[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Paraphyly and sister taxa (was Re: AMARGASAURUS SATTLERI)



Newly incarcerated in the innermost fold of the germ layer of Hell, Tech's
own Jeffrey Martz wrote:

>    Paraphyly is implicit at EVERY NODE where the sister taxa are not
>identical, which means at every node except where individual specimens
>assigned to the same genus and species are sister taxa.  Unless we are
>willing to place every single organism in the same genus and species, there
>is no possible system of taxonomy that can eliminate paraphyly.  Taxonomy is
>useless WITHOUT implicit paraphyly.
        All right, there are two entirely separate problems which
participants in this discussion appear to be addressing. I will handle each
in turn:

1) Species may be paraphyletic.
        Well, I think this one is pretty obvious. It may interest y'all to
know that there are some systematists who actually advocate recognizing only
monophyletic species, and others who, recognizing that this then amounts to
recognizing a species "rank," advocate the abandonment of species
altogether. This would result in a nested series of progressively less
inclusive clades of organisms. Note, of course, that, with respect to their
component germ cell lineages, an organism is paraphyletic). Also, I do not
know of a criterion for monophyly among biparental organisms.
        In any case, if one holds to the idea of a species as being an
entity composed of interbreeding organisms with some sort of intergrated
historical componenent, one must, in my opinion, accept species paraphyly.
Note that I do not take this position rashly, nor am I alone in adopting it.
There are a number of systematists who accept species as a class of
(not-necessarily monophyletic) entity, perhaps separate from clades, perhaps
inextricably linked. To my knowledge, it bothers none of these individuals
that species are paraphyletic, nor does it bother them that an ancestor may
be represented as having a sister-relationship with its descendant on a
cladogram.
        Remember that a cladogram is a diagramatic representation of
hypothesized propinquity of descent, NOT a phylogeny sensu stricto, i.e., it
is a graphical representation of relationships, not a graphical model of HOW
those relationships came about. There is nothing implicit in the topology
(A,(B,C)) which obviates the possibility that B might be ancestral to C, nor
that C might be ancestral to B, nor that A might ancestral to both. (Yes, I
know there are other possibilities, but I have more important things to do
right now.)

2) Genera may be paraphyletic.
        This problem is currently being addressed by groups within the
systematic community, although (IMHO), this discussion is perhaps not always
as explicit as it could be. The problem is that, under the ICZN code,
referral to a genus-rank taxon is obligatory for all species. It doesn't
take much thought to realize that, if a genus is mandatory for every
species, and some species are paraphyletic, then some genera are
paraphyletic too.
        Some recent systematists have begun insisting upon monophyletic
genera. While this is a good start, it requires the recognition of numerous
additional genera (e.g. Einiosaurus, Gorgosaurus), each of which is by no
means necessarily monophyletic itself. One might respond that they are also
not demonstrably paraphyletic, but this stems only from the fact that there
is no catagory of positive evidence (AFAIK) for ancestry-descent
relationships among fossil species. Further, recognizing these genera often
simply adds more monotypic (and therefore redundant, and therefore useless)
genera to an already overflowing list.
        In any case, as far as I can tell, this is really no surprise to
anyone. It does not, however, represent a flaw in phylogenetic systematics,
but rather a flaw in Linnean nomenclature, that being obligatory genera. The
obvious solution is to abandon that principle, and move to a system of
nomenclature, either uni- or binomial, which does not require two separate
taxa for each species, and which makes a clear distinction between species
and clades (assuming you want to use species at all). As I mentioned above,
this is being worked on. I can get you a reference or two if you want.

        Later,

        Wagner
--------------------------------------------------------------------------------
     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi