Basal exclusivity

[Mike Crisp]

As can be gleaned from the correspondence from Thomas Schlemmermeyer and
David Orlovich, there is some confusion caused by the use of the term
'basal' in relation to phylogenetic species concepts.  Usually 'basal' is
used in the sense given below by Schlemmermeyer, although this is a sloppy
notion, and confuses placement of groups on a tree relative to the root
with character evolution (whether pleisomorphic or apomorphic).  However, I
dont mean to engage in debate about this.

When used in the context of species concepts, 'basal' means something
different - it means the smallest recognisable group, perhaps monophyletic,
perhaps paraphyletic, and it would be a terminal on a tree, which is the
opposite of 'basal' as used by Schlemmermeyer.

One derivation of 'exclusive' dates from: Baum, D. (1992), Phylogenetic
species concepts. Trends in Ecology and Evolution 7: 1-2, and has been used
subsequently in papers by Baum and Donoghue.  Baum was trying to
distinguish between monophyly (as defined by descent from a single common
ancestor) and 'exclusivity', defining a group whose members are all more
closely related to each other than to any member of another group.  He
makes the distinction to deal with reticulation among lineages.  Thus, he
says that the group comprising himself, his cousins and all the other
descendants of his maternal grandmother is monophyletic but not exclusive
because his cousins have closer relationships outside that group.  This
distinction is the consequence of sexual reproduction or hybridisation.  My
own view is that the distinction is not a useful one.

A slightly different slant on exclusivity is given by Rieppel, O. (1994,
Species and history, in R.W. Scotland, D.J. Siebert & D.M. Williams, eds.,
Models in Phylogeny Reconstruction, Systematics Association Special Volume
52 pp. 31-50. Clarendon Press: Oxford):

"..Species have, indeed, been viewed as individuals (Ghiselin, Hull), but
then again Frost et al. emphasize that the 'analogy of organismal
reproduction to lineage origination is not gestation and birth; it is
mitosis or schizogyny'.  These analogies however, are just what they are -
analogies rather than homologies (of concepts)!  The problem results from
the fact that the genealogical relation between parent(s) and offspring as
much as between ancestor and descendant is *exclusive* (sequential in time
and space), whereas cladistic methodology is a 'discovery procedure'
(Nelson) for *inclusive* relationship of groups (taxa) within groups
(Nelson and Platnick).  Exclusive relations between parent and offspring
are material and historical, inclusive relations of groups within groups
are logical and a-temporal." (p. 34).

He is distinguishing phylogenetic species as historical entities (lineages)
from phylogenetic species as pattern (minimum diagnosable units).  To him,
species in the same lineage but separated in time are 'exclusive', whereas
in a phylogeny, taxa have relationships that are 'inclusive', because a
phylogeny consists of nested groups.  I prefer this concept of
inclusivity/exclusivity.

Mike Crisp
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On Tuesday, 11 Apr 2000 12:50:50 -0300,
Thomas Schlemmermeyer wrote:

>The term "basal exclusivity" is new to me. But as far as I know wha
>phylogeneticists want to say when they say "basal" is that the group under
>question is supposed to be relatively primitive or generalized. That means
>that
>it shows many characters of the supposed hypothetical ancestor, that it is an
>early off-shoot close to the root of the tree, and that it may be old and that
>it has not many autapomorphies or that it has very peculiar ones, not shared
>with other members of the more inclusive group.
>
>On (Tue, 11 Apr 2000 09:33:33 +1200),  David Orlovich
><David.Orlovich at BOTANY.OTAGO.AC.NZ> wrote:
>
>>
>>Hello Taxacom people.  I've been reading a bit on phylogenetic
>>species concepts and am having trouble understanding 'basal
>>exclusivity'.  Can anyone enlighten me?  From what I can work out, a
>>group is basal if all it's members are more closely related to each
>>other than to others, and exclusive if it contains no other less
>>inclusive group.  I can't understand what the difference is between
>>this and saying that a species is monophyletic and has
>>autapomorphies.  I am wondering if using the terms basal and
>>exclusive are just a way of avoiding cladistic terminology - but then
>>I would have thought that to determine basal exclusivity would
>>require a phylogenetic analysis of individuals anyway.  The term
>>basal seems also to be confusing since it seems to refer to a
>>terminal taxon in the 'basal exclusivity' sense and to something
>>quite different in a cladistic sense.  Any help appreciated!  Cheers,
>>David Orlovich.
>>

_____________________________________
Dr Michael D. Crisp
Reader in Plant Systematics
Division of Botany and Zoology
Australian National University
Canberra ACT 0200 Australia

Phone + 61(0)2 6249 2882
Fax   + 61(0)2 6249 5573
E-mail mike.crisp at anu.edu.au
http://online.anu.edu.au/BoZo/staffandstudents/staffprofiles/crisp.html
_____________________________________