The flightless birds known as ratites are oddballs of evolution. Research using phylogenomic analysis and other methods has so far failed to result in a clear picture of their phylogeny.
Ratites are large, flightless birds found on different continents throughout the Southern Hemisphere. Their ranks include the ostriches of Africa, the kiwis and extinct moas of New Zealand, the emus and cassowaries of Australia, the rheas of South America, and the extinct elephant birds of Madagascar.
The leading hypothesis on ratite phylogeny is that they are a monophyletic group, meaning they share a common ancestor. Because all ratites are flightless yet they are found on so many continents, biologists have proposed that flight loss occurred long ago, before the ancient supercontinent Gondwanaland broke up. According to this hypothesis, the closest sister group to ratites is the tinamous, a group of ground-dwelling, flighted birds from South America.
Support for Ratite Monophyly
A 2001 phylogenetic analysis of ratite mitochondrial DNA, conducted using the principles of cladistics, found that the ratites are indeed a monophyletic group (Haddrath & Baker, 2001). The researchers in this study took the complete mitochondrial genome from two preserved moa specimens, five other ratites species, and two tinamou species. This analysis resulted in a phylogeny tree in which tinamous are indeed the closest sister group to ratites. In order from most basal to most derived, the ratite groups are moas, rheas, ostriches, kiwis, emus, and cassowaries, with no nested subtrees.
Under this view, ratite speciation would have occurred through biogeographic vicariance. As Gondwanaland broke up, populations of the last common ancestor of the ratites were physically isolated and evolved independently. This last common ancestor is assumed to have been flightless, with flight loss occurring once after the divergence between the tinamou and ratite lineages.
Ratite Monophyly Hypothesis
Ratite Polyphyly?
A recently published cladistics study looked not at the mitochondrial genome, but at various unlinked genes from nuclear DNA of ratites and tinamous (Harshman, Braun, Braun, et al., 2008). The researchers were unable to reconcile their results with the monophyly hypothesis of ratite phylogeny. Their results indicate that the last common ancestor of ratites was flighted, that tinamous are within the ratite phylogeny tree, and that flight loss must have occurred at least three different times in different ratite lineages. According to this analysis, ratites are a polyphyletic group, meaning they cannot be placed nicely into their own phylogeny tree.
Ratites share many similarities in appearance and in behavior. This new hypothesis explains similarities as the result of convergent or parallel evolution, not common ancestry.
The phylogeny of ratites continues to be an area of research for biologists. Despite studies using biogeography, morphology, mitochondrial DNA, and nuclear DNA, and taking full advantage of the science of cladistics, evolutionary biologists have still not reached a consensus on how these fascinating birds evolved.