defining relict plant species

[pierre deleporte]

A 16:33 19/06/2003 +1000, Jasmyn Lynch wrote:
>The concept of a relic(t) plant species has been used as an important 
>character of a species in conservation assessments in Australia.  This 
>concept relates to the conservation of species and environments that were 
>more common in the past and that are indicative of evolutionary 
>processes.  Relict species are defined in Lincoln et al. (1998, p. 260) 
>as:  “1. Persistent remnants of formerly widespread fauna or flora 
>existing in certain isolated areas or habitats;  relic.  2. A phylogenetic 
>relict;  the existence of an archaic form in an otherwise extinct taxon.”
>
>While phylogenetic relicts are relatively easy to identify, it is more 
>difficult to define which taxa now occur in remnant distributions.(...)

Is a "phylogenetic relict" so easy to identify ? Is an "archaic form" not a 
relict when the taxon is otherwise "not extinct" ? Is a "derived form" not 
a relict when the taxon is otherwise extinct ? And how should "the" taxon 
be circumscribed in order to check whether it is completely extinct or not ?

Suggestion: "relict" would apply in a more relevant way to characters than 
to taxa... (e.g. concern plesiomorphic-looking features rarely carried by 
living taxa, by contrast to frequently encountered apomorphic-looking 
features...?).

Extant descendants (...all extants are "descendants"...) are susceptible to 
carry a mixture of "ancient" (unchanged) and "modern" (recently modified) 
features. Focusing on the features and their evolutionary scenarios helps 
avoiding the misleading confusion between characters and taxa.
A taxon carrying 100% characters interpretable as being in a plesiomorphic 
state relative to to those carried by another taxon would be a possible 
"living fossil" (= "complete relict"?).
The relevant "taxon" (the one possibly extinct in definition 2 above) would 
be the group of organisms carrying the feature or series of features of 
interest (possibly paraphyletic under this definition).

Pierre

>   Even the related concept of distinguishing between neo-, holo- and 
> palaeoendemics is rarely attempted.
>
>However, categorisations of relict plants were made in the 1970s based on 
>the retention of (a) floral characters generally accepted by taxonomists 
>and morphologists as primitive, and (b) early stages in the evolution of 
>the leaf, shoot and inflorescence morphologies.  There are some 592 relict 
>seed plant species retaining primitive floral characteristics listed for 
>Australia by Specht et al. (1974), and 381 species of angiosperms 
>retaining primitive morphological characters.
>
>Australian examples given in Melville (1973) of (a) include Eupomatia 
>laurina and E. bennettii (Eupomatiaceae), Himatandra (Himatandraceae), 
>Austrobaileya (Austrobaileyaceae) and Blepharocarya 
>(Anacardiaceae).  Examples of (b) include Stirlingia spp. 
>(Proteaceae);  to the extent that Melville related characters of S. 
>tenuifolia, S. simplex, S. abrotanoides and S. teretifolia to branch 
>systems found among plants of the Psilophytales of Silurian to Devonian 
>age, and S. latifolia leaf characters to fossil leaves of Gangamopteris of 
>Permo-carboniferous age.  However, the relationship of characters of 
>Palaeozoic plants to angiosperm lineages that did not begin to evolve 
>until the Cretaceous, and species extant in the Holocene (some 200 million 
>years later) must be tenuous.  The concept is more tenable in terms of 
>groups such as the Cycads and conifers that are presumed to be much older, 
>however, can these lineages still be used to infer that extant descendants 
>are primitive?
>
>I would welcome any comments in regard to relict species (and refugia), 
>and related (preferably more recent) literature, as the relationships 
>between species and their degree of environmental adaptation is of 
>particular interest to me.
>
>Jasmyn Lynch
>PhD student
>University of Queensland
>Email:  s4023888 at student.uq.edu.au

Pierre Deleporte
CNRS UMR 6552 - Station Biologique de Paimpont
F-35380 Paimpont   FRANCE
Téléphone : 02 99 61 81 66
Télécopie : 02 99 61 81 88