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made internally cohesive by reticulating ('tokogenetic') relationships among its 
component organisms, but no such connections exist between lineages •— they are 
mutually exclusive. Species are the internodes of a phylogenetic tree and speciation 
is the point at which lineages split permanently. Hennig's model has been reproduced 
many times, with modifications to show details or complications such as temporary 
versus permanent splits, reticulation and extinction (Kornet 1993a; Kornet 1993b; 
O'Hara 1993; Frost & Kluge 1995; Graybeal 1995). Most importantly, a lineage species 
is a model of evolutionary process. It is viewed as a real entity that evolves in time 
and space, has a definite beginning and end, and may be the ancestor of lineages 
comprising one or many species. It has been called the 'evolutionary species concept' 
(Wiley 1981; Frost & Kluge 1995) and the 'internodal species concept' (Nixon & 
Wheeler 1990; Kornet 1993a). 
Some authors have been preoccupied with 'exclusivity' of lineage species (Donoghue 
1985; de Queiroz & Donoghue 1988; de Queiroz & Donoghue 1990a; de Queiroz & 
Donoghue 1990b; Baum 1992; Baum & Shaw 1995; Graybeal 1995). (This is often 
called 'monophyly' but strictly monophyly refers to a taxon diagnosed by an 
autapomorphy.) A lineage is exclusive only if all its members are more closely 
related to one another (by ancestry) than to any member of another lineage. For 
example, the lineage of descendants of Queen Victoria is not exclusive because some 
descendants are more closely related to members of other lineages (by marriage, e.g. 
cousins). This discussion can reduce to the absurd because any lineage may be 
shown to be non-exclusive if examined minutely enough, even body cells (Frost & 
Kluge 1995). In her formalisation of a lineage concept of species, Kornet (1993a) 
shows that internodal species are miitualli/ exclusive partitions of the genealogical 
network. Whilst this is a different notion of exclusivity from that discussed above, 
Kornet shows the latter problem to be irrelevant by using descent rather than ancestry 
as the criterion of group membership. 
Species as taxa 
A major problem with species conceived as lineages is that they have poor empirical 
content (Kornet 1993a). When we find two allopatric populations that are essentially 
similar, we have no way of judging their fate — whether they are the basis of new, 
historically separate lineages, whether either will become extinct, or whether they 
will reunite and become reproductively, tokogenetically cohesive again. Therefore, 
systematists have also proposed concepts of species that have an empirical basis. In 
this view, species are part of a pattern of similarity among organisms: the hierarchy 
of internested groups that are called taxa (Nelson & Platnick 1981; Nelson 1989b; 
Rieppel 1994). The internested groups or taxa are recognised by shared similarity in 
characters, known as synapomorphy or homology. This hierarchy is represented as 
a tree (cladogram or phylogeny), but it is an abstract representation of pattern. The 
branches of the tree represent taxonomic groups which are internested, static and do 
not evolve. Thus the stem at the base of the angiosperms represents not the ancestral 
species of all angiosperms, but the most inclusive set of all taxa that we call 
angiosperms, recognised by the set of characters that all angiosperms share, and 
marked on the stem. Rieppel (1994) suggests that species conceived as lineages and 
species conceived as taxa are 'complementary but incompatible'. (Frost & Kluge 
(1995) refer to this distinction as the 'scalar' hierarchy versus the 'specification' 
hierarchy.) If taxa are also considered to be ancestors and descendants, then we are 
confronted with a paradox (Nelson 1989b). For example, does the subordination of 
the angiosperms to the seed plants imply that the seed plants are the ancestors of 
the angiosperms? Surely not, because the angiosperms are also a part of seed plants. 
