Biology Reference
In-Depth Information
taxa displayed in the tree, which are descendants of an internal node
(therefore, this node is the cenancestor of the terminal taxa in the
grouping in question); this grouping forms a monophylon. Accordingly,
a holophylon is a special, complete kind of monophylon.
It is possible to define a group in which some descendants of its
cenancestor are not included. In other words, a systematic choice is made
(consciously or not) to exclude some siblings from a group. Such group-
ings come in three different varieties, some of which are more acceptable
than others. They are discussed in the remainder of this section.
4.6.1. Paraphylon — Acceptable with caveat
When a grouping of symplesiomorphic taxa — assembled on the basis of
some shared ancestral characteristic(s) — excludes from its midst some sib-
lings, i.e. descendants from the cenancestor of this group, it is called para-
phyletic and constitutes a paraphylon. These siblings have been excluded
either because they do not share the plesiomorphy of the group (some
ancestral characteristic(s) which they have lost) or because they have devel-
oped their own apomorphy (a unique trait), which sets them apart.
Though phyletically incorrect, paraphylons are generally consid-
ered acceptable if they are helpful from a classification point of view.
A classical example is the exclusion of birds and mammals from pres-
ent-day reptiles.
On the basis of a number of “primitive” traits, present-day reptiles
are defined as including the tuatara (
Sphenodon
, a “living-fossil” rhyn-
chocephalian), Chelonia (turtles), crocodilians, as well as Squamata
(lizards and snakes), and excluding birds and mammals. However, nowa-
days, crocodilians are generally considered to be phyletically closer to
birds than to any other existing reptile (see right dendrogram in Fig. 14);
accordingly, the
Reptilia
taxon is a paraphylon. Nevertheless, it is still a
taxonomically useful grouping because it marks the evolutionary and
anagenetically important boundary between the two taxa of fully
homoiothermic amniotes (
Mammalia
and
Aves
) and poikilothermic
amniotes (
Reptilia
).
There is another classical example of useful paraphylon. As opposed
to “primitive”, cartilaginous, sea-dwelling fish (
Chondrichthyes
), bony
