species concepts (this is long!)

[warrena at bcc.orst.edu]

Happy Thanksgiving!!

If anyone is interested in complete citations to the 
literature references in the text below, I would be 
happy to provide them.  They are all available in a 
recent (2000) book edited by Quentin D. Wheeler and 
Rudolph Meier: Species Concepts and Phylogenetic 
Theory, A Debate.  New York: Columbia Univ. Press. 
230pp.  I highly recommend this book to any taxonomist 
with strong feelings about their own species concepts.  
It forces you to analyze and be critical of your own 
beliefs on the subject; something I think we can all 
benefit from, especially if we criticize taxonomic 
decisions of other workers.    

There is certainly no concensus among systematists as 
to what species concept should be used. As James Kruse 
has correctly pointed out(in quotes below,)the vast 
majority of systematists outside of Lepidoptera do not 
recognize subspecies.  Subspecies are only used in a 
few animal groups (when nomenclature for the entire 
spectrum of life is considered).  The use of subspecies 
became popular in these animal groups (mostly birds, 
mammals, and insects) after Mayr (1957)proposed his 
Biological Species Concept (BSC); previous species 
concepts used back to the time of Plato are frequently 
referred to as Morphological or Typological Species 
concepts. Subspecies have become less and less popular 
since the works of Hennig (1966: Phylogenetic 
Systematics) and the subsequent rapid rise of 
cladistics and the use of cladograms to summarize 
relationships (as well as the rapid rise of competing 
species concepts).  Subspecies are essentially never 
used in most animal and plant groups. I know of no 
formal definition for a subspecies that has been 
proposed that may be applied in all cases for all or 
most organisms (please let me know if you know of 
one!).  Most taxonomists agree that species exist and 
can be identified, and most agree that species in 
separate groups of organisms should be equivalent 
taxonomic units.  This is not a universal reality for 
species (because of different species concepts), but it 
is not even possible for subspecies since they 
necessarily cannot be equivalent because there is no 
standard definition for them.  All workers that use 
subspecies have different working definitions of 
subspecies, and I have not seen any definitions clearly 
presented in a way that can be applied in the real 
world to all or at least most organisms (asexual 
organisms present a challenge to most species concepts; 
the BSC for example amdittedly cannot accomodate 
asexual organisms).  All of this means that in the 
business of taxonomy, subspecies are entirely 
subjective.  They may be useful in summarizing 
variation, but cannot be objectively applied. Workers 
who propose subspecific names should strive to reach a 
standard definition for a subspecies to remove some 
subjectivity from their decisions.  There is a whole 
body of literature that is literally missing here...  

The species problem is big enough!  Mayden (1997) 
identified at least 22 species concepts in the 
contemporary literature.  Wheeler and Meier (2000)edit 
a debate bewteen proponents of what are probably the 
six most widely applied species concepts: 

Biological Species Concept (by Mayr), 
"Biological species [are]groups of interbreeding 
natural populations that are reproductively isolated 
from other such groups."

The Hennigian Species Concept (by Meier and Willmann), 
"Species are reproductively isolated natural 
populations or groups of natural populations.  They 
originate via the dissolution of the stem species in a 
speciation event and cease to exist either through 
extinction or speciation."

The Phylogenetic Species Concept (sensu Mishler and 
Theriot) by Mishler and Theriot, 
"A species is the least inclusive taxon recognized in a 
formal phylogenetic classification.  As with all 
hierarchical levels of taxa in such a classification, 
organisms are grouped into species because of evidence 
of monophyly.  Taxa are ranked as species rather than 
at some higher level because they are the smallest 
monophyletic groups deemed worthy of formal 
recognition, because of the amount of support for their 
monophyly and / or because of their importance in 
biological processes operating on the lineage in 
question."

The Phylogenetic Species Concept (sensu Wheeler and 
Platnick), by Wheeler and Platnick, 
"Species are the smallest aggregation of (sexual) 
populations or (asexual) lineages diagnosable by a 
unique conbination of character states."  

and the Evolutionary Species Concept (by Wiley and 
Mayden),
"An evolutionary species is an entity composed of 
organisms that maintains its identity from other such 
entities through time and over space and that has its 
own independent evolutionary fate and historical 
tendencies."  

