Unit content
Shared derived characters, outgroups and clade diagnosis
Phylogenetic inference compares character states: alternative forms of a heritable feature.
A derived character state is one that evolved after divergence from an earlier ancestral state. A shared derived character, or synapomorphy, is a derived state inherited by multiple lineages from their most recent common ancestor. Synapomorphies are especially useful for identifying clades.
To decide which state is ancestral and which is derived, biologists often compare the focal group—the ingroup—with an outgroup, a related lineage known to have diverged earlier.
Example
Suppose species A, B and C are the ingroup and species O is an outgroup. For one character:
| Species | Character state |
|---|---|
| O | 0 |
| A | 0 |
| B | 1 |
| C | 1 |
If state 0 is present in the outgroup, state 1 is reasonably inferred to be derived within the ingroup. Its presence in both B and C supports a clade containing B and C.
Not every shared character is phylogenetically informative. A state inherited from a much older ancestor may be shared broadly and fail to distinguish relationships within the ingroup.
A monophyletic group, or clade, contains a common ancestor and all of its descendants. Synapomorphies diagnose such groups because they mark evolutionary changes that occurred on ancestral branches.
The logic is therefore:
$$\boxed{\text{outgroup comparison} \rightarrow \text{character polarity} \rightarrow \text{shared derived states} \rightarrow \text{clade hypothesis}.}$$