Principios de Taxonomia

36j 2 Why is there a Species Problem?
Figure 2.6 The phylogenetic tree consists of two elements: the lineage and the bifurcating clade.
species but not Felidae, which is instead considered a family? Both groups of
organisms have common ancestors so this cannot be what makes them different. A
common ancestor cannot define a species because families also have a common
ancestor. Why are all organisms not just a single species?
Where are the limits drawn? A taxonomy based on common ancestry (monophyly)
faces the problem of defining beginnings and ends (Mallet, 1995). When
does a branch begin and when does it end? Until what point is a species a species,
and when is the ancestor distant enough for the group to be a genus? In addition,
evolution proceeds with different speeds. There are evolutionarily younger and
older species; one cannot simply declare that an evolutionarily older species is a
genus and that organisms with even older common ancestors are families, orders,
and so on. Cladistics alone cannot define a species; it needs to borrow some of the
classification principles of other species concepts to define a species.
Cladistics attempts to resolve this problem with phylogenetic bifurcations. The
branches of the phylogenetic tree fork repeatedly, and there are lineages and clades
(Figure 2.6). A species concept is defined according to this basic pattern of a
phylogenetic tree, termed the cladistic species (de Queiroz, 1998). Therefore, a
species is simply a lineage. As long as the branch continues as a lineage throughout
the propagation of generations, the species continues to exist. However, as soon as
the branch splits, the life of the species ends, and two new species (daughter
species) begin which live on until new branching events stop their existence. With
every bifurcation, two new species begin, and with every additional bifurcation of
the two daughter branches, each of the two species ends and a new species begins.
This appears to be a simple definition.
However, the entire notion of a cladistic species concept changes with the
definition of a bifurcation. We certainly cannot define the species as the part of a
branch between two bifurcations in the phylogenetic tree without first defining a
bifurcation. The definition of a bifurcation (split) is very difficult because each birth
of two siblings is a bifurcation from the parents into two daughter branches. Two
brothers or two sisters differ with regard to many traits; the creation of a filial
generation (F1) from a parental generation (P) is, of course, a cladistic bifurcation
because the filial generation consists of individuals that differ in traits. What, then,