Principios de Taxonomia

7.9 The Concepts of Monophyly and Paraphyly cannot be Applied to Speciesj207
founder of cladistic taxonomy, avoided applying the cladistic unit of a monophylum to
the category of species and restricted the application of the monophyly concept to
higher taxa (Hull, 1997; Ereshefsky, 1999). In contrast, Hennig s successors have
extended the concept of monophyly to the level of species (Ax, 1995; de Queiroz, 1999;
de Queiroz and Donoghue 1988; Mishler and Donoghue, 1994).
The problem in this case is that the concepts of monophyly and paraphyly are tied
to bifurcations (Figure 7.5). Without bifurcations, there is no mono- and no paraphyly.
However, biparental species as a gene-flow community do not represent a
bifurcating system. Within a unit in which reproductive connections between
organisms exist, there can be no bifurcations. A bifurcation within a species always
splits the species into two separate gene flow communities, thus representing the
origin of two new species.
Accordingly, the terms monophyly and paraphyly cannot be applied to biparental
species because there is no bifurcation within such species. Mono- or paraphyla must
be groups that include distinct branches. The logic of the paraphylum always requires
at least two bifurcations, which lead to three branches with dissimilar kinship
relationships that would then be expressed in the paraphylum (Figure 7.5).
Two branches alone cannot form a paraphylum; and a gene-flow community that
always is a group of organisms without any separated branches, never can be a monoor
paraphylum. Bifurcations only exist at higher levels of taxonomic hierarchies
above the species level.
This is explained by the following example (Figure 7.11). On the Canary Islands of
Tenerife and Gran Canaria, there is an endemic species of finch that only occurs on
these two islands, the Blue Chaffinch (Fringilla teydea). The Blue Chaffinch is a sister
species of the Common Chaffinch (Fringilla coelebs). It is thought that the Blue
Chaffinch separated from the continental Common Chaffinch only a relatively short
time ago evolutionarily.
Let us assume that the stem species of the contemporary Common Chaffinch
and Blue Chaffinch inhabited a wide geographical range from northwest Africa
through Europe to West Asia, similar to the distribution of the contemporary
Common Chaffinch. Let us also assume that during that time, different races
arose that remained connected to each other via continuing gene flow (open
circles within the stem species in Figure 7.11). When population e then reached
the Canary Islands, the gene flow cohesion with the continental populations
ceased due to allopatry.
Two sister species arose: the recent Common Chaffinch and the Blue Chaffinch.
It does not make sense to view the Common Chaffinch as a paraphylum, as its current
races, a through d, are connected by gene flow and, thus, belong to a single gene-flow
community. Gene flow cohesion excludes the possibility of a more recent common
ancestor or a less recent common ancestor of populations a through e because all
populations are connected and do not consist of a system of separated branches.
Therefore, the Common Chaffinch cannot be a paraphyletic species. The concepts of
monophyly and paraphyly cannot be applied to species as long as species are
understood as gene-flow communities.