Cladistic theory

[Ken Kinman]

Bryan,
     Your post is a fantastic and insightful overview of the issues.  I
think I might print it off and hang it on my wall.
     I think your choice of approach to intra-specific lineages is the best
one.   As for your quandry over lineage recognition in general, remember
that it is no longer just a choice between whether or not to recognize
paraphyletic groups.  A third "middle ground" approach is semi-paraphyletic
groups.  If you have questions how Kinman markers could optimize one of your
"grass" classifications, e-mail me (privately would probably be better than
through Taxacom), and I can tell you how I would approach it.
                      Cheers, Ken Kinman
********************************************************
>From: Bryan Simon <Bryan.Simon at ENV.QLD.GOV.AU>
>Reply-To: Bryan Simon <Bryan.Simon at ENV.QLD.GOV.AU>
>To: TAXACOM at USOBI.ORG
>Subject: Cladistic theory
>Date: Sun, 6 Feb 2000 23:14:44 -0600
>
>I am relatively new to using cladistics to assist in the systematic study
>of
>grasses, using morphological characters.  This has led me into chasing up a
>lot of literature concerning cladistic theory, from which I have come to
>realize just how much diversity there is in this field, as recent postings
>on "Farewell to species" has revealed and at looking at past issues of
>Cladistics and Systematic Zoology.
>
>Two basic concerns I have at present are
>a)the recognition and nomenclature of lineages following dichotomy and
>b) the treatment of lineages that arise at the tokogenetic level within a
>species.
>Both topics are well covered in the literature and on previous Taxacom
>threads, but it is proving difficult to discover some sort of uniform
>treatment in dealing with them.   As an herbarium botanist I feel it vital
>that I am able to explain the classifications I produce (using cladistic
>methods) to the users of these classifications in the real world.
>
>a. Recognition of lineages.  Some clarification on this issue has been
>presented in the recent thread, but I still not clear exactly which path to
>take.  Much has been written in recent times about the recognition or not
>of
>paraphyletic groups. Adherents of both philosophies have presented
>persuasive arguments.   It appears to all hinge on what happens at the
>point
>of lineage splitting, be it of a dichotomous nature or budding off from an
>existing lineage.  The cladistic purists would say that at the point of
>lineage splitting two new entities "evolve", whether they be recognizable
>or
>not. A good example on Taxacom a few months ago was the example of a
>population of Homo sapiens becoming  isolated on Mars into a new species H.
>ares.  From the instant this happens the purists say, the extant Earth
>species H. sapiens has also changed (although there is no autapomorphic
>character present) to H. terrestris, say.  This is a very difficult concept
>to put across to practically minded people, let alone the fact that the
>different populations of H. terrestris (or extant H. sapiens) are now
>paraphyletic.
>
>b. Treatment of intra-species lineages.  This essential concerns the
>phylogenetic species concept and the necessity of recognising
>infra-specific
>taxa.  Some recent postings on Taxacom have suggested 1) that if the PSC
>(more recently called the diagnosability sc) is strictly adhered to there
>is
>no need to recognize infra-specific ranks at all.  Other workers have
>suggested 2) that where fixation of one or more character states has not
>become complete, there are grounds for recognising the peaks of variation
>that may be detected phenetically at infra-specific rank  (subspecies in
>preference to var.).  Yet other workers think 3) there is no essential
>difference  between species and lineages within species, and that terminal
>taxa on a cladogram may be represented by a genus, section, species,
>subspecies or population.  The second choice seems preferable to me as a
>practical herbarium taxonomist.  The idea of working with clades at a rank
>lower than species would surely lead to all sorts of problems, including
>the
>recognition of possible paraphyletic populations within the borders of a
>species.
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