AP Syllabus focus:
‘An outgroup is the least closely related lineage; shared derived characters indicate common ancestry among groups.’
Outgroups and shared derived characters are tools for inferring evolutionary relationships. They help biologists root phylogenetic trees, distinguish ancestral from derived traits, and justify clades using evidence-based character comparisons.
Core ideas: outgroups, ingroups, and character states
Outgroup vs. ingroup
Outgroup: a lineage that diverged prior to the group of interest and is therefore the least closely related lineage used for comparison.
The ingroup is the set of taxa whose relationships you are trying to resolve. The outgroup is chosen to be close enough to allow meaningful comparisons, but outside the ingroup.
Ingroup: the taxa under study whose evolutionary relationships are being inferred relative to one another.
Character states: ancestral vs. derived
Phylogenetic inference often depends on determining whether a trait is ancestral (present before the ingroup diversified) or derived (evolved within the ingroup). The outgroup provides a reference point to polarize characters—i.e., infer the direction of evolutionary change.
If a character state is present in both the outgroup and some ingroup taxa, it is usually inferred to be ancestral for the ingroup.
If a character state is absent in the outgroup but shared by some ingroup taxa, it is often inferred to be derived within the ingroup.
Shared derived characters and common ancestry
Synapomorphies define clades
Shared derived character (synapomorphy): a derived trait shared by two or more taxa and inferred to have arisen in their most recent common ancestor.
The AP focus statement links synapomorphies to inference: shared derived characters indicate common ancestry among groups. In practice, synapomorphies are used to justify clades (monophyletic groups) on a tree.
This figure compares monophyletic groups (true clades) with polyphyletic and paraphyletic groupings that fail to capture a single ancestor plus all descendants. It helps students see why clade membership is evaluated by branching structure (shared ancestry), not by superficial similarity or leaving out descendant lineages. Source
A clade is supported when its members share one or more synapomorphies not found outside the clade.
Synapomorphies can be morphological, developmental, or molecular (e.g., specific DNA base changes), but the logic is the same: shared novelty implies shared ancestry.
Shared ancestral vs. shared derived
Not all shared traits are equally informative.
Shared ancestral character (plesiomorphy): a trait shared by taxa because it was present in a distant ancestor, not because it evolved in their most recent common ancestor.
Shared ancestral characters do not reliably define a clade within the ingroup because they can be widespread across many lineages.
Example logic (no worked example): if both the outgroup and multiple ingroup taxa have the trait, it may be ancestral and therefore weak evidence for a specific subgroup within the ingroup.
By contrast, a synapomorphy is a “new” trait relative to the outgroup, so it is stronger evidence for grouping.
How outgroups are used to root phylogenetic trees
Rooting and direction of evolution
An unrooted tree shows relationships without specifying which splits happened earlier. An outgroup is used to root the tree, identifying the basal branch and allowing interpretation of evolutionary sequence.
Rooting a phylogeny with an outgroup converts an unrooted network of relationships into a rooted tree with a defined direction of ancestry. The figure contrasts an unrooted layout, a rooted layout, and a rooted layout that includes the outgroup—highlighting that the root is placed on the branch leading to the outgroup, and the ingroup root is where the ingroup attaches. Source
The root is placed on the branch connecting the outgroup to the ingroup.
Once rooted, you can infer which character states likely appeared earlier (closer to the root) versus later (toward the tips).
Choosing an appropriate outgroup
Outgroup choice strongly affects character polarization and therefore clade support.
Use a lineage that is outside the ingroup but closely related enough to share homologous characters.
Avoid outgroups that are too distantly related, which can increase misinterpretation of similarity due to coincidental change.
When possible, include more than one outgroup taxon to strengthen inferences about ancestral states.
Common pitfalls: homoplasy and misleading similarity
Homoplasy can mimic shared ancestry
Homoplasy: similarity in traits that arises independently (e.g., convergent evolution or reversal) rather than from common ancestry.
Homoplasy can create apparent “shared derived” traits that are not truly synapomorphies.
This comparative figure shows that bird and bat wings share homologous bones (reflecting common ancestry of forelimb skeletal elements), while insect wings are superficially similar but structurally distinct. It visually reinforces that similarity can arise by convergence (homoplasy), so shared function alone is not sufficient evidence for a synapomorphy. Source
This is why phylogenetic hypotheses are strengthened when multiple independent characters support the same clade.
Convergence: unrelated lineages evolve similar solutions under similar conditions.
Reversal: a derived trait returns to an ancestral-like state, obscuring history.
Interpreting support
Clade support is strongest when:
the outgroup clearly indicates which state is ancestral,
multiple synapomorphies agree on the same grouping,
characters being compared are likely homologous rather than analogous.
FAQ
They look for a balance: distant enough to be outside the ingroup, but close enough that many characters are confidently homologous.
Signals of “too distant” include extensive missing/comparable characters and highly ambiguous alignments in sequence data.
Yes. Multiple outgroups can reveal that a trait seen in one outgroup is unusual, helping avoid misclassifying a state as ancestral.
This can stabilise rooting and character polarisation.
Rooting becomes misleading, and ancestral/derived assignments can be flipped.
This can incorrectly redefine clades because synapomorphies are identified relative to the chosen outgroup.
They can be single states or structured features, but they must be comparable and heritable.
In molecular datasets, a synapomorphy might be a specific nucleotide at a particular aligned position.
Common approaches include checking whether the same grouping is supported by many independent characters and inspecting whether a trait plausibly evolved multiple times.
High homoplasy often appears as conflicting character support across the dataset.
Practice Questions
Define an outgroup and state one reason an outgroup is used when constructing a phylogenetic tree. (1–3 marks)
Defines outgroup as a lineage least closely related/outside the ingroup used for comparison (1).
States it is used to root the tree and/or infer ancestral vs derived character states (1).
Mentions polarising character change direction (1).
Explain how shared derived characters are used to infer common ancestry within an ingroup, and describe two limitations that could lead to incorrect grouping. (4–6 marks)
Identifies shared derived characters (synapomorphies) as derived traits shared by taxa due to a most recent common ancestor (1).
Explains they are used to define clades/monophyletic groups within the ingroup (1).
Links outgroup comparison to identifying the derived state (1).
Limitation: homoplasy via convergence (1).
Limitation: character reversal or incorrect/outgroup too distant leading to wrong polarity (1).
Notes stronger inference when multiple independent characters support the same clade (1).
