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Re: Princeton Field Guide
> I have 1) a general statement on the matter, and 2) a direct reply
to one of Greg's arguments.
I mostly agree with Jaime here, although I differ slightly in
interpretation. I think that "species" can be defined so as to make
it a term describing real entities (phylogenetic species concept,
i.e. distinct evolutionary destinies), but I would also say that
there are, practical sense, so many hurdles to actually identifying
and measuring species as distinct entities - and to applying any
single species concept across all of biology, without special
pleading for umpteen different subfields - that the concept of
"species" in any hard-and-fast sense is useless. It is good only as
an approximation to ease communication somewhat.
I think the real problem is Linnaean nomenclature, which requires us to
shoehorn every organism into a species (and a genus) if we are to put it
into any kind of system at all.
If we ignore this requirement, and the PhyloCode will let us, everything
suddenly looks a lot different. For instance, "phylogenetic species"
turns out to be just a synonym of LITU (Least Inclusive Taxonomic Unit =
smallest recognizable clade; a term not used by the PhyloCode, but fully
compatible with it). Why use the separate term "species" for it? There
is no reason.
Other species concepts, too, describe real entities. If you're
interested in gene flow between populations, the good old Biological
Species Concepts (both of them*) describe useful entities (ring species
included), laborious as they are to discover; they're just often not
congruent with ecological species, morphological species, or
phylogenetic species. All these concepts have nothing in common except
the _word_ "species". _There is no_ Platonic ideal of "species" that
these concepts approach, as so many people still seem to think.
* _Can_ they produce fertile offspring in captivity vs. _do_ they do so
in the wild.
I believe "species" can be addressed logically, as stated, but not
observed and measured adequately. That still gives us some basis for
saying it is a real concept. "Genus," however, has never been
measured, nor to my knowledge has it ever been given any definition
that can validate its existence through logic alone.
There is a definition of "genus". It's an extrapolation of the
Biological Species Concepts: if they (can) have fertile offspring,
they're in the same species; if they (can) have sterile offspring,
they're different species in the same genus; if they can't have
offspring at all, they're in different genera.
Unsurprisingly, almost nobody has ever tried to use this definition.
Like the BSC, it's impossible to use for anything but extant sexually
reproducing organisms, and very difficult in practice to apply even to
those; like the BSC, it would trigger a lot of lumping, in some cases to
the point that several traditional subfamilies would have to be merged
into a single genus; and in addition to ring species, we'd get sphere
The concept is somewhat useful as a sort of "handle" by which we can
mentally "pick up" various similar species-ish taxa simultaneously -
a sort of shorthand, really - but that's about it. It lets us
conceptually grab bunches of species together instead of listing the
whole lot or constantly laying out the definition of their clade.
The _name_ of the clade in question does this, not the _rank_ of genus
(or any other rank).
Genera are therefore subjective. If we agree on their content by
convention, then so be it, because we know what we mean by it and it
hypothetically facilitates communication.
Problem is, there is no consensus, and the ICZN doesn't even try to help:
There are certain underlying principles upon which the Code is based.
These are as follows:
(1) The Code refrains from infringing upon taxonomic judgment, which
must not be made subject to regulation or restraint.
(2) Nomenclature does not determine the inclusiveness or exclusiveness
of any taxon, nor the rank to be accorded to any assemblage of animals,
but, rather, provides the name that is to be used for a taxon whatever
taxonomic limits and rank are given to it.<<
Anyone can publish a classification that assigns however many species to
any genus as they damn well please. And then anyone else can come along
and publish a classification that contradicts it. And to all eternity
both are correct, and neither is wrong, provided they keep the type
species in their genera and obey priority.
(...Never mind that the mandatory suffixes for family-group ranks and
the mandatory formats for genus-group and species-group names contradict
Principle 2 rather blatantly.)
So, if I say "*Bufo*", do I mean the mid-20th-century *Bufo* which
includes all hundreds of bufonids that aren't too weird, or the much,
much smaller Frost et al. (2006) concept of *Bufo* which is limited to a
couple of Eurasian species, so that the cane toad is *Rhinella marina*?
The name alone doesn't tell you, and neither does the (unchanged) genus
rank of that name.
If we start talking about inclusion or exclusion of taxa according to
phylogeny as elucidated by traits, we're applying objective reasoning
to a subjective construct. That does not work. Only subjective
consensus applies to defining subjective phenomena. So saying that we
should or shouldn't include taxa in a genus based "only" on traits
like frill horns or whatnot is absurd, because we can subjectively
attach as much or as little significance to any given trait as we
collectively agree to do.
We don't even need to agree. It's enough that the ICZN lets us.
Greg's subjective preference that we give less value to such
features is not wrong by any means, it's just not the standard agreed
upon by a larger set of people-interested-in-the-issue.
There isn't a standard in the first place.
Greg's standard has therefore been excluded by subjective consensus;
That consensus consists only of which species to include in which genera
of Mesozoic dinosaurs. There's not much, if any, of a general concept
Some point-by-point responses to a particular line of reasoning:
"It is not possible to define species either, but species are real."
As far as science is concerned. all real things have a definition.
"It is not possible to define life, but it is real."
"Life" is a loaded word. I would argue that reasonable definitions
for it do exist, as long as we're careful about "unloading" it first
to avoid confusion.
It's a continuum that our languages fail to express. Therefore, where in
the continuum we decide to draw the line is completely subjective. Are
viruses living beings? Depends on how we choose to define "life". :-| If
we want viruses to count as "alive", we'll likely define "life" along
criteria of information; if we don't want them to count, we'll likely
use things like independent metabolism as defining. It's up to us.
"There is no way to precisely define battleship versus battlecruiser
(HMS HOOD for example) versus heavy cruiser (Alaska class for
That's because, philosophically, there's no such thing as a
"battleship," "battlecruiser, or "cruiser." Let me clarify. The
designs for those vessels exist; the vessels themselves exist as
physical objects. They are real and they are mathematically
definable. But the categorizations for them are ultimately arbitrary
human labels. Yes, one could say "semi-arbitrary," because tonnage
and certain other factors are measurable as portions of a continuum,
which is what such terms are attempting to encapsulate. Yet, like
language in general, the labels themselves have no fixed meaning, and
further, there are so many continua being lumped together (tonnage,
dimensions, # of guns, etc) that a huge number of variations along
one or all continua are possible. We could define every possible
combination along every last axis if we had the time and inclination
to do so, but it's such a complex question that no one has the
wherewithal to attempt it, and that would still leave us with a
radically different form of classification.
In point of fact, there is no *empirical* dividing line along any
continuum at which one category turns into another, except in our
own minds due our own subjective preferences (because in this case
we *want* categories, so we delineate them at our convenience. We
could choose to lump them all into a single category if we desired to
do so, or we could break them down until we only talk about specific
designs, specific mission profiles, or specific vessels). The
physical world only shows that one ship has X guns and another ship
has Y, *not* that this difference has significance in any absolute
sense, and therefore *not* that this difference demands any
*specific* nomenclature for categorization. The only significance is
in our minds, and hence the terms we apply to individual designs are
still arbitrary designations - except insofar as we wish to
communicate useful information (so while we could call a
"battlecruiser" a "flying spaghetti warship" if we so wished, we
prefer to use the term that will convey meaning more effectively to
those who might otherwise be confused).
You are, thus, saying that, "philosophically, there's no such thing as"
I'm fine with that.
"Many terms are approximations, it does not mean that they cannot be
used to describe and distinguish basic, comparable types."
Right. Approximations. General notions. Not things we can be
definite about. So not, for example, things we can definitely say
include or exclude any one taxon.
Phylogenetic nomenclature escapes this by using objectively discoverable
events to define clade names. Rank-based nomenclature doesn't even try.
That's because rank-based nomenclature is deliberately theory-free. Both
Charles Darwin and Richard Owen, together with many others, are among
the authors of the 1842/3 Strickland code of zoological nomenclature, so
it's a worst-of-all-worlds compromise. It treats organisms as
battleships, battle cruisers and heavy cruisers. Phylogenetic
nomenclature, on the other hand, assumes evolution and uses
tree-thinking; this makes everything easier.