Farewell to Species - reticulation

[Hubert Turner]

On Wed, 02 Feb 2000 10:43:17 -0500, Thomas DiBenedetto wrote:

>I would say that we have two species, but three taxa. The original species
>should not be "relegated to nomenclatural extinction", but recognized as a
>higher taxon. My view of a taxon is that it represents a lineage (branch, or
>system of branches)emanating from a common ancestor. The original taxon
>still exists, and will continue to exist until all of its descendants are
>extinct. The original taxon is, however, more complex than a simple lineage
>branch; that original single branch has given off a sublineage. Thus the
>original taxon (the branch emanating from the original common ancestor) is a
>higher taxon. The name applied to the lineage emanating from that common
>ancestor should always remain with the lineage (system) emanating from that
>ancestor.

The issue here is, that there are three unsplit branches between (terminal
or internal) nodes in the cladogram of lineages:

 \    /
 1\  /2
   \/
   /
  /3
 /

Lineage 1 has an apomorphy, hence it can be recognised as a separate
species. Lineage 2 does not. Lineage 3 (the 'ancestor') also has an
apomorphy. The problem comes when the question is asked which lineages
should be grouped together, either for nomenclatural purposes, or for
cladistic purposes. To begin with, I see no problem with uniting 2 and 3
under the same name (nomenclature); at any rate the specimens in the two
cannot be recognised as belonging to the one or the other. In cladistic
analysis the issue becomes: is it allowed to take 2 and 3 together as a
single unit, next to lineage 1 (and outgroup lineages, of course). I don't
see anything wrong with doing that, and am backed by the analysis of the
"species problem" made by Kornet & McAllister (J. Theor. Biol.
164:407--435, and Kornet's PhD thesis from Leiden University). Of course,
if one would use both 2 and 3 as separate units in a cladistic analysis,
the resulting tree would be:

1   2   3
 \  |  /
  \ | /
   \|/
    |

1, 2, and 3 share an apomorphy (the one that characterised 3), and lineage
1 in addition has an autapomorphy. Under no circumstance will a cladistic
analysis tell you that one of the lineages is the ancestor to one of the
others. The analysis is simply not designed to do such a thing.
Back to nomenclature: 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 sudden
become the name for a clade (1+2+3). At worst it is the name to be
restricted to members of lineage 3 (or 2), and the other lineage should get
a new name. The clade (all three lineages) could also be named separately.

>-----------------
>Richard Jensen:
>My view is that, unless both daughter species have evolved
>autapomorphies with respect to the original species, then after the split
>there are still two species: the original is unchanged and there is the
>new isolated species that takes off on its own evolutionary pathway.
>------------------
Exactly.
>But the name of the original taxon was meant to apply to all descendants of
>the common ancestor of the original taxon.

When it was named, lineages 2+3 were assumed to be a single lineage (or:
lineage s 1 and 2 did not exist)

> The original taxon is a higher taxon.

No! It is still a single lineage between two nodes: but now, the
terminating node is not terminal, but internal. In your view, Archaeopteryx
( or maybe Protoavis) is not a genus, but an order (=Aves). Try getting
that past a nomenclaturist!

Hubert Turner


*******************************************************
Dr. Hubert Turner
EEW, Sect. Theoretical Biology & Phylogenetics
PO Box 9516, 2300 RA  Leiden, The Netherlands
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E-mail: turner at rulsfb.leidenuniv.nl
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FROM 18 JANUARY TILL MID-APRIL I WILL BE VISITING THE NEW YORK
BOTANICAL GARDEN AS VISITING SCHOLAR, DOING MOLECULAR
PHYLOGENETIC RESEARCH ON ANACARDIACEAE. THIS E-MAIL ADDRESS
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