Paraphyly: the continuing saga

Thomas DiBenedetto tdib at OCEANCONSERVANCY.ORG
Wed Jan 16 11:42:23 CST 2002

-----Original Message-----
From: Curtis Clark
... While I appreciate the
theoretical views of Byron and Tom, it seems extraordinarily likely that
the evolutionary fate of the "parent" species is affected no more by the
successful new species than by the failed ones.
... It would be nice if species "behaved" and were always monophyletic, but
fact the interest and importance of species is that they are the interface
between tokogenetic relationships and phylogenetic relationships.
I think this argument returns us to a point I was trying to make to Ken,
about being somewhat trapped by our concepts. I refer particularly to the
term "species". Unfortunatly the term has taken on a rich and complex set of
meanings and this fact provokes much confusion. There is a clear meaning for
the term that is rooted in empirical systematics - the meaning I was pushing
in previous posts - namely that "species" is simply a rank, applied to an
empirically determined taxon - often seen as the fundamental unit of
diversity, or  a terminal branch in the hierarchy of taxa. But "species" is
also defined in reference to a host of biological processes - a reproductive
nexus, an ecological unit etc. etc. Other biologists are pretty content to
leave it to systematists to define "genus", or "family" etc. but
systematists do not have clear ownership of the term "species". In addition,
since systematists are also biologists, we all understand and to some extent
subscribe to these other meanings, and I think that has consequence for how
we deal with "species" in systematics.
If two individuals in a species simply die, then clearly the species is not
affected in any way that has any real consequence for our conception of what
the species is. So I understand of course the argument that if these two
individuals, instead of dying, go off to the other side of a mountain and
start a new isolated breeding population, that then evolves into a new
"species", the "parent" grouping is similarly unaffected. So it is easy, and
seemingly logical to conclude that "species" are not necessarily
monophyletic - for here we have a species that budded off a new species, and
yet persists effectively unchanged (i.e it remains the same "species").
But, I would argue, the entity that we judge to be unchanged is a "species"
in all of the non-systematic meanings of the word. From a systematic point
of view, we demarcate groups (species) relative to other groups. The
criterion we use to delineate groups are the relationships that we infer to
exist between the indiviuals in the groups, and between the groups
themsleves.  Although we necessarily use characters as evidence of these
relationships, it is the relationships that are the ultimate criterion.
Before going over the mountain, the two individuals in our example had a
certain type of relationship with other members of the group, and these
relationships caused us to recognize that they all constitute a single
species. After the journey over the mountain, the groupings on either side
have this same relationship within their group - there are now two groups,
two species. Is either one identical _in terms of relationships_ to the
parent group that existed yesterday? I would say - clearly not. In fact they
both are made up of individuals that are descendants of the individuals in
the parent - and this descendant relationship is exactly the same in both of
the two groups. In addition, the parent species had a historical
relationship with some third group not previously mentioned - that other
group being the one that shared a most recent common ancestor with the
parent. But this type of historical relatioship is what the two new species
share with each other, not with that third group. Clearly there is a novel
set of relationships throughout this situation.
I am not saying that systematists must reject non-systematic meanings of
"species" - but perhps we need to do that when we are actually doing
systematics. In systematics, "species" are taxa, and they are defined in
reference to the relationships within and between the individuals and groups
of individuals we percieve. Natural species-level taxa are monophyletic in
the same way that higher taxa are - it is only if we forget about the
criterion of relationship do we begin to concieve of "species" remaining
unchanged while some of their descendants form new species - i.e. of species
being paraphyletic. When relationships change, relationship-based taxa
change, even if an ecologist can't tell the difference.

Tom DiBenedetto

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