Farewell to Species - reticulation

[Curtis Clark]

At 11:41 AM 00.02.04 -0500, Thomas DiBenedetto wrote:
>I dont understand what you find to be ludicrous though. Obviously species
>give off isolates all the time, only some of which become new "species".
If
>they dont become new species, then what is the issue? They wont be seen
and
>recognized and nothing will change taxonomically. They will just be 
>isolated populations that sputter out.

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? When does he become paraphyletic?
Conception? Birth? What about miscarriages?

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?

Michael didn't become a higher taxon when his child was born. Species that
spawn peripheral isolates are themselves no more affected by the failed
isolates than by the successful ones. Yes, in theory perhaps it would be
nice to formalize the historical pattern, but in doing so, we have to say
that at some point Michael (read "a given species") was no longer an
individual (read "species") but had become a higher taxon, that we know for
sure that the change had taken place by the time his first child was born
(read "observed speciation"), but that we can't rule out that it may have
happened earlier (read "unobserved speciation"), so that we can never say
with any assurance whether during his earlier existence he was a man or an
institution (and I think I hear Schrödinger's cat meowing in the
background...).

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.

> If we are erecting a taxonomy based on the
>real history of lineages, then the question of whether a lineage has in 
>fact divereged seems to be absolutely a relevant concern.

If we set ourselves up so that we have to recognize whether lineages have
diverged in order to do classification, I think we are in trouble. A
missing species won't make us doubt that humans are mammals, but a missing
peripheral isolate could cause us to mistake a higher taxon for a species,
and cause us to fail to distinguish that higher taxon from the otherwise
identical species that it gave rise to.

>I guess I agree that shoehorning observed biology into a system that
>misrepresents it, is a very bad thing. I was not aware that that is what I
>was doing. And I still dont see how I am.

I hope I've made it clear (whether you agree or not): to me the pattern
that cladistics describes so well starts to break down at the level of
species.

>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.

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Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
California State Polytechnic University      FAX: (909) 869-4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu