Crisp & Chandler, Paraphyletic species 
815 
and angiosperms are not ancestors of themselves, any more than I am part of my 
grandfather. However, it cannot be disputed that some member of the seed plants 
evolved into the first angiosperm. The resolution of this paradox is to recognise taxa 
as units of an hierarchical pattern, not as part of the evolutionary process. By logical 
extension, species belong to this hierarchy. This pattern, when reconstructed, may 
be used as a framework for hypotheses about the evolutionary process, e.g. that a 
seed plant with certain characteristics gave rise to the first angiosperm. Thus the 
role of ancestor is restricted to lineages and their subunits, such as individuals or 
populations (Rieppel 1994) or 'internodons' (Kornet f993b). If species are treated as 
taxa, then they are not different in kind from higher taxa. They are simply the least 
inclusive units in the systematic hierarchy. 
Recent concepts of species as phylogenetic taxa derive from Nelson & Platnick (1981: 
12), who define species as 'the smallest diagnosable cluster of self-perpetuating 
organisms that have unique sets of characters'. This is known as the 'phylogenetic 
species concept', 'irreducible unit' or 'minimum diagnosable unit' (Cracraft 1989; 
Nixon & Wheeler 1990; Nixon & Wheeler 1992). However, a unique or diagnostic 
character may be either an apomorphy or a plesiomorphy, and a group diagnosed 
solely by the latter is not monophyletic, which is anathema to authors such as 
Nelson (1989a; 1989b). Such species are not simple internodal partitions of a 
phylogenetic tree. They 'survive' a speciation event in which an autapomorphic 
species branches off from the phylogenetic stem (Nixon & Wheeler 1992: fig. 4.7B). 
Contrast this with Hennig's (1966: fig. 6) methodological extinction of ancestral 
species at branch-points. Neither Cracraft nor Nixon and Wheeler confront the 
paraphyly issue, but instead imply that paraphyletic species are acceptable, provided 
that they manifest unique and fixed character combinations. Under the phylogenetic 
species concept, speciation is the point at which a lineage acquires an apomorphy, 
or more precisely when a new character is fixed (Nixon & Wheeler 1992: fig. 4.7). 
This is true even of species diagnosed by a plesiomorphy, because at some point 
earlier in history, every plesiomorphy was an apomorphy. A problem with the 
notion of an 'irreducible unit' is that it is prone to regress (cf. de Queiroz & Donoghue 
1990b). Peripherally isolated populations in which trivia! genetic characters can easily 
become fixed would be diagnosed as species, contrary to general practice. One 
solution to the paraphyly problem is the monophyletic species concept, under which 
species have at least one autapomorphy (Rosen 1979; Donoghue 1985; Nelson 1989a; 
Nelson 1989b). However, this concept is unsatisfactory because ultimately taxa are 
not necessarily divisible into monophyletic sister-taxa (Smith 1994b). The smallest 
autapomorphic unit may have as its sister-group an unresolved symplesiomorphic 
cluster of organisms. The autopomorphic species concept consigns these to limbo, 
outside any species, but they can scarcely be ignored. 
Some authors have taken the pragmatic view that phenetic clusters may be treated 
as species. This approach has been termed the 'phenetic species concept', although 
it is actually an empirical criterion, free of assumptions about evolutionary process. 
Such units have been termed 'phena' (Mayr 1969; Smith 1994b), to distinguish them 
from theoretically based 'species'. In fact, the phenetic species concept is the formal 
equivalent of the traditional 'taxonomic' or 'morphological' species concept, under 
which species are circumscribed by intuitively perceived similarity among their 
members (Sneath & Sokal 1973: 364-5). Sometimes this concept is conflated with the 
phylogenetic species concept; however, clustering by overall similarity is not the 
same as clustering by diagnostic (fixed) characters. Clusters in phenetic space may 
share no diagnostic character; usually they are circumscribed by a series of partially 
correlating (polythetic) characters. Nevertheless, some authors have argued that 
phenetic clusters may be equivalent in practice to phylogenetic species (Theriot 
1992; Crisp & Weston 1993). 
