Editing Ratite (section)

== Evolution ==

The longstanding story of ratite evolution was that they share a common flightless ancestor that lived in [[Gondwana]], whose descendants were isolated from each other by [[continental drift]], which carried them to their present locations. Supporting this idea, some studies based on morphology, immunology and DNA sequencing reported that ratites are [[monophyletic]].<ref name ="Haddrath, O & Baker, A (2001)"/><ref>{{Cite journal |last=Roff |first=Derek A. |year=1994 |title=The evolution of flightlessness: Is history important? |url=http://link.springer.com/10.1007/BF01237847 |journal=[[Evolutionary Ecology]] |lang=en |volume=8 |issue=6 |pages=639–657 |doi=10.1007/BF01237847 |bibcode=1994EvEco...8..639R |s2cid=13524994 |issn=0269-7653|url-access=subscription }}</ref> Cracraft's 1974 biogeographic vicariance hypothesis suggested that ancestral flightless paleognaths, the ancestors of ratites, were present and widespread in Gondwana during the Late Cretaceous. As the supercontinent fragmented due to [[plate tectonics]], they were carried by plate movements to their current positions and evolved into the species present today.<ref name=Cracraft/> The earliest known ratite fossils date to the [[Paleocene]] epoch about 56 million years ago (e.g., ''[[Diogenornis]]'', a possible early relative of the rhea).<ref name = Laurinetal2012 /> However, more primitive [[paleognathae|paleognaths]] are known from several million years earlier,<ref name = Leonardetal2005 /> and the classification and membership of the Ratitae itself is uncertain. Some of the earliest ratites occur in Europe.<ref name=Buffetaut2014/>

Recent analyses of genetic variation between the ratites do not support this simple picture. The ratites may have diverged from one another too recently to share a common Gondwanan ancestor. Also, the Middle Eocene ratites such as ''[[Palaeotis]]'' and ''[[Remiornis]]'' from Central Europe may imply that the "out-of-Gondwana" hypothesis is oversimplified.

Molecular phylogenies of the ratites have generally placed ostriches in the [[Basal (phylogenetics)|basal]] position and among extant ratites, placed rheas in the second most basal position, with Australo-Pacific ratites splitting up last; they have also shown that both the latter groups are monophyletic.<ref name = "Harshman" /><ref name = "Mitchell2014" /><ref name = Baker2014/> Early mitochondrial genetic studies that failed to make ostriches basal<ref name = "Haddrath, O & Baker, A (2001)"/><ref name = Cooper/> were apparently compromised by the combination of rapid early radiation of the group and long terminal branches.<ref name = Baker2014/> A morphological analysis that created a basal New Zealand clade<ref name = Bourdon/> has not been corroborated by molecular studies. A 2008 study of nuclear genes shows ostriches branching first, followed by rheas and tinamous, then kiwi splitting from emus and cassowaries.<ref name = Harshman/> In more recent studies, moas and tinamous were shown to be [[sister group]]s,<ref name = Phillips2010/><ref name = Allentoft2012/><ref name = Baker2014/>  and elephant birds were shown to be most closely related to the New Zealand kiwi.<ref name = Mitchell2014/> Additional support for the latter relationship was obtained from morphological analysis.<ref name = Mitchell2014/>

The finding that tinamous nest within this group, originally based on twenty nuclear genes<ref name = "Harshman" /> and corroborated by a study using forty novel nuclear loci<ref name = Smith2013/> makes 'ratites' [[Polyphyly|polyphyletic]] rather than monophyletic, if we exclude the tinamous.<ref name = Hackett/><ref name=Sackton/> Since tinamous are weak fliers, this raises interesting questions about the evolution of flightlessness in this group. The branching of the tinamous within the ratite radiation suggests flightlessness [[parallel evolution|evolved independently]] among ratites at least three times.<ref name =Harshman/><ref name = Holmes/><ref name=Sackton/> More recent evidence suggests this happened at least six times, or once in each major ratite lineage.<ref name = "Mitchell2014" /><ref name="Sackton"/> Re-evolution of flight in the tinamous would be an alternative explanation, but such a development is without precedent in avian history, while loss of flight is commonplace.<ref name = Harshman/><ref name="Sackton"/>

{{cladogram
|title= [[Cladogram]] based on Mitchell ''et al.'' (2014)<ref name = Mitchell2014/><br/> and Yonezawa ''et al.'' (2016)<ref name=Yonezawa2017/>
|clades={{Clade
   |style=width:35em;
   |label1=recent [[paleognath]]s
   |1={{Clade
       |1={{clade
          |label1=
          |1=[[Struthionidae]] (ostriches, 2&nbsp;spp.)
          }}
       |label2=
       |2={{Clade
          |1={{clade
               |label1=
               |1=[[Rheidae]] (rheas, 2~3&nbsp;spp.)
               }}
          |label2=
          |2={{Clade
             |1={{clade
                |label1=
                |1={{Clade
                   |label1=
                   |1=†[[Dinornithiformes]] (moa)
                   |2=[[Tinamidae]] (tinamous, 46&nbsp;spp.)
                   }}
                }}
             |label2=
             |2={{Clade
                |label1=
                |1={{Clade
                   |label1=
                   |1=†[[Aepyornithidae]] (elephant birds)
                   |2=[[Apterygidae]] (kiwi, 5&nbsp;spp.)
                   }}
                   |2={{Clade
                      |label1=
                      |1=[[Casuariidae]] (cassowaries, 3&nbsp;spp.)
                      |2=[[Dromaiidae]] (emus, 1&nbsp;sp.)
                      }}
                }}
            }}

