Five subspecies currently are recognized (Folch 1992, Dickinson 2003), although Blake (1977) cautions that "the characters and ranges of the several races of americana are tentative":
Rhea americana americana widespread in Brazil, except for extreme south and southwest
Rhea americana intermedia southern Brazil (Rio Grande do Sul) and Uruguay
Rhea americana nobilis eastern Paraguay
Rhea americana araneipes western Paraguay, Bolivia, southwestern Brazil (southern Mato Grosso)
Rhea americana albescens northern Argentina
The complete mitochondrial DNA molecule contains 16,710 nucleotides, and its organization most resembles that of the ostrich and chicken (Sales 2006).
Rheas, ostriches, emus, cassowaries, and kiwis comprise the group of large flightless birds called ratites. The relationships between the different ratite groups long has been controversial. The distribution of ratites across southern land masses has suggested that the group originated from the large ancient paleocontinent Gondwana, which is believed to have broken into several of the land masses ocupying the Southern Hemisphere. Ratites often are considered to be monophyletic, paired with the sister group of flighted tinamous, which suggested a single loss of flight in the ratite ancestry. A recent phylogenetic analysis (Harshman et al. 2008) suggests tinamous should be counted among the ratites, making the group polyphyletic. This study proposes that the loss of flight evolved separately at least three times in ratites, and explains many of their morphological and behavioral similarities. It also calls for a reconsideration of the proposed link between ratite evolution and continental drift.
Divergences between the rhea and other members of the order Struthioniformes have been estimated as follows:
Greater Rhea/Lesser Rhea: 13.7 million years (Haddrath and Baker 2001).
rheas/emus: ca 45 million years (Harlid et al. 1998).
rheas/ostrich: ca51 million years (Harlid et al. 1998).
75-80 million years (Sibley and Ahlquist 1990).
The relationships of rheas within the ratites remains unresolved. Mitochondrial DNA sequencing of representative ratites suggests that rheas are grouped with emus while ostriches are basal (Van Tuinen et al. 1998). A separate sequence analysis suggests that rheas, not ostriches, are basal (Lee et al. 1997). Most molecular-based trees place the rhea as the most basal living ratite (Zelenitsky and Modesto 2003). Harshman et al. (2008) favor a topology in which rheas are sister to tinamous, although some results from their research suggest that rheas are basal to a clade of tinamous + Australasian ratites (kiwis, emus, and cassowaries).
Linnaeus described the Greater Rhea in 1758.
The Greater Rhea also is known as the Gray Rhea or the Common Rhea. In Spanish, it is called "Ñandú" or "Ñandú Común".