Galliformes - Systematics and Evolution

Systematics and Evolution

The living Galliformes were long divided into some 7 or more families. More recently, it is generally believed that the quite distinctly appearing grouse and turkeys do not warrant separation as families due to their quite recent origin from partridge- or pheasant-like birds. As it seems, the turkeys became huge after their ancestors colonized temperate and subtropical North America where pheasant-sized competitors were absent. The ancestors of grouse, on the other hand, adapted to harsh climate and thus grouse could colonize subarctic regions. Consequently the Phasianidae are expanded in current treatments to include the former Tetraonidae and Meleagrididae as subfamilies.

The Anseriformes (waterfowl) and the Galliformes together make up the Galloanserae. They are basal among the living neognathous birds, and normally follow the Paleognathae (ratites and tinamous) in modern bird classification systems. This was first proposed in the Sibley-Ahlquist taxonomy and has been the one major change of that proposed scheme that was almost universally adopted. On the other hand, the Galliformes as they were traditionally delimited are called Gallomorphae in the Sibley-Ahlquist taxonomy, which splits the Cracidae and Megapodidae as an order "Craciformes". This is not a natural group however, but rather an erroneous result of the now-obsolete phenetic methodology employed in the Sibley-Ahlquist taxonomy. Phenetic studies do not distinguish between plesiomorphic and apomorphic characters, which leads to basal lineages appearing as monophyletic groups.

Historically, the buttonquails (Turnicidae), mesites (Mesitornithidae) and the Hoatzin (Opisthocomus hoazin) were placed in the Galliformes too. The former are now known to be shorebirds adapted to an inland lifestyle, whereas the mesites are probably closely related to pigeons & doves. The relationships of the Hoatzin are entirely obscure, and it is usually treated as a monotypic order Opisthocomiformes to signify this.