Other concepts discussed by various contributiors 
include the Recognition Species Concept (Patterson, 
1973 to 1986), the Cohesion Species Concept (Templeton, 
1989, 1994), and the Ecological Species Concept (Van 
Valen 1976).

There are huge differences between some of these 
species concepts, and in their application and 
consequences.  A strong proponent of any one species 
concept should be familiar with other competing 
concepts as well, to understand the strengths and 
weaknesses of them all so that their own beliefs can be 
better justified.  

Even though I believe in a certain species concept (it 
just happens to be one of those elaborated upon in the 
Wheeler and Meier (2000) volume), and personally think 
it is the most justifiable concept available, I respect 
taxonomic work presented by workers with other species 
concepts, and avoid "chuckling to myself" as others 
apparently do, when they see taxonomic combinations 
that are not consistent with their own species concepts 
[I only chuckle when someone claims that their taxonomy 
is "correct"].  If other workers can defend and justify 
their use of alternate species concepts, they have 
every right to use them.  Their taxonomy is no 
more "correct" or "incorrect" than any other taxonomy 
justified under another species concept (assuming all 
ICZN rules are met in all cases).  It seems appropriate 
to me that workers should have a defined, specific 
species concept that is applied in their research, and 
that the concept be consistently applied.  This enables 
workers to defend each and every taxonomic decision 
they make on the grounds of their species concept.  

The following are a few quotes (followed by my replies)
from the past week or two illustrating viewpoints 
expressed on Leps-l about what people think species and 
subspecies are.  This is not meant to be a personal 
attack on anyone, but is meant only to show the 
diversity of species concepts among out list members, 
and to clarify some statements (presented as factual) 
that have been made on this topic that cannot be 
justified.  

On Nov. 9, Ron Gatrelle wrote:
"Too many subspecies have been described just on the 
basis of how they look
alone.  Evolutionary and biogeographic factors are very 
important and often
neglected factors in taxonomic study.  I still laugh to 
myself when I see
some of the stuff the lumper lists have as subspecies --
 based totally on
how a couple populations look to the human eye....  
That is why I chuckle - their listings are totally 
based on
human vision and totally non scientific."

My reply:
So what is totally non scientific about using 
morphological characters to differentiate populations?  
Since there is no scientific definition of a subspecies 
(we only have ICZN rules dictating how to deal with 
them), there cannot be anything "non scientific" about 
how a worker treats them or what characters they use to 
differentiate them.  "Evolutionary" and biogeographic 
factors are not always known with certainty, and only 
add more ad hoc hypotheses to the to the theory of a 
subspecies; making the hypothesis of a subspecies less 
testable.  In the eyes of many scientific philosophers 
(as explained in works by Popper and Farris among 
others) the best scientific hypotheses are those that 
are the most testable, those with the fewest associated 
ad hoc hypotheses (this is to say, the most 
parsimonious exalanation is the best).

On Nov. 20, Ron Gatrelle Wrote:
"Phenotypic (how critters look to people)
subspeciation is the weakest and least significant type 
of subspecific
delimitation."

My reply:
Again, there are no rules, or even published 
suggestions as to which characters are more useful in 
determining subspecific relationships than others.  To 
give meaning to the above statement, a formal 
definition of a subspecies, along with information on 
what characters should be used in the taxonomic 
analysis must be presented.  

On Nov. 20, 2001, Jaska Kullberg wrote:
"In general I feel that frigga is a good species to 
represent a holarctic
species  as it lives abundantly and commonly in 
northern vet tundra
thorought Europe and N.Siberia." 

My reply: 
This type of statement is easily justifiable in the 
eyes of the Biological Species Concept, or the 
Ecological Species Concept, for example.  There is 
nothing "wrong" with this point of view, if those 
species concepts seem reasonable and are justified by 
their proponents. 

On Nov. 20, 2001, Kenelm Philip wrote:
"I might hesitate to split off an allopatric species 
based on such
a single character--but it would be interesting to know 
how the European
population (if consistent across Europe) grades into 
the Asian populations."

