Biology Reference
In-Depth Information
To keep with the tree analogy, the terminal nodes (C, D, and E here)
are often termed leaves; and the node of origin, the root. The most
recent common ancestor of a set is called their cenancestor, or direct
ancestor (their last ancestor before the speciation event from which the
two or more lineages were born). Thus, in Fig. 1, node B is the cenances-
tor of D and E; while node A is the cenancestor of C, D, and E.
Trees can also be represented in a parenthesis format: each internal
node is represented as a pair of parentheses with the descendant nodes
between them separated by commas, only the terminal nodes being
labeled. Generally, a semicolon is used to indicate the end of the tree.
The dendrogram in Fig. 1 can therefore be represented as
(
C
, (
D
,
E
)).
As we are beginning to see, an entire family of terms and concepts has
evolved around phylogenetics, and we will look at some of these in more
detail as we progress through the chapter. Phylogenetics first developed
as a science for classifying organisms into phylons and phyla
b
according
to their evolutionary history. A phylon is a set of organisms that normally
includes all of the descendants from a given ancestral node and some or
all of the ancestral forms they share. In phylogenetics, an organism or a
group of organisms, irrespective of its position in the taxonomic hierarchy,
is often referred to as a taxon.
c
It must be stressed that a taxon can rep-
resent a single organism (e.g. subspecies
Homo sapiens sapiens
) or a group
of organisms (e.g. hominoids).
The term “phyletic” refers to the nodes of lineage, without addressing
the manner in which phylons were formed — a phyletic tree is a simple
dendrogram, while a phylogenetic tree is a graphical construction which
displays anagenetic information in addition to the nodes of the tree (the
cladogenetic part). This supplementary information may be an estimate of
b
The term “phylum” has come to mean, particularly in zoology, a higher-order rank
of living organisms whose cenancestor is situated relatively early in the tree of life
(again, the phylum usually includes the cenancestor and any other ancestor descend-
ing from it). Phyla are thus particular kinds of phylons.
c
Both “taxons” and “taxa” are used as the plural form.
