Principios de Taxonomia

192j 7 The Cohesion of Organisms Through Genealogical Lineage (Cladistics)
In no part of the generation sequence do we see a parental generation giving birth to a
new species or, especially, to a new genus. If from one mother several daughters
descend, then, although a kind of cohesion is given between mother and daughters,
this connection does not delimit any taxa. For the mother is in turn also a daughter of
her parents, and her daughters give birth again to additional offspring. Accordingly,
one can trace back the genealogical cohesion as far as the beginning of all life, without
ever having encountered a boundary that could signify a taxon s end.
Because a majority of scientists are convinced that all organisms on Earth have a
common root (at least all complex organisms), all organisms are related to each other.
Cladistics shows us continuing bifurcations. Yet, what turns a bifurcation into the
birth of a taxon? Is the birth of multiple children by the same parents not already a
cladistic bifurcation? Where does one species end and the other begin? When does
one descent community stop and a new one begin? Taxonomy cannot evade these
questions. It is not initially clear how the descent community, which is a progressing
continuum, can be linked to taxonomy, which must create delimited groups.
Taxonomy faces the difficult task of classifying by common ancestry as well as by
group, but a group can only be formed if there are also delimitations against
neighboring groups.
Criteria for the decision when one species ceases and a new species begins must be
borrowed from other species concepts. Usually species borders are defined through
the alteration of traits and/or through the separation of traits (apomorphies, see
below). If the traits change, then this is rated as the origin of a new species. If the
group splits into two groups with different traits (autapomorphies, see below), then
this is rated as the origin of two new species.
Traits by themselves, however, cannot be the reason for classifying organisms
into taxa. Traits are never a species definition. They only can be used to distinguish
species which previously are defined by other criteria than traits (Chapter 2). A
sorting of the organisms according to trait similarity is always a class formation
(Chapter 3). Classifying organisms by their traits is something different from
classifying organisms by their relational connection, and a descent community is a
grouping of the organisms according to relational cohesion. Cladistic taxonomy
cannot be typology, the species concept that forms taxonomic groups according to
trait similarity.
This difference is elucidated by following example (Figure 7.4). Nine children hold
hands with each other and in doing so form a group called A. At the end of a certain
time (toward the top in the figure), group A splits into two separate groups of five and
four children (B and C, respectively). While the group consisting of four children (C)
remains as it is, the group of five children (B) splits again into two additional groups of
two and three children (D and E, respectively). This example makes clear that the
children s group cohesion is given only by the fact that they hold hands with each
other. To understand what the groups are, which group has originated from which
and who is the ancestor of the particular groups, one need not assume that any one of
the children changes any of its traits at some point in time.
This example should be understood as a parallel for the descent community
in taxonomy. It is supposed to elucidate the fact that the currently living groups