Principios de Taxonomia

7.5 Descent is not the Same Thing as Kinship: The Concepts of Monophyly and Paraphylyj197
While the statement several organisms have the same ancestor should not be
understood quantitatively (there is no stronger or weaker descent), the statement
several organisms are related to each other should certainly be understood
quantitatively because there are closer and more distant kinships.
This becomes clear in the following simple example. All son/daughter descendants
(F1 descendants) of a pair of parents are equally related to each other. The four
grandchildren of a pair of parents (F2 generation), however, are no longer equally
related to each other (Figure 7.2). They are either siblings or cousins and so are related
to each other to different degrees. While the four grandchildren in Figure 7.2 form a
coherent descent community, they separate into two groups as kinship communities:
siblings and cousins. If a number of organisms were grouped according to equalranking
descent, then this would result in a different group than if a number of
organisms were grouped according to equal-ranking kinship. The group of organisms
possessing equal-ranking descent only in the F1 generation (son/daughter
generation) is congruent with the group of organisms possessing equal-ranking
kinship, but it is no longer congruent in the F2 generation (grandchild generation).
This line of thought is the starting point for understanding the concept of the
monophylum, which contains more than just a group of organisms with a common
descent. Besides having a common ancestor, the monophylum must be complete
(Figure 7.5a). A monophylum is a descent community that has to fulfill two
conditions. First, it must be a group of descendants with a common ancestor.
Second, all the descendants of this common ancestor must belong to this group
without exception, for there is no conclusive justification for excluding some portion
of the descendants from the group and to include them in another group. For this
reason, Ereshefsky stresses: A [monophylum] must contain a single ancestral
species as well as all and only its descendant species (Ereshefsky, 1999). A monophylum
always has to follow the rule that all sister taxa must be of the same rank.
Figure 7.5 A monophylum is a group of
descendants with a recent common ancestor. In
addition to having a common ancestor, a
monophylum also must contain all of the
descendants of the common ancestor. If a
branch is excluded, the remaining group is a
paraphylum. In a paraphylum, the sister taxa (A
and B þ C) are of unequal ranks.