Relative apparent synapomorphy analysis, or RASA, is a method that aims to determine whether a given character is shared between

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

due to

shared ancestry Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...

or due to

convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen *Convergence (comics), "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that ...

. A

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...

is a shared trait found among two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. RASA assigns a score to the character based on its potential to be informative.

Limitations

The method performs poorly when used to select an outgroup taxon, to quantify the amount of

phylogenetic signal Phylogenetic signal is an evolutionary and ecological term, that describes the tendency or the pattern of related biological species to resemble each other more than any other species that is randomly picked from the same phylogenetic tree. Cha ...

present, or to identify taxa that may be prone to

long branch attraction In phylogenetics, long branch attraction (LBA) is a form of systematic error whereby distantly related lineages are incorrectly inferred to be closely related. LBA arises when the amount of molecular or morphological change accumulated within a lin ...

References

Taxonomy (biology) {{biology-stub