Farewell to Species - reticulation

Thomas DiBenedetto TDibenedetto at DCCMC.ORG
Sun Feb 13 18:13:52 CST 2000

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 -- and in response to a rather
obvious point I made about how a species level taxon evolves into a higher
What rubbish!!!!! 3 is a branch on the network. When it was terminal,
it was a species (by your own convention!) When it split into
branches 1 and 2, it became an internal branch. According to the
phylog. species conc., branch 3 stopped being a species then.
No. You just dont seem to get it. 3 does not become an internal branch. It
becomes an internal branch with 2 terminal branches subtended from the
internal branch. In other words, its descendants are PART OF IT. Just like
ALL of the descendants of the original mammal species are part of Mammalia.
Just like Homo and Pan etc. are PART OF Primates (as you seem to
understand!). It is only possible to see taxon 3 as an internal branch only,
by arbitrarily cutting off the descendants. 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.
tdib previously:
>If a fossil species were ever to be identified as actually being
>ancestral to later species, then I would have no problem saying that it WAS
>a species at some time. Now it may just be part of a higher taxon.
No, that particular chunk of the genealogical network still is a
species! In addition it is part of a higher taxon, but so is a
terminal species extant right now (Homo sapiens is part of the taxon
Primates is part of the taxon Mammalia is part of the taxon
Chordata). These are not incompatible designations.
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. 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.
tdib previously:
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.

Tom DiBenedetto
tdib at dccmc.org

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