Farewell to Species - reticulation

Thomas DiBenedetto TDibenedetto at DCCMC.ORG
Sun Feb 6 12:05:47 CST 2000

Curtis Clark wrote:

Many years ago, Michael Donoghue made a remark that he was unsure whether
he wanted to be a parent, because he would become paraphyletic. Of course
many of us see *only* humor in that remark, but let's assume it were true.
How is he different before and after?
Unfortunatly, Donaghue wasn't joking. And I think his musing display quite a
bit of confusion about important issues. Paraphyly is not a natural process;
it is a condition which results from a human decision to draw certain limits
around a named grouping. If there were no humans around to name things,
there could not be any such thing as a paraphyletic group, because something
is paraphyletic only when a human decision is made to exclude some
descendant members of a group from the set that is to be called by a certain
name. The birthing process has nothing to do with making anyone or anything
paraphyletic. Paraphyly is simply a result of how you choose to apply names
to things.
But of course, it is also inappropriate to analogize the relationships
within a tokogenetic network with those in a phylogenetic hierarchy (and
that, I suspect is the focus of your disagreement with Donaghue. If so, I
agree with you).
At least with individual people, we could (arbitrarily!) choose birth as
the instant of "speciation", but with species, at what point do we count a
peripheral isolate as a species for "historical purposes"? And even if we
(arbitrarily!) chose a criterion, how could we show that such an event had
never occurred?
But this is a different issue! Of course, the speciation process is usually
an extended, and often abortive "tearing away". Hennig's famous figure 6
diagrammed that very well. At some point though, everyone would agree that
speciation has occured. And we are discussing how to recognize that in our
naming system.
I am saying that the name of the ancestor (of the ancestral taxon) applies
to all of its descendants. That ancestral taxon is no longer ranked as a
species, because it is no longer a terminal taxon. It has diversified; it is
now a higher taxon. How can you claim that some of the descendants are at
one with the ancestor and others are not (by giving some the same name as
the ancestor and others a different name?). THere is no difference between
one descendant and another in terms of their relationships to the ancestor.
They are both descendants. We might not be able to recognize this right
away, so obviously there is a period during which we have to deal with
"meta-species" and things like that. But that does not mean that the
undiagnosable descendant is the same thing as the ancestral species. It does
have a different sister group than the ancestor did. Its sisteer-group is
one of the ancestors descendants.
Species that
spawn peripheral isolates are themselves no more affected by the failed
isolates than by the successful ones.
It depends what you mean by "affected". I am not "affected" in a physical
way by the fact that my sister, a thousand miles away, has given birth, or
whether her child survives or not. But my set of relationships to those
closely related to me has certainly changed. I am now an uncle.
Our classifications are, I think rather obviouly, based on the notion that
the groupings refer to _relationships_ of one group to other groups.
Classification is all about relationships.
If a species gives off an isolate that withers and dies, then the species is
not changed in any way. But if the isolate survives and develops its own
evolutionary fate, then the _relationships_ of the parts change, even if the
groups seem otherwise unaffected.  The ancestral "species" has a set of
descendants that includes populations that do not breed with eachother. This
makes for a different set of relationships. The whole lineage, including the
ancestral populations, the contemporary populations in the "main"
population, and the isolate, all combine to form a higher taxon. And how can
that higher taxon be named anything but the name that it had before it had
descendants? (i.e the name of the ancestral species, now promoted to higher
I hope I'm also hearing some of you saying, "But cladistics can't possibly
work within species of biparental, sexually reproducing organisms."
Exactly! Cladistics has a realm in which it works (exceptionally/passably)
well. It is important to understand the limits of that realm. IMO species
are one of those limits, and that is exactly why species are biologically
interesting. I know there are those who disagree, and who are accepting of
Michael's paraphyly. But that view makes me wonder if I should save my
breath when I defend cladistics as not being essentialist.
I agree with you that cladistics is not, nor has ever pretended to be
suitable for non-hierarchial relationship systems.
I have not been talking about intra-specific issues in this discussion, only
cases in which a permanent divergence has taken place (even if we cant see
it yet).
>they all are still descendants of that ancestor? The name adheres to the
>ancestor and all of its descendants.

My disagreement is that one of those descendants is biologically
indistinguishable from the ancestor.
I understand your point, but not why you disagree with me. Obviously in that
case we have to wait for an apomorphy to arise before everything is cut and
dried. In the meantime, we use various conventions, like metaspecies to
indicate that the group has a unique set of relationships to other groups,
but is for now undiagnosable to that level.

Tom DiBenedetto
tdib at dccmc.org

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