Subject: Subspecies or species? [long]

In a recent post Shyamal wrote:

> I am not well read in the topics of taxonomy, cladistics, phenetics etc
> but it appears that I have a lot more faith in the good-old-(Ernst)
> Mayrian taxonomy. I dont believe that they could have gone that badly
> wrong on the classification of the bushlarks. There could certainly be
> a large amount of variation. I imagine that any species with a wide
> distribution would show local differences due to genetic drifts and
> isolation , and sometimes the differences would be visible(as perhaps in
> the case cited by Ramana). Maybe if one looked hard at the right
> characters we could always say where a particular population came from
> just as can be said of Common Langurs (and their tails) or the many
> different color forms of Malabar Giant Squirrels. What I would like to
> know is where are lines drawn in subpecies nomenclatures.

I must confess that I do not share Shyamal's admiration for Ernst Mayr's
brand of systematics, nor do I think the biological/polytypic species
concept and the relegation of clearly diagnosable (identifiable) forms to
subspecies status useful. The goals of systematics are (a) the discovery
and description of biological diversity and (b) the search for patterns
of relationship among the unit entitities (species) of this diversity.
Polytypic species are not unit entities but are themselves composed of
two or more subspecies. A very considerable number of birds (for example)
currently regarded as 'subspecies' are actually distinct forms which are
clearly diagnosable from similar forms. To regard them as subspecies of
polytypic species obscures real biological diversity. Since polytypic
species are composed of subspecies which have been grouped together on
non-phylogenetic criteria (reproductive compatability, intergradation and
geographic representation), they are often unnatural (non-monophyletic)
and therefore cannot function as terminal taxa in phylogeny analysis.

A couple of years ago during a discussion on the same subject on another
list I wrote the following:
---------------------------


Perhaps no single issue in systematic ornithology is as controversial as
the concept of species. To most systematists species are specific kinds of
animals or plants, "[t]he smallest detected samples of self-perpetuating
organisms that have unique sets of characters". In the 19th century, due
to a tendency among some European ornithologists to give scientific names
to almost every minor variation of a bird, the concept of subspecies was
found useful since it enabled the absorption of a multitude of named minor
variations without threatening near-chaos in the names of even common
species. Later, especially at the turn of the century and in the early
20th century, the subspecies concept began to gain a wider meaning. If two
or more clearly distinct and separable forms appeared to intergrade among
each other in such a way that intermediates could be found this was 
considered sufficient reason for placing all the individual forms as
subspecies within one species: intergradation became the cornerstone of
subspecies. Soon some ornithologists (e.g., Ernst Hartert) went even
beyond this idea of species: they began to give it a purely geographical
meaning. Related forms which were clearly distinct morphologically but
which occupied non-overlapping ranges (allopatric) were regarded as
geographical representatives of one another and therefore grouped
together as subspecies of a single species. Hand in hand with these views
of species went the old idea of reproductive compatibility. As a result,
there has been, in effect, a steady retrogression in species-level bird
taxonomy: the great strides made by 18th and 19th century ornithologists
in documenting the world's bird diversity were reversed, as more and more
clearly distinct forms were lumped together to conform to the theory-laden
species concept of certain misguided systematists (e.g., Ernst Mayr).

The so-called biological species concept (BSC) is, of course, the
culmination of this retrogressive trend and it is instructive to examine
some of the ideas that have shaped the BSC.

(1) Reproductive compatibility: This is more often assumed than
demonstrated, but even if two or more distinct forms can interbreed and
produce viable offspring what does this prove? In a differentiating
population reproductive compatibility is a primitive feature, which may be
lost in one or more forms as they speciate. Thus, to group by reproductive
compatibilty is to group by a primitive feature and can only lead to
artificial assemblages of forms.

(2) Geographic representation: Evidence from the distribution of closely
related forms suggests that the allopatric mode of speciation is the
usual manner in which populations fragment and diversify. Geographic
disjunction (allopatry) in the ranges of two or more closely related forms
is, therefore, the norm, and sympatry evidence of dispersion and secondary
overlap. Thus it makes little sense to lump together such forms simply
because they are considered "geographic representatives" of one another.

(3) Intergradation: Apparent intergradation and clinal variation are very
often the artefacts of inadequate sampling. When sampled much more finely
these "clinal species" are frequently revealed to be composed of a number
of distinct forms, each of which is quite homogeneous. In other cases
intergradation is the result of hybridisation brought about by secondary
contact between two or more previously allopatric species. The existence
of such intermediates is quite irrelevant to the question of the validity
of the species involved.

Of course this does not mean that all avian subspecies should be regarded
as valid species. The widespread use of the subspecies category in
ornithology has resulted in the acceptance of many forms on very
inadequate data. Such forms which are not clearly diagnosable are best
not recognised at all.

The literature on this subject is very extensive but I can recommend the
following three papers as especially useful from an ornithological
perspective:

Cracraft, J. 1983. Species concepts and speciation analysis. Current
Ornithology 1: 159-187.

McKitrick, M. C. & Zink, R. M. 1988. Species concepts in ornithology.
The Condor 90(1): 1-14.

Zink, R. M. & McKitrick, M. C. 1995. The debate over species concepts and
its implications for ornithology. The Auk 112(3): 701-719.

Priyantha
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dilrukshan Priyantha Wijesinghe       Email:dwijesin@email.gc.cuny.edu
Department of Entomology              Fax: 212-769 5277
American Museum of Natural History
Central Park West at 79th Street
New York, NY 10024