ANT333 Lecture #14 – Cladistics & Phylogenetic Systematics
Method classifies taxa into groups called clades, which consist of ancestor organism and all
All characters shared between taxa can be divided into three types:
1. Homologies & homoplasies,
2. Shared ancestral homologies, and
3. Shared derived homologies.
Homoplasies and Homologies
Homoplasy (bad) is a convergent character - one that is shared between species but that was
not present in their common ancestor.
Homology (more helpful) is character shared between species that was also present in their
Shared ancestral homologies (symplesiomorphy - bad): found in ancestor of a group of species
but only in some of its descendants. Not what we want.
Shared derived homologies (synapomorphy - good): unique to a particular group of species (and
their ancestor). Only shared derived homologies indicate phylogenetic groups.
Only shared derived characters gives information about phylogeny (cladistics or phylogenetic
Taxa that share many derived characters are grouped more closely together than those that do
Relationships are shown in branching hierarchical tree (cladogram).
Want to use as many diverse traits as possible
Cladogram: A branching diagram depicting hierarchical arrangement of taxa defined by
distribution of shared derived characters.
Cladogram constructed such that number of changes from one character state to next is
Principle behind this is rule of parsimony - any hypothesis that requires fewer assumptions is a
more defensible hypothesis.
Character: A feature or thing we can examine or label. It is important that feature be heritable.
Homologous Character: Character which is shared by taxa by descent.
Analogous Character: Shared resemblance between characters by other means than descent,
such as adaptation. This is often called a homoplastic character.
Shared Character: A character shared by all members of group
1 Plesiomorphy: A character which is in state shared by common ancestor of group. (primitive)
Apomorphy: A character in a new state, not primitive one.
Synapomorphy: A character which is in a new state and shared by all member of group. – helps
Autapomorphy: A character found only in that taxon. Is a synapomorphy when discussed at level
of members of a taxon. – links members of different taxa
Some Other Key Definitions
Out Group: A group outside groups in question which is used to define polarity of character
transformations (primitive to derived). VERY IMPORTANT!
Sister Group: A monophyletic group more closely related to group under examination than any
other group. (side by side in tree)
Ancestor Problem: All possible ancestors are regarded as sister groups.
A group containing an ancestor and all of its decedents. Defined by one or more
A group consisting of an ancestor but not all of its descendants. It is defined by what it does not
have. Difficult to deal with.
A group that does not include common ancestor of group. Common ancestor is place in another
group. Worst one.
It’s All About Tree Topology!
Tree can be flipped or rotated as long as relationships aren’t broken
Trees are NOT Ladders!
No direction in phylogeny
Consists of interdependent but distinct steps
1. Taxa to be classified are chosen.
2. Characteristics to provide evidence for relationship are chosen.
3. Characteristics are analyzed to reconstruct relationship among taxa