            }}

        }}
     }}
 }}

By 2014, a mitochondrial DNA phylogeny including fossil members placed ostriches on the [[Basal (phylogenetics)|basal]] branch, followed by rheas, then a clade consisting of moas and tinamous, followed by the final two branches: a clade of emus plus cassowaries and one of elephant birds plus kiwis.<ref name = Mitchell2014/>

[[vicariance|Vicariant speciation]] based on the [[plate tectonics|plate tectonic]] split-up of Gondwana followed by continental drift would predict that the deepest phylogenetic split would be between African and all other ratites, followed by a split between South American and Australo-Pacific ratites, roughly as observed. However, the elephant bird–kiwi relation appears to require dispersal across oceans by flight,<ref name = Mitchell2014/> as apparently does the colonization of New Zealand by the moa and possibly the back-dispersal of tinamous to South America, if the latter occurred.<ref name = Phillips2010/> The phylogeny as a whole suggests not only multiple independent origins of flightlessness, but also of gigantism (at least five times).<ref name = "Mitchell2014" /> [[Cope's rule|Gigantism]] in birds tends to be [[Island gigantism|insular]]; however, a ten-million-year-long window of opportunity for evolution of avian gigantism on continents may have existed following the [[Cretaceous–Paleogene extinction event|extinction of the non-avian dinosaurs]], in which ratites were able to fill vacant herbivorous niches before mammals attained large size.<ref name = Mitchell2014/> Some authorities, though, have been skeptical of the new findings and conclusions.<ref name = Zimmer2014/>

Kiwi and tinamous are the only palaeognath lineages not to evolve gigantism, perhaps because of competitive exclusion by giant ratites already present on New Zealand and South America when they arrived or arose.<ref name = "Mitchell2014" /> The fact that New Zealand has been the only land mass to recently support two major lineages of flightless ratites may reflect the near total absence of native mammals, which allowed kiwi to occupy a mammal-like nocturnal [[Ecological niche|niche]].<ref name="Le DucRenaud2015"/> However, various other landmasses such as South America and Europe have supported multiple lineages of flightless ratites that evolved independently, undermining this competitive exclusion hypothesis.<ref name=Agnolin2016/>

Most recently, studies on genetic and morphological divergence and fossil distribution show that paleognaths as a whole probably had an origin in the northern hemisphere. Early Cenozoic northern hemisphere paleognaths such as ''[[Lithornis]]'', ''[[Pseudocrypturus]]'', ''[[Paracathartes]]'' and ''[[Palaeotis]]'' appear to be the most basal members of the clade.<ref name=Yonezawa2017/> The various ratite lineages were probably descended from flying ancestors that independently colonised South America and Africa from the north, probably initially in South America. From South America, they could have traveled overland to Australia via Antarctica,<ref name = Tambussi1994/> (by the same route marsupials are thought to have used to reach Australia<ref name = Nilsson2010/>) and then reached New Zealand and Madagascar via "sweepstakes" dispersals (rare low probability dispersal methods, such as long distance rafting) across the oceans. Gigantism would have evolved subsequent to trans-oceanic dispersals.<ref name=Yonezawa2017/>

===Loss of flight===

Loss of flight allows birds to eliminate the costs of maintaining various flight-enabling adaptations like high [[Bird anatomy#Muscular system|pectoral muscle]] mass, hollow bones and a light build, et cetera.<ref name = Mcnab1994 /> The basal metabolic rate of flighted species is much higher than that of flightless terrestrial birds.<ref name="Cubo"/> But energetic efficiency can only help explain the loss of flight when the benefits of flying are not critical to survival.

Research on flightless rails indicates the flightless condition evolved in the absence of predators.<ref name=Mcnab2006/> This shows flight to be generally necessary for survival and dispersal in birds.<ref name=Diamond1991/> In apparent contradiction to this, many landmasses occupied by ratites are also inhabited by predatory mammals.<ref name=Mitchell2014/> However, the [[K–Pg extinction event]] created a window of time with large predators absent that may have allowed the ancestors of extant flightless ratites to evolve flightlessness. They subsequently underwent selection for large size.<ref name=Phillips2010/> One hypothesis suggests that as predation pressure decreases on islands with low raptor species richness and no mammalian predators, the need for large, powerful flight muscles that make for a quick escape decreases. Moreover, raptor species tend to become generalist predators on islands with low species richness, as opposed to specializing in the predation of birds. An increase in leg size compensates for a reduction in wing length in insular birds that have not lost flight by providing a longer lever to increase force generated during the thrust that initiates takeoff.<ref name = Wright2016/>