Farewell to Species - reticulation

Hubert Turner turner at RULSFB.LEIDENUNIV.NL
Sat Feb 5 17:43:27 CST 2000

On Fri, 04 Feb 2000 11:20:39 -0500, Thomas DiBenedetto
>Hubert Turner wrote:
>[in reference to an example where '3' refers to the base of
>a Y-shaped
>lineage, and'1', and '2' refer to the arms]

>>If a name was applied to specimens in lineages 2 & 3
>>before the existence of lineage 1 was known, it does not all of a
>>become the name for a clade (1+2+3).
>Huh? Why not?

Lineages 2 & 3 were seen as a single terminal lineage
(species). I don't see a reason for retaining the name of a
particular lineage (such as 3) when that lineage branches up into
several new ones. Indeed, 3 becomes the ancestral lineage of the
clade (1+2+3), but that does not mean that the name of 3 must remain
attached to the clade.
 As you write yourself:
>The name adheres to a taxon. A taxon
>represents an ancestor and all of its descendants. All of lineage 1
>is a descendant of the ancestor of 2 & 3.
When did a terminal branch(no descendants!) stop being a
taxon??? And what about all those fossil species? Do you really think
that they are either dead ends in evolution, or actually not chunks
of the genealogical network between permanent splits (those are what
I would call a branch, although actually only permanent splits
accompanied by fixation of an evolutionary novelty in at least one of
the descendant branches (i.e. apomorphies)are the ones that matter in
demarkating sets of branches that together can be called species) but
rather that all their descendants should be included?
>>No! It is still a single lineage between two nodes: but now, the
>>terminating node is not terminal, but internal.
>Huh? This seems to imply that descendants can somehow no longer
>belong to
>their history!
No, rather that bits of lineages between speciation events (i.e.
permanent splits accompanied by fixation of a novelty) can rightfully
be carriers of a unique name, just like you still have your own
unique name even when you reproduce.
>I think it is a fundamental point that,
>from an evolutionary
>perpsective, taxa derive their identity
>from their history..
>archosaurs, so too taxon 1 and taxon 2
>in our example are part of the
>lineage emanating from taxon 3. If taxon 3 were named at any point,
>this name applies to taxon 1 and 2 as well.
>If taxon 3 were the primoridal mammal, and taxon 1 was the
>monotreme and taxon 2 the primordial therian, then is it not the case
>all three are part of the higher taxon Mammalia?

Sure, agreed, but why can't taxon 3 bear its own
(specific) name, rather than the very vague 'taxon 3' you would like
to use? Of course, the clade (1+2+3) can also be named, not
necessarily with a particular rank.
>This is of course, why ranks are being
>abandonded. ....Knowing that means that we know something about the
>evolutionary process. I am not sure what the system of ranks applied
>to taxa tells us about evolution.
Names are just useful ways of referring to particular entities,
and they do not necessarily tell us anything about evolution (at
least not in the Linnean system). I fear that's where many
cladistically inclined taxonomers go wrong, in insisting that
nomenclature and taxonomy be fully compatible and phylogenetically
based. That's mixing two functions and inevitably it will not lead to
a stable nomenclature, one of tye other gaols of nomenclaturists. Why
not accept the fact that our ideas about the evolutionary history can
change with every new scrap of evidence (position 442 in gene XYZ is
a G rather than an A!), and if the name of a taxon is to accurately
reflect its position on the tree of life, that too will have to
change. Linnean nomenclature is primarily a means of conveying
information on how similar species look by means of their
classification. And it has proven to be very useful in that respect.
Everybody nows what a reptile or a dicot looks like!

Hubert Turner

Dr. Hubert Turner
EEW, Sect. Theoretical Biology & Phylogenetics
PO Box 9516, 2300 RA  Leiden, The Netherlands
Visiting address: Van der Klaauw Laboratory, Kaiserstraat 63, Leiden
Phone: +31-71-5274904    Fax: +31-71-5274900
E-mail: turner at rulsfb.leidenuniv.nl
WWW: http://wwwbio.leidenuniv.nl/~turner/index.html


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