The De-riving Force of Cladogenesis1
Andrew J Petto University of Wisconsin, Milwaukee, and
Editor, National Center for Science Education

Cladogenesis is the term used to describe the branching off of new taxa. These
branches - or clades - are based on several criteria which make the
descendants along a particular branch different from their ancestors and from
related taxa on other branches. Each new branch exhibits a combination of novel
characteristics that are unique to that branch mixed with some "familial"
characteristics which this branch shares with its evolutionary ancestors. Although
certain novel traits may be diagnostic for members of an evolving lineage, it is
often the combination of unique and shared characteristics that defines new
branches.

The basis of constructing a valid cladogram is the ability to identify the
characteristics of the ancestral population and those of the descendants
(http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_06). Characteristics
found among the ancestors and shared by most or all members of related taxa
are referred to as primitive. In cladistic studies this word is understood as
"original" or "primal" and not as "crude" or "simple". In order to avoid confusion,
some writers refer to these traits as conservative or simply ancestral. Shared,
conservative traits link the members of related branches to a common ancestor.
On the other hand, characteristics that are found in various evolutionary branches
that differ from those of the ancestors are considered derived. In many cases
these derived characteristics are unique modifications of widely shared ancestral
characteristics. Derived traits distinguish the members of one evolutionary branch
from the members of another branch.

A cladogram is constructed on these combinations of ancestral and derived
characteristics in related taxa by organizing and diagramming the pattern and
sequence in which they could have arisen. Ideally, we want a cladogram based on
branches defined by uniquely derived characters that emerge once in an evolving
lineage and are shared by all subsequent descendants. This helps us to test our
hypotheses about common descent in evolving lineages. A branch that includes
all the organisms descended from the same ancestral population is said to be
monophyletic.

Because living organisms are a complex combination of traits, however,
sometimes it is possible to draw more than one cladogram that might reflect the
evolutionary history of a group of organisms. There is a variety of methods that
researchers use to evaluate these options, and the appropriate choice depends on
the kinds of data available and the specific hypothesis to be tested. The goal,
however, is to find the tree that best explains the phylogenetic relationships
among the organisms included in the tree.

Two fundamental principles used in evaluating cladograms are parsimony
(http://evolution.berkeley.edu/evolibrary/article/0_0_0/phylogenetics_08) and
robusticity. First, when there is more than one way to draw a cladogram, and
when there are no other data that suggest one of these is more likely than the
others, we tend to choose the one in which derived traits are re-invented in
different branches the fewest number of times. Second, we prefer trees that
maintain their basic form, even when different options are applied to the
sequence of changes in one or more of their branches. However, when more data
are available about the history or the origin of a particular feature, these data are
more important tools in determining which of the alternative trees is better. In
contrast to exercises in mere classification, we want to base our taxonomy on the
cladogram. The guiding principle is that our taxa should be monophyletic.
Each evolutionary branch must contain all descendants of a common ancestor.
One of the chief criticisms against the "classical" taxonomy that places humans
on one branch of the hominoid family tree and the great apes (African apes and
the orang utan) on another is that this arrangement fails on the criterion of
monophyly. Based on fossil data, comparative anatomy, and molecular biology,
humans and African apes share a recent common ancestor and so a monophyletic
clade would include humans and African apes together. Any branch that combines
Asian apes (such as the orang utan) with African apes, but excludes the human
branch, is invalid because it does not include all the descendants of the common
ancestor of Asian and African apes (see http://tolweb.org/hominidae/16299).

There is, of course, a uniquely human clade containing all the hominins (species
of the genera Homo, Australopithecus, and Ardipithecus) descended from the first
upright walkers among the African apes; however, no clade that excludes humans
but includes African and Asian groups is phylogenetically valid because it fails on
the basic criterion of monophyly: it must include the most recent common ancestor of all
the organisms in the tree and all the descendants of that most recent common
ancestor.

Fossil data help to refine cladistic analysis by providing information about the
sequence or order in which certain derived traits emerged. Cladistic analysis helps
to resolve the "problem" of the so-called "missing links" or the intermediate
specimens, because it does not require that fossil species evolve into any related
species which emerge later. Instead, it represents the evolutionary history of an
evolving lineage in terms of a collection of characteristics which can be passed
along to descendant populations - or not!
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1This explanation of cladogenesis, cladistics and cladograms was first published in
the Reports of the National Center for Science Education, May/June 1999, Vol. 19,
No. 3, page 13. Printed here with the kind permission (and some clarifying
adjustments) from the author. We encourage you to join the NCSE (US $30),
support its program of protecting the integrity of science education, and receive
its always interesting journal (6 issues per year): http://www.natcenscied.org/.

Andrew J Petto, PhD, Editor
National Center for Science Education
editor@ncseweb.org
and
Senior Lecturer
Department of Biological Sciences
University of Wisconsin­Milwaukee
PO Box 413
Milwaukee WI 53201-0413
ajpetto@uwm.edu
http://www.uwm.edu/~ajpetto