Principios de Taxonomia

level of organisms with the next-higher hierarchical level of species. To understand
the application of a phylogenetic tree to organisms with biparental sexuality, one must
first realize that it is necessary to switch between different hierarchical levels of
biological organization (see below). Because biparentally reproducing organisms do
not only cleave into daughter organisms, but also fuse by sexual conjunction, a
phylogenetic tree of biparental organisms cannot be displayed at the organismic level;
it only can be displayed at the taxon level (de Queiroz, 1999).
To achieve a bifurcating phylogenetic tree for biparentally reproducing organisms,
one has to abandon the phylogenetic tree at the organismal level and display the tree
at the next-higher hierarchical level. In a biparental phylogenetic tree, the stem and
the daughter branches refer to taxa (shaded as a shrouded stem in Figure 6.1), not to
organisms. However, the reticular cross-connections within the shrouds refer to
branches of the individual organisms; they are not species branches (Figure 6.1).
This consideration makes clear the actual difference between uniparental and
biparental propagation (Chapter 6). Biparentality means the transformation of a
bifurcating phylogenetic tree into a network. A network means that there are lateral
connections, and this is exactly what makes a species. If there are no lateral
connections, then the individual organisms are not cohesively linked to each other.
If they are not linked, they cannot be species. Only biparentally reproducing
organisms can form species in nature. The fusion of phylogenetic branches is
almost a definition for that what a species is. Species transform bifurcating trees into
reticular networks at the organismic level.
7.3
What are Species Boundaries in Cladistics?
7.3 What are Species Boundaries in Cladistics?j191
A phylogenetic tree means the arrangement of organisms according to common
descent. However, how can a phylogenetic tree define groups? Taxonomy requires the
formation of groups. It does not suffice to ascertain that particular organisms belong
with each other by sharing a common ancestor. All life on Earth shares a common
ancestor. This is not taxonomy.
Group formation presumes not only cohesion criteria but also delimitation
criteria. If groups are to be formed, then one also needs rules that specify the criteria
on which an entity is delimited from another entity, not only rules that specify their
cohesion (Mishler and Donoghue, 1994). By doing so, the problem arises of how
cladistics and taxonomy can at all be linked to each other.
The problem of a species concept as a descent community consists of having to find
criteriaaccording towhich differenttaxa canactually bedefined.The descent cohesion
is at first the connection of consecutive generations. Descent cohesion is genealogical
cohesion, and genealogical cohesion knows no boundaries. As long as the organisms
reproduce,thethread doesnot breakoff. Everybirthof a daughter organism maintains
its current pace without presenting criteria for taxon delimitations.
Descent is the genealogical sequence of parental generations and continuing filial
generations. Parents give birth to children, and children give birth to grandchildren.