Principios de Taxonomia

2.12 Species Pluralism: How Many Species Concepts Exist?j35
the formation of the species (de Queiroz, 1999 5323). The concept does not
conceive of groups of organisms as natural entities (Hull, 1997).
(2) Secondly, there is an infinite number of characteristics that can be labeled as
different traits between two organisms (Chapter 4). In the phenetic concept,
there are no criteria regarding which traits are biologically relevant and which
are not (Dupre, 1999). A major objection to the concept is that traits are so
heterogeneous that any consideration of quantifying degrees of resemblance
is impossible (Ghiselin, 1997). How similar must two organisms be so that
they are similar enough to belong to the same species (Hull, 1999)? No
pheneticist can answer this question. Accordingly, many authors think that
phenetics is not a science (Mahner and Bunge, 1997). Mayr considered
phenetics to be, at best, a starting point for further taxonomic analyses (Mayr
and Ashlock, 1991).
The species concept of phenetics is not very different from the species
concept that underlies the barcoding approach (Chapter 4). Both concepts
ignore biological processes and only look for differences. However, barcoding
is based on DNA sequence differences, whereas phenetics mainly considers
phenotypic differences. This is an important difference because it protects
barcoding taxonomy from being confounded by convergent (parallel-evolved)
trait similarities. However, beyond this one point, barcoding taxonomy is a
type of phenetics.
The phenetic species concept is the only species concept that can be equally
applied to all phyla of living organisms and thus offers the broadest applicability
(Dupre, 1999; Hull, 1999). It is a very simple concept that can easily be
applied in practice because it avoids conflicts with controversial opinions
regarding descent or gene flow relationships. Perhaps a modest confinement
to pure phenetics would have protected taxonomy from becoming an eternally
disputed never-ending story (see below).
2) The cladistic species concept: The cladistic species concept is that of descent and
therefore of evolutionary kinship (Hennig, 1966). Species are groups of organisms
that have the same common ancestor. Every organism that has a common ancestor
(i.e., organisms are related to each other) belongs to one species. In strong contrast
to the phenetic species concept, organisms are not grouped according to trait
similarities but are defined according to their genealogical relationships to each
other (Peters, 1998). The cladistic species concept does not recognize a change in
traits along an evolutionary line. Alterations in traits (anagenesis, see Chapter 7) do
not give rise to new species (Figure 2.3). If a group of organisms does not bifurcate,
it cannot become a new species because determining a change in traits is liable to
subjective standards of perception and evaluation. In contrast, bifurcation is
considered an objective fact (cladogenesis, see Chapter 7).
The unresolved problem of the cladistic species concept is that all organisms on
Earth are of common descent. Where does one species end and another begin?
All Felidae (the cat family) possess a common ancestor, just as all Lions (Panthera
leo) ranging from South Africa to India have a common ancestor. Why is the Lion a