Biology Reference
In-Depth Information
Fig. 1.
Dendrogram illustrating the evolution of present-day forms C, D, and E,
from ancestral forms B and A.
called terminal nodes — are the present-time forms of an object capable
of duplication and evolution. Some time ago, an ancestral form B bifur-
cated into two diverging forms, ending in present-day forms D and E.
An earlier ancestral form A had previously bifurcated into two diverging
forms, resulting in the ancestral form B and the present-day form C.
In addition, one must keep in mind that an evolving object can be subject
to extinction (e.g. if there were no points D and E, with B an end node
representing an ancient form which is now extinct).
Because of its branching structure, this kind of graph is called a tree
or dendrogram. Formally, it can be characterized as a graph presenting a
single path between any two nodes. A branch is normally the segment
connecting two adjacent nodes (e.g. branch [AB]).
a
Branching occurs at
internal nodes, where branches bifurcate (in terms of organisms, it
denotes a speciation event). A series of successive branches that form a
lineal descent is called a lineage.
a
The term “branch” can also be used more loosely to identify a terminal subtree,
e.g. the branch formed by the set of nodes A, B, D, E and their interconnecting
branches (with D and E as terminal nodes).
