The guans are a number of bird genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 which make up the largest group in the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Cracidae

Cracidae

The chachalacas, guans and curassows are birds in the family Cracidae.These are species of tropical and subtropical Central and South America. One species, the Plain Chachalaca, just reaches southernmost Texas in the USA...

. They are found mainly in northern South America

South America

South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

, southern Central America

Central America

Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent...

, and a few adjacent Caribbean

Caribbean

The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...

 islands. There is also the peculiar Horned Guan

Horned Guan

The Horned Guan, Oreophasis derbianus is a large, approximately 85 cm long, turkey-like bird with glossed black upperparts plumage, red legs, white iris, yellow bill and a red horn on top of head. The breast and upper belly are white, and its long tail feathers are black with white band near...

 (Oreophasis derbianus) which is not a true guan, but a very distinct and ancient cracid with no close living relatives (Pereira et al. 2002).
The evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 of the group is fairly well resolved due to comprehensive analyses of morphology

Morphology (biology)

In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

, biogeography

Biogeography

Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...

, and mt and nDNA sequence

DNA sequence

The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...

s (Pereira et al. 2002, Grau et al. 2005).