They are quoted as example for discontinuous distribution. This discontinuity is believed to have happened millions of years ago due to continental drift.
It is believed by scientists that the most flightless birds evolved in the absence of predators on islands and lost the power of flight, because they had few enemies, although this is likely not the case for ostrich, emu and cassowary as all have claws on their feet/wings to use as a weapon against predators.
Many ratites live on isolated oceanic islands where there are no predators. Structurally these birds’ sternum (breast bone) is without keel to which flight muscles could be anchored.
All species of ratites are thus unable to fly. They have more feathers than flying birds. The wings of these birds may be considered as the vestigial organs like pinna andappendices of human beings. New Zealand has more species of ratites like kiwi and penguin. One reasonis until the arrival of humans about 1,000 years ago there were no largeland predators in this island.
But yet we are not able to answer this question as ratites are also found in South America and South Africa where predators are common.
Studies by different authors on the phylogeny and biogeography of ratites inferred from DNA sequences do not reveal any clue. Alistair Dawson put forth a theory that these ratites are neotenous descendents of flying birds as Axolotyl larva of Salamander ( Amphibia).
That is, the young birds of flying birds ( Neognathae) that have attained sexual maturity even before the body growth is completed. Why these ratitese volved in this way is open to speculation.
Source: The Hindu
