Large Flightless Birds on Isolated Continents and Islands
Scientists have discovered that many species of flightless birds of the infraclass Palaeognathae lost their ability to fly independently of one another. The ancestors of the ostrich evolved into large flightless birds in Africa, the ancestors of the emu and cassowary did the same in Australia, and so forth.
These flightless birds are traditionally known as ratites. The ratites are simply the flightless members of the infraclass Palaeognathae. Palaeognathae includes the ratites: the African ostriches, the South American rheas, the Australian emu and cassowaries, the New Zealand kiwis and extinct moas, and the extinct elephant birds of Madagascar. Also in Palaeognathae are the various South American species of tinamous that do fly, but usually only for very short distances.
Previously it had been believed that all these birds but the various species of tinamous had evolved from a flightless bird on the southern continent of Gondwana, also called Gondwanaland. When Gondwana broke up forming Africa, South America, Australia, Madagascar, New Zealand, and other land masses the different flightless birds evolved into their present species and the recently extinct moas and elephant birds. Both the moas and elephant birds were driven into extinction by humans in the past few thousand years.
New ResearchRecent research published in the Journal Science by Kieren J. Mitchell, Bastien Llamas, Julien Soubrier, Nicolas J. Rawlence1, Trevor H. Worthy, Jamie Wood, Michael S. Y. Lee1, and Alan Cooper argues that the ancestors of these flightless birds flew into their separate land masses after Gondwana broke up and independently evolved into large flightless forms.
Not everyone will be able to read more than the abstract on the Science site. I believe unpaid access is limited to members of the AAAS, American Association for the Advancement of Science. For the intelligent layman you can read an article posted by NPR, National Public Radio here.
Why did this happen? A Just So StoryAs