Character Polarity and Outgroup Comparison

Determining character polarity establishes which state of a character is ancestral and which is derived, a prerequisite for recognizing the synapomorphies that define clades.

Definition

Character polarity is the inferred direction of evolutionary transformation between states of a character; outgroup comparison infers the ancestral state as the one present in close relatives outside the group under study.

Scope

This topic covers the concept of polarity (ancestral versus derived states), the outgroup comparison method as the primary means of inferring polarity, the role of multiple and successive outgroups, and alternative or historical criteria such as ontogenetic and paleontological evidence.

Core questions

• Why must character states be polarized before they can group taxa?
• How does outgroup comparison infer the ancestral state?
• What considerations guide the choice of outgroups?
• What other criteria have been proposed for determining polarity?

Key theories

Outgroup criterion

The state found in appropriate outgroups is inferred to be ancestral for the ingroup, so the alternative state shared within the ingroup is derived and can serve as a synapomorphy.

Ontogenetic criterion

Hennig and others proposed that the more general, earlier-appearing developmental state is ancestral, offering an additional, if debated, line of polarity evidence.

Clinical relevance

Correct polarity is the difference between recognizing a genuine evolutionary novelty and mistaking a retained ancestral trait for evidence of relationship, which matters wherever inferred relationships guide downstream biological decisions.

History

Polarity assessment was central to Hennig's argument-scheme; outgroup comparison emerged as the dominant operational criterion, largely supplanting the more contentious ontogenetic and paleontological criteria as explicit algorithms and rooting procedures were developed.

Debates

Relative reliability of polarity criteria

Outgroup comparison is the standard, but disputes persist over the use of ontogeny and fossils to determine polarity, and over how to handle conflicting outgroup signals.

Key figures

• Willi Hennig
• E. O. Wiley

Seminal works

• hennig1966
• wiley2011
• schuh2009

Frequently asked questions

What is the difference between an ancestral and a derived character state?

An ancestral (plesiomorphic) state is the older condition inherited from earlier ancestors, while a derived (apomorphic) state is a newer modification; only shared derived states group taxa.

How is an outgroup chosen?

An outgroup is a taxon known to fall outside but reasonably close to the group of interest, so its character states can stand in for the ancestral condition of that group.