Farewell to Species - reticulation
turner at RULSFB.LEIDENUNIV.NL
Tue Feb 15 00:10:11 CST 2000
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No. You just dont seem to get it.
YOU just dont seem to get it!!! All I want is to be able to refer to a
particular internal branch by a particular name. And that name is sometimes
already available, namely a species name. That at a certain point
Archaeopteryx lithographica branched onto several descendant species and at
that very moment stopped existing doesn't matter. The specimen that was
covered with limestone in the deposits near what was later to become the
city of Solnhofen was part of a lineage characterised by a number of
synapomorphies, but the animal did not yet display further apomorphies that
were to develop only later in the lineage (e.g. webbed feet, such as
present-day ducks have). What is the objection against referring to that
particular branch (starting at a split and ending at the very next split)
by a name at species level? When that branch was terminal, it was referred
to as a species, so why not continue to do so? Of course it is also part of
a larger clade which also includes all of its descendants, but it is also
part of a still larger clade that includes all vertebrates, which are part
of an even larger clade that includes all metazoans, etc. etc. But the same
goes for e.g. Parus major, or Homo sapiens, two terminal branches which you
don't seem to mind naming as species. But just because we do not know if
Parus major is split into different entities (Parus aus and Parus bus) does
not mean that it will never do so. In fact, it may already have split up,
only we haven't recognised it (yet).
It is probably the most basic
point of phylogenetic systematics, that taxa are meant to refer to real
monophyletic groups. When a taxon, ranked as a species (because it is
terminal) diverges, then it is not equal to the internal branch, it is equal
to, as always, the original ancestor and all of its descendants. It has
evolved into a higher ranking taxon.
Yes, it has evolved into a higher taxon, consisting of the ancestor
(branch 3) and its descendants (branches 1 and 2). But the clade is not the
same thing as the ancestor, at least not in terms of taxonomic rank. The
clade is the same thing as the lineage that started with branch 3, I'll
give you that. But I don't see how you can talk about the original ancestor
as a separate unit, but do not want to be able to refer to it by any other
name than the very vague 'original ancestor' (BTW, what is an unoriginal
ancestor? ;-) ). My point is that the branch in the phylogeny that you
identify as the ancestor should bear the taxonomic rank of species, and
should be named accordingly. Following Hennig, as I pointed out before,
any segment of a lineage between to branching points could be called a
species, whether it terminates in the present, or by extinction, or by
By what logic or definition must it still be considered a species? What is
so problematical about using the past tense to describe something that no
longer exists? Species is a rank. As a rank, it has conventionally been
applied to taxa deemed to be terminal.
But it has conventionally been applied to Archaeopteryx lithographica and
other assemblages of individual specimens that might have been ancestors of
later species, and as such were not deemed to be terminal.
When a taxon is terminal and
unbranched, it is ranked as a species. When it aint, it aint. When it is
branched it is a higher taxon. When it is internal, and defined in isolation
from its descendants, it is an artificial, paraphyletic assemblage.
The taxon is the ancestor and all its descendants.
As I said, rubbish!!! In traditional (i.e. nomenclatural) usage,
taxon is any grouping of species, from the least inclusive (1
species) to the most (phyla). Don't try to redefine the meaning of
Its not me doing any redefining. The definition I presented has been
standard for quite some time.
I feel you confuse the term 'taxon' with the term 'clade'. I see no
reason not to give names (at the species level) to internal branches. I
agree that it would not be desirable to formally name paraphyletic grades
(such as reptiles, or something like 'Protomagnoliaceae'). But for genera
the codes leave us no other choice (apart from just not naming ancestral
taxa) than to erect paraphyletic genera.
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
FROM 18 JANUARY TILL MID-APRIL I WILL BE AT THE NEW YORK
BOTANICAL GARDEN AS VISITING SCHOLAR, DOING MOLECULAR
PHYLOGENETIC RESEARCH ON ANACARDIACEAE. MY REGULAR E-MAIL ADDRESS
WILL REMAIN FUNCTIONAL DURING THAT PERIOD.
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