In cladistic analysis, what does "outgroup" refer to?

In cladistic analysis, the term "outgroup" refers to a reference group used for comparison. An outgroup is a taxon that is closely related to the ingroup (the main group under study) but is not a part of it. The purpose of including an outgroup is to help determine the characteristics that are derived (evolved) within the ingroup compared to the outgroup, which is usually assumed to share the ancestral traits. By examining both groups, researchers can better understand evolutionary relationships and the characteristics that are unique to the ingroup. This comparative approach helps clarify how specific traits evolved and assists in constructing accurate phylogenetic trees. Using an outgroup effectively delineates which traits are ancestral and which are derived, allowing for a clearer interpretation of the evolutionary history of the organisms being studied.