If you were using cladistics to construct a phylogenetic tree of birds, what is the best outgroup?

Answer: In cladistics, an outgroup is a species or group of species that is closely related to but not part of the group of interest (the ingroup). The outgroup provides a reference point for the ingroup to help determine which characteristics are ancestral and which are derived within the group. When constructing a phylogenetic tree of birds, the best outgroup would be a species or group that shares a common ancestor with birds but diverged from their lineage before the common ancestor of all modern birds.

The most appropriate outgroup for birds (class Aves) would likely be the crocodilians (family Crocodylidae), such as crocodiles and alligators. Birds and crocodilians both belong to the larger group Archosauria, and together they form a clade known as the Archosauriformes. Crocodilians are the closest living relatives to birds, and their divergence point from the bird lineage occurred before the last common ancestor of all existing birds. Using the crocodilians as an outgroup can help identify which traits in birds are uniquely avian and which are shared with their closest relatives, giving insight into the evolutionary history of birds.