[Taxacom] Invisible evolution, paraphyly

[Curtis Clark]

On 2007-06-12 07:33, Richard Zander wrote:
> I see. A paraphyletic species remains integral, held together by
> something, but amniotes minus birds and mammals isn't held together by
> anything and so is fair game for splitting.

I must not be understanding you. *Everyone* splits the Reptilia, into 
orders, families, genera, and species. Even Ken. One can split 
paraphyletic species, intot subspecies, varieties, and formas, or into 
populations, races, or ecotypes. So what?

> Yes, we have a species definition problem. A species is a basic unit of
> taxonomy with usually two kinds of further special definitions, taxon
> based and phylogeny based. I think both the idea that a species must be
> one genetic unit and also that it must be monophyletic classification
> wise is a great burden on systematics.

I agree. The species level marks a fundamental change in pattern for 
sexual organisms, from reticulate to dendroid relationships. Mike 
Donoghue once made the statement that he was uncertain about having 
children, because then he would be paraphyletic. That's ridiculous on 
the face of it (and I hope he meant it that way), because one doesn't do 
phylogenies of people (in the Hennigian sense), one does genealogies.

> Right now there are important strictures that hobble thinking about
> evolution in the systematics context. There is a general acceptance that
> species are governed by the biological (genetic) species concept,

I've always disagreed with this. Certainly most of the plants I've 
worked with don't "play nice" in that arena.

> and
> that classification must be monophyletic. 

I see no reason why fuzzyness or uncertainty at the species level 
provides a justification for non-monophyletic higher taxa. It's like 
saying that non-coding DNA means we can no longer analyze dihybrid crosses.

> However, in my opinion:  A species may consist of populations that are
> isolated genetically, yet in expressed traits be essentially identical.
> A species may consist of populations that are tracked by neutral base
> changes to reveal a different species popping off a population somewhere
> in the middle of a phylogenetically complex group of populations. 

Fair enough. That's one of the reasons people study evolution at the 
species level.

> This is because of a switch in systematics from viewing expressed traits
> as critical to an evolutionary classification to depending on non-coding
> base changes. It's easy to use non-coding base changes. They sort of
> track evolution as guessed at with morphology, they provide lots of data
> that while ignoring dozens of assumptions provide statistically 100%
> certainty of inferred dichotomous genealogies, throwing in the necessity
> of monophyly connects non-coding base changes with classification where
> previously there was none, and assuming the biological (genetic) species
> concept wraps in all up in a nice package.

IMO phylogeny is the relationships among *species*. Species are the 
fundamental units of phylogeny, and unless one has at least a working 
hypothesis of species, it's hard to do effective phylogeny unless one is 
fairly certain of having only a single exemplar per species.

> A paraphyletic species, in my opinion, is exactly like a paraphyletic
> higher taxon, being commonly a phylogenetically complex unit most
> pragmatically defined by expressed traits, and which is not changed in
> any way by peripheral segregation (not necessarily isolation!) of a new
> taxon. A taxonomist may choose not to split a paraphyletic species when
> perceived as a single genetic unit, but split a paraphyletic higher
> taxon because it is vulnerable. On the other hand, many organisms do not
> fit the biological species concept. Also, there is now a tendency to
> recognize fully cryptic species out of phylogenetically complex species
> totally on the basis on non-coding traits alone.

I'm not, nor am I likely ever to be, a nominalist. Whether species are 
"real" or not, the processes that give rise to them can be studied, and 
they are extremely useful constructs. Thus I can never equate 
paraphyletic species with paraphyletic groups of species, and I believe 
that anyone who does is missing some interesting biological questions. 
But then that's just me.

-- 
Curtis Clark            http://www.csupomona.edu/~jcclark/
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