Paraphyletic species 
Michael D. Crisp and Greg T. Chandler 
813 
Abstract 
Crisp, Michael D. and Chandler, Gregory T. (Division of Botany and Zoology, The Australian National 
University, Canberra ACT 0200, Australia) 1996. Paraphyletic species Telopea 6(4): 813-844. We 
present evidence, mainly from plants, that many recognised species and subspecies are 
paraphyletic. Whilst some cladists have argued that species are like other taxa, and should be 
monophyletic, it is clear that even cladists either implicitly or explicitly recognise non- 
monophyletic species. Moreover, species concepts such as the phylogenetic species concept 
and the composite species concept predict non-monophyly of many species. Whenever a 
monophyletic species is circumscribed, it is possible that a paraphyletic or metaphyletic 
'residual' species is simultaneously recognised. Furthermore, attempts to place all organisms 
in a monophyletic taxon at every rank regress to the population level where monophyly is no 
longer applicable, leaving paraphyletic residuals. These groups of organisms can hardly be 
ignored, unless one wishes to define them out of existence (as in the monophyletic species 
concept). It has been argued that paraphyly is only an artifact of the Linnean system, which 
requires all organisms to be classified in certain ranks, e.g. species. However the phenonemon 
of regress shows that this is incorrect, because paraphyly is inherent in species. The solution 
to this conundrum is to recognise species as special taxa, which may be monophyletic or 
paraphyletic. (Higher taxa should always be monophyletic, and can be made so.) This requires 
the acceptance of a species concept that allows paraphyly, such as the phylogenetic species 
concept or the composite species concept. The monophyletic species concept, which does not 
allow paraphyly, is not acceptable. The special nature of species derives from their basal 
position in the phylogenetic system. Theoretically, the proportion of paraphyletic and 
metaphyletic species may be 50% or higher. Empirical estimates range from 20% to 50%. Use 
of non-monophyletic species in historical applications such as biogeography is widespread 
but may not be invalid, depending upon the assumptions made. 
Introduction 
In recent years, systematists have sought a species concept that is compatible with 
a phylogenetic system. They have rejected the biological species concept because of 
its reliance on the single criterion of reproduction. Entities which are distinct in 
many evolutionary, biological, and ecological features are nevertheless capable of 
interbreeding (Endler 1989, pp. 629-30). The biological species concept has never 
dealt satisfactorily with the conundrum of potentially (but not actually) interbreeding 
allopatric populations. Above all, the biological species concept is based on 
contemporary micro-evolutionary processes and cannot be reconciled with a 
phylogenetic system, in which taxa are viewed as historical units, extended in time 
and the units of a nested hierarchy (Rosen 1979; Donoghue 1985; Cracraft 1989; 
Vrana & Wheeler 1992; Frost & Kluge 1995). 
Species as lineages 
Systematists have debated whether species should be viewed as lineages or taxa 
(Nelson 1989b; Rieppel 1994; Frost & Kluge 1995). Recent views of species as lineages 
hark back to a model presented by Hennig (1966: fig. 6), showing a lineage of 
sexually reproducing organisms splitting into two daughter linages. Each lineage is 