My reply:
This sounds like the BSC to me.  Proponents of any of 
the phylogenetic species concepts would have no problem 
splitting off an allopatric species if it had a unique 
and totally (as far as is known) universal character 
that can be used to identify it.  It wouldn't even 
necessarily matter what that character is...  There is 
nothing wrong with Ken's point of view, since it is 
easily justified under the BSC, at least.  

On Nov. 20, 2001, Norbert Kondla wrote:
"Species as biological and evolutionary entities
are not defined by the scale of visual differences to 
our human eyes -- they
are defined by a shared gene pool that has evolved 
separately from other
organisms and there may or may not be huge visual 
differences as seen
through our simple human eyes that only perceive a very 
small part of the
electromagnetic spectrum." 

My reply:
I am not sure which species concept these views are 
consistent with.  I ask how a researcher would know if 
the organisms they are studying are united by a shared 
gene pool that has evolved separately from other such 
gene pools without exhaustive DNA analyses?  This may 
be a good theory but it cannot be applied to the field 
without a huge grant and labor force.  In fact, some 
species concepts do define species on the basis of 
visual differences (to the human eye). 

On Nov. 20, 2001, Ron Gatrelle wrote:
"Now that mtDNA
studies have shown them to "be the same" (and if only 
knowing the dna and
visual factors) we would now say these were the same 
species.
There has to be a holistic understanding of each 
organism to reach the
"correct" conclusion.  Since so many have abandoned 
subspecies, it is no
wonder that they now seem to be lumping the species 
too."

My reply:
Two things here.  Just because researchers failed to 
find differences in a segment of mtDNA between 
eurymedon and rutulus does not mean that the study 
found the two taxa to be the same.  They looked at a 
small fragment on a single gene (one of well over a 
thousand genes).  Just because mtDNA segments in the 
two taxa are the same does not prove the species are 
the same.  I think I am agreeing with Ron here, but the 
DNA study did not conclude that the two taxa should be 
treated as one; it concluded that the two taxa are very 
closely related (I deal with interpretations such as 
this in my own research all the time).
There can be a "correct" taxonomic conclusion (not 
including ICZN-related issues here) only under the 
context of a single and specific species concept.  
However, to claim a "correct" taxonomic conclusion is 
to claim that the species concept it is based on is 
also correct.  The huge body of literature summarized 
in the Wheeler and Meier (2000) volume (which goes way 
beyond the species concepts they elaborate upon)clearly 
demonstrates that there is no one, single, correct 
species concept.  No concensus has been reached among 
systematists.  Therefore there can be no universally 
correct taxonomic decision (an no taxonomically correct 
listing, in the eyes of all species concepts).  Use of 
the word "correct" in this context can really only be 
justified when dealing with taxonomical issues 
regulated by the ICZN.  

On Nov. 20, 2001, Chris Durden wrote:
"A vote for the "Holistic Species Concept" I take to be 
equivalent to a vote 
for the "Ecological Species Concept"."

This "holistic" view does sound like the Ecological or 
the Evolutionary Species concept.  

On Nov. 20, James Kruse wrote:
"I think that in most cases subspecies probably 
represent clinal variation.
Individuals within a population are sometimes more 
variable than individuals
between populations in several accepted subspecies. 
Additionally the
"boundary lines" between subspecies often lay in 
uncollected territory. In
the rest of the cases that do not lend themselves to 
the above, I ask, if it
is so distinct why not name it a new species? Indeed, 
workers in butterflies
stand in stark contrast against the rest of systematic 
zoology on the point
of subspecies.
Because I think that subspecies are (generally) not 
very useful, since you
typically need to know where they are from to know what 
they are, does not
mean that I want to lump species. In fact, I wouldn't 
lump a subspecies
unless I had the kind of evidence that I think is 
necessary to make the
decision to raise it in the first place."

My reply:
Here we have a Phylogenetic species concept.  

The point of this posting is to to demonstrate that 
what makes a good species (or subspecies) depends on 
your point of view.  There is no right or wrong in the 
world of species concepts, and until there is, 
taxonomic lists will remain necessarily subjective and 
tied to specific species concepts.  

All the Best,

Andy Warren
PS: I would be interested in learning of any 
publication that formally defines subspecies and how to 
identify them in practice.  
 





 
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