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species musings

There were several posts on the subject of species and phylogenetics
awaiting me this morning.  Here's a random assemblage of them, with some

>From Jeff Poling:
>Is it for the same reason put forth by Chris Brochu,
>that you also use the phylogenetic species concept

I don't think I "use" the phylogenetic species concept as such.  In fact, I
view the evolutionary and phylogenetic species concepts as part of the same
mental process - one states what they are, the other states how we go about
identifying them (in principle).

For most vertebrate paleontologists, there is no operational difference
between a phylogenetic species approach and the traditional "morphological"
species concept that Tom Holtz mentioned.  In both cases, species are
smallest diagnosable units, and it's VERY rare that anyone actually puts a
species name on an internodal branch.  In fact, very little of the debate
surrounding what a species should be has actually changed the way things
are done by workaday taxonomists this century.

Things get fuzzier when you deal with species-level relationships among a
group of closely-related animals that might very well represent both
lineage splitting events and anagenetically-evolving lineages.  Ask me
about Mio-Pliocene Alligator or Bridgerian-Uintan crocs sometime.

>From Didymus:

>I had always thought that the definition was a group of
>interbreeding individuals.  And two different 'species' cannot interbreed
>and produce viable offspring.  How does this apply to the defining of a
>dinosaur species.  Obviously we don't have the means to breed dinosaurs, so
>we have to rely on anatomy to define the species.  How do the two relate?
> Do we have a good handle on what anatomical differences are enough to fit
>into the definition?  Or am I 'full of weeds' with the species definition??

You're not "full of weeds," but you're running face-first into a problem
with how the profession is taught.  There are perhaps as many approaches to
what a species is as there are people making that approach.  We used to
have a systematics discussion group at UT that, normally, was very
congenial.  We had hardcorce geljocks, botanists, paleoanthropoligists, and
morphologists discussing things calmly - unless the s-word came up, and all
gloves came off.

The biological species concept is probably the only one taught in most
basic courses in biology or paleontology, and the controversy surrounding
it is rarely broached.  THe book Jim Farlow recommended is excellent; I
also recommend Ghiselin's new book on species and metaphysics, as well as
the Otte and Endler volume *SPeciation and its Consequences* (a classic)
for more on the issues involved.

And you've pointed at one real problem with over-reliance on interbreeding
- the dead no longer interbreed.

Another from Jeff Poling:

>   This does, I hate to say, bring to mind an argument I saw from our
>bestest buddy "Cal King" before I stopped reading his posts.  He argued
>that MRCA (a node on a cladogram) was a flawed concept because there was no
>direct fossil evidence of this animal.  However, this "mystery animal" is
>accepted by "cladists" (for lack of a better term) as having existed, even
>though there's no fossil evidence.  Yet "cladists" are not willing to
>accept the hypothesis that one species begat enother because of a lack of
>fossil evidence.  I'm not sure I see the distinction.

The statement that "cladists are not willing to accept the hypothesis..."
is not universally true.  Hardcore pattern cladists accept the concept of
common ancestry, but recognize the difficulty in recognizing the ancestors
themselves.  For more on cladistic approaches for hypothesizing actual
ancestors (they really exist, despite C*l K*ng's denial), take a look at
Archibald's paper on mammalian turnover across the K-T boundary in
Paleobiology from, I think, 1992, as well as Nelson's chapter in the
Homology volume edited by Hall.

>From Dinogeorge:

>I'd love to drive a stake through the heart of that Phylogenetic Species
>Concept, one of the most ridiculous ideas to dribble out of cladistics
>(poisoning the whole system, if you ask me). Imagine you have a population of
>animals that becomes divided in two by, say, a river that keeps one group from
>interbreeding with the other. The Phylogenetic Species Concept declares that
>>at this point< the original species becomes extinct, and two daughter
>species, both identical to the parent species, come into being.

Not entirely.  Species, like all monophyletic groups, are viewed as
individuals and not as classes.  One of the characteristics of an
individual (philosophically speaking) is "fuzzy boundaries."  Can anyone
say when, to the second, you became an individual?  Was it when the sperm
entered the egg, when the chromosomes mixed, when your heart started
beating, when you drank your first beer...?  Classes, defined on the basis
of some possessed property, can be sharply bound, but not individuals.  As
such, the phylogenetic species concept does not (at least in principle)
specify a *point*.

Talk about
>>arbitrary<! And if the river dries up later, and the two populations merge,
>what then? Two species going extinct and a fourth species created?

Perhaps.  It also depends on the limits to reticulation you accept over
time.  It might be said that they were not really species at all, since
over time the lineage simply experienced a large-scale reticulation.  (I'm
not going to pretend to have the answer here, but neither do I think any
other species concept avoids ambiguity in this case.)


Christopher Brochu

Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL  60605  USA

phone:  312-922-9410, ext. 469
fax:  312-922-9566