Principios de Taxonomia

7
The Cohesion of Organisms Through Genealogical
Lineage (Cladistics)
7.1
Preliminary Remarks on Descent Connection
This chapter deals with the problem of converting phylogenetic trees into taxonomic
groups. This is a difficult problem because phylogenetic trees are unbounded
continua without borderlines, while taxonomic groups are delimited groups
(Mallet, 1995). The cohesion of the individual organisms in a genealogical connection
through the parent-child-grandchild lineage is a constantly and steadily progressing
continuum without any boundaries. The biological processes that are relevant for the
production of an F1 and an F2 (and so on) generation from the parental generation
cannot define taxon barriers. Nothing that connects the F1 generation with its parents
can define the birth of a species. Genealogical cohesion implies the birth of children
and grandchildren, not the birth of taxa. Nothing in the succession of consecutive
generations constitutes the end of one taxon and the beginning of another taxon
(Simpson, 1961).
Taxonomy means the classification of organisms into groups. Without cohesion,
no species can be defined. However, what is just as important and sometimes
overlooked is that delimitation is of similar importance. Without delimitation, there
can be no species. A species is a group of organisms that are connected to each other
and delimited from other such groups. Each group formation requires criteria of
cohesion, but at the same time, also criteria of delimitation against neighboring
groups (Mishler and Donoghue, 1994).
This reveals the problem of how to delimit species in cladistics. What are the
species borders in cladistics? Where does one species begin and another cease? What
defines the concept of a speciation in cladistics? Can the bifurcation into two sister
branches be defined by the criteria of the genealogical succession of generations, or is
species delimitation only possible by the criteria of other species concepts as the
typological concept or the gene-flow concept?
At first glance, the cladistic delimitation of species is not perceived as a problem.
Cladistics is the theory of phylogenetic bifurcations, and every phylogenetic tree
displays the origin of new species as the bifurcation of the evolutionary tree. Horses
(Equus caballus) and Donkeys (Equus asinus) descend from a common ancestor, a
Do Species Exist? Principles of Taxonomic Classification, First Edition. Werner Kunz.
Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.
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