turner at BIO.UVA.NL
Fri Jan 5 09:47:02 CST 1996
>Date: Thu, 4 Jan 1996 14:43:51 -0500
>Reply-To: 06JLSMITH at BSUVC.BSU.EDU
>Sender: Biological Systematics Discussion List
><TAXACOM at CMSA.BERKELEY.EDU>
>From: 06jlsmith at BSUVC.BSU.EDU
>Subject: need suggestions
>To: Multiple recipients of list TAXACOM <TAXACOM at CMSA.BERKELEY.EDU>
> I am doing so taxonomic work with African violets (Saintpaulia) and
>need some advice on what others have done with plant groups that show
>similar taxonomic problems.
>African violets belong to a genus with a relative small geographical
>range (parts of Tanzania and Kenya). Although 20 species have been
>described, it is now becoming apparent that most of these "species"
>were described from small populations of individuals. When brought
>into cultivation, almost every species will cross easily with another
>and give fertile offspring. There does not appear to be any genetic
>barriers between the species.
>Question - in other plant groups, what guidelines have been used to
>define a "species" in situations like this? Should a species be one
>of the local populations that can be identified by morphological
>characteristics, or should the ease with which the plants hybridize
>be taken into account?
>The pollinators appear to be thrips (very small flying insects) so
>crossing between geographical populations in the wild is doubtful.
>I'd appreciate any advice and throughts on how to define the species
>under these circumstances.
>06jlsmith at bsuvc.bsu.edu
D.J. Kornet, from Leiden University, has proposed a species concept based
on two nested criteria. The first criterion is 'permanent splits in the
genealogical network' [Kornet, 1993. Permanent splits as speciation events:
a formal reconstruction of the internodal species concept. J. Theor. Biol.
164:407--435]. She has been able to prove mathematically that permanent
splits partition the genealogical network exhaustively into mutually
exclusive entities, a very desirable property for any candidate species
concept [Kornet and Metz, 1995[?]. Internodons as equivalence classes in
genealogical networks. J. Math. Biol. -- I don't have the exact reference
at hand]. However, the partitionings, which she calls internodons, are
believed to be too short-lived to qualify as species. Therefore she applies
a secondary criterion, quality *Q*, with which to designate 'originator
internodons.' A species then consists of an originator internodon and all
its descendant internodons that are not themselves originator internodons
(and their respective descendants). As quality *Q* she suggests using the
(majority) fixation of an evolutionary novelty in an internodon, but other
criteria could be conceived, e.g. loss of ability to interbreed with
members of other internodons. Since internodons are mutually exclusive
entities, such sets of internodons ('composite species') by necessity have
the same property [Kornet, 1993. Reconstructing species. Demarcations in
genealogical networks. PhD thesis, Leiden University -- previously
mentioned refs. are part of this thesis].
Obviously, all the above is highly theoretical, but the concept can be
applied in practical studies if the investigator can be reasonably certain
that the morphological units s/he recognizes in the material studied are
permanently split parts of the genealogical network, as might be the case
for allopatric units whose members have poor dispersal capabilities.
As a corollary of this species concept it follows that species cannot
hybridize, because that would violate the first criterion of permanent
splits. In the case of Saintpaulia this would mean that all hybridizing
'species' should be merged into one variable species. However, in a
recently published paper from the Rijksherbarium in Leiden [Duistermaat,
1996. Monograph of Arctium L. (Asteraceae). Generic delimitation (including
Cousinia Cass. p.p.), revision of the species, pollen morphology, and
hybrids. Gorteria Suppl. 3:1--143] the author applied the composite species
concept (with majority fixation as quality *Q*), but disregarded
hybridization if it occurs only under special circumstances such as severe
disturbance of the habitat or in cultivation.
Hope this helps.
Institute for Systematics and Population Biology/Zoological Museum
Dept. of Entomology
University of Amsterdam
P.O. Box 94766 Plantage Middenlaan 64
1090 GT Amsterdam 1018 DH Amsterdam
phone: +31 20 525 6245 fax: +31 20 525 6528
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