Principios de Taxonomia

7.10 Paraphyly and Anagenesis are Mixed Classificationsj209
employed. When is it sufficient to determine species membership only based on trait
characteristics, and when is it necessary to make use of additional criteria, such as
descent or gene flow relationships? There is no underlying theory that determines
which species concept should be consulted in individual cases.
It is not easy to accept mixed classification, although this is the current, wellestablished
practice. However, different species concepts are based on different
biological laws. This must evoke conflicts, as documented in the monophyly-paraphyly
conflict, as well as the anagenesis-cladogenesis conflict.
Evolution consists of two different processes. First, the traits of organisms change;
no organism today looks as it did ten millennia ago. This process is called
anagenesis (Figure 2.3a) (Rensch, 1947). Second, group cohesion may become
lost in the course of evolution; organisms that are laterally connected to each other by
gene flow will split and form separate groups. This process is called cladogenesis
(Figure 2.3b) (Hennig, 1966).
These two processes are not directly correlated with each other. Anagenesis may
occur without cladogenesis, and cladogenesis may occur without anagenesis. In the
course of evolution, trait alterations can occur within a population without the
population splitting up. In turn, bifurcations can take place without trait alterations
(Peters, 1998). For this reason, it is difficult to unite trait alterations and cladogenetic
splits with the same precision into a single taxonomic system. A decision must be
made regarding which of the two processes represents speciation. It is contradictory
to consider both processes as speciation, as two different evolutionary processes
would then be united into one concept. It would be more consistent to avoid mixed
classifications and to consider only bifurcation into separate groups as speciation and
to ignore alterations of traits.
Anagenesis is a qualitative type of change: a single species exhibits alterations in its
traits, whereas the number of species does not change. In contrast, cladogenesis is a
numerical type of change: one species becomes two. This difference is comparable to
the well-known saying of the ancient Greek naturalist Heraclitus You cannot step
into the same river twice. If you understand the same river to represent an identity
regarding quality, then Heraclitus s saying is true. However, if you understand this to
be a numerical identity, then it is always the same river because it does not have any
daughter rivers; that is, it always remains the stem river over the course of time.
Based on the same reasoning, a species cannot become a new species by solely by
undergoing changes in its traits if it is understood as a numerical entity. A new
species can only arise, if a stem species separates into two daughter species, which is a
change in the number of species, not in their quality.
If a biological species is considered as an individual (not as an artificial class of traitequivalent
organisms) (Ghiselin, 1997) (Chapter 3), then anagenetic speciation has to
be rejected. An individual of relationally connected organisms cannot become a new
individual just because of trait alteration. Why should an individual not change its
traits? It makes no sense to state that it becomes a new individual just because it has
changed its traits. As an individual human being does not become a new human by
changing his traits, a biological species as a unique historical product of evolution,
that is, as an individual, also cannot become a new individual, that is, a new species,