is indicated by
morphological and ecological data is still inferring the relationships of
the populations of all species in a "conceptual historical context", in a
cladistic sense of branching lineages.  However, in the cladogram depicting
branching lineages based on molecular divergence, morphologically divergent
sets of populations originate from WITHIN a clade formed by morphologically
and ecologically uniform populations of another, widespread diagnosable
taxon.  I can't justify, any better than anyone else might, subdividing
phenotypically indistinguishable sets of populations to make them
monophyletic units to be called species, to "relieve" Viola canadensis of
paraphyly.  Your argument presupposes a phylogenetically "even" split in
which everything works out according to theoretical niceties and two
monophyletic lineages (based on all existing data?) result.

I accept that one can diagnose any set of organisms to be a particular
taxon, at some level, depending on one's criteria, although I still opt for
an "evolutionary" species concept because it matches the patterns of
variation one finds in nature with the plants I work with--and with most
others I'm familiar with.  But I'm talking about a more involved issue:
when populations appear to belong to precisely the same taxon
phenotypically, yet when analyzed cladistically using other types of data
(molecular data) one arrives at a paraphyletic arrangement of some
populations of the SAME taxon within which are embedded populations of
other phenotypically divergent taxa (each forming a monophyletic group of
populations that are phenotypically indistinguishable among themselves).
Two sources of data "collide" but provide potential illumination when
considered in an evolutionary context.  I confess that I don't see where I
am neglecting the "historical" context.  On the contrary, the historical
(or at least the geological and phylogenetic) context suggests derivation
of the monophyletic sets of populations from different marginal populations
of the more widespread and phenotypically diagnosable species.  Which set
of data is right regarding monophyly--or don't I have to choose?  I suggest
that it's a larger issue than just choosing monophyly over paraphyly, or
dumping a biological species concept or another that is not strictly
cladistic in interpretation.

I wrote:
>(2) We biologists worldwide, and individually, should be exerting
>much MORE effort to catalog and characterizing biodiversity from an
>alpha taxonomic standpoint before it slips from our grasps in the
>next few decades, and to leave taxa UNNAMED means to leave them
>uncharacterized--a suggestion that I find unconscionable and cowardly
>in the face of the biodiversity crisis assailing us.

Tom wrote:
>I dont think this is a strong argument. One can certainly name any
>specimen one comes across, if one can distinguish diagnsotic
>characters for it. If not, then what? Or if the "species" proves,
>under histroical analysis to be undiagnosable, then its name is
>rendered in quotation marks, or otherwise designated as undiagnosable
>within a larger context. I dont see how this need have any adverse
>effect on conservation-type isssues. Unless you mean we should
>pretend to know more than we do...

I referred to previously published suggestions of leaving non-monophyletic
(in this case paraphyletic) taxa such as species unrecognized.  Again, this
particular situation would presumably arise by accumulation of data other
than phenotypic that showed a paraphyletic assemblage of populations.  I
admit that I'm also against leaving "orphan" species--those whose
phylogenetic position is not fully resolved--to rest unassigned, without a
"home".  Why not take a conservative approach and leave them where they
were originally placed until further data clarifies their position (or
doesn't, as the case may be).

I wrote:
>(3) Considering that organic evolution proceeds in a
>"non-monophyletic" fashion a goodly share of the time, systems of
>classification that refuse to recognize paraphyletic species reject a
>substantial proportion of the products of evolution and, in essence,
>create an "unnatural" classification system.

Tom wrote:
>How can you say this? A paraphyletic group is a group which YOU
>choose to recognize, because of your own criterea, and which consists
>of an ancestor and some, but not all of its descendants. How can you
>claim that nature produces such groups?

Again, I concur that at SOME level I'm making the judgment about which
populations belong to which taxon (on the other hand, if they're identical
in hand and very different from everything else you've got, that rather
reduces the problem to simple terms).  If we dispense with the word or
taxon assignment "species", I suppose that we might consider accepting
every individual organism as a distinct taxon in its own right, which we
could call "species", but that doesn't get us very far.  In this instance,
a group of phenotypically indistinguishable units is rendered paraphyletic
by other groups of indistinguishable units.  We could make up names for
each of the sets of indistinguishable units, or turn to microcharacters to
try to resolve differences.  But assuming that we find no other phenotypic
features that allow us to reassign the paraphyletic set to a number of
different monophyletic assemblages such that every set is monophyletic, how
do we deal with the situation?  All the evidence suggests a particular
evolutionary event: speciation around the periphery of a widespread taxon
represented by a series of populations across its range.  Cladistically
this creates a discomforting pattern that isn't rectifiable on a molecular
phylogenetic level with phenotype.  What now?  My argument is that this
performance receives a frequent encore throughout geologic history.  How
will a usable PHENOTYPIC nomenclature be developed to accomodate this as
well as "clean" and balanced phylogenetic splits that result in two sets of
populations that are essentially indistinguishable amongst themselves and
divergent phenotypically from other related populations.

Harvey Ballard


********
Harvey E. Ballard, Jr., Ph.D.
Postdoctoral Researcher, USDA, ARS
Horticulture Department, University of Wisconsin
1575 Linden Drive, Madison WI 53706
phone: (608) 262-0159
fax: (608) 262-4743
and
Honorary Fellow, University of Wisconsin Herbarium
Botany Department, University of Wisconsin
132 Birge, 430 Lincoln Drive, Madison WI 53706

AFTER 31 JULY:
Assistant Professor
Department of Environmental and Plant Biology
Porter Hall
Ohio University
Athens OH 45701
Phone: (614) 593-4659
Fax: (614) 593-1130




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