Cladistics and "Eclecticism"
turner at RULSFB.LEIDENUNIV.NL
Tue Feb 5 18:21:57 CST 2002
[Tom DiBenedetto wrote:]
> ..... 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.
It seems to me that your species concept is strictly internodal
("Hennigian"). If you conceive of each internodon as a separate
species, then I agree that the ancestral internodon no longer exists
after the split, having given rise to two daughter internodons. But
it is not necessarily possible to identify to which internodon a
specimen belongs. For example, to which internodon ("species") do the
two founder specimens belong? The parent or a descendant? If they
belong to the descendant, did they already belong to that descendant
before they migrated? Or have they changed taxon status after 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.
Sure, but as I wrote, it is in practice frequently not possible to
determine to which internodon ("species") a specimen belongs, without
exact knowledge of the genealogical (parenthood) relations.
>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.
So, yesterday I was Homo sapiens, tomorrow I'll be on my way to Mars
and have changed specific status: I'll be Homo futuris or whatever??
This goes against all common sense and all rules of nomenclature. (Of
course the rules can be changed if necessary)
>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.
Perhaps we should make a distinction between species as taxa that can
be recognised by apomorphic character states at whatever level (as
far as I'm concerned a single fixed state is sufficient, e.g. A, in
position umpteen of gene such-and-such, where the ancestral state was
C, G, or T; of course it must be fixed throughout the entire set of
individuals for the evolutionary fate to be determined inescapably,
save further new changes) and internodons, however real they might
be, whose parts (members) cannot be identified with certainty
without prior knowledge of the genealogical network. And with that
knowledge, we already know what the phylogeny is, and we do not need
to reconstruct it any longer!
> Natural species-level taxa are monophyletic in
>the same way that higher taxa are -
What do you mean by "monophyletic" here? If ten American couples and
ten Dutch couples go off to Mars to form a new "species", is that new
species monophyletic? After all, the Americans and the Dutch are not
more closely related to each other than they are to their siblings
that stay behind. To me monophyletic is meaningless without
specifying the object and the level to which it pertains: A genus is
a monophyletic set of species (a common ancestor species and all its
descendant species), but a species is not necessarily a monophyletic
set of internodons, and certainly not a monophyletic set of
individuals (for sexually reproducing species this is impossible
anyway, because there is no single ancestral specimen to begin with!).
> 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.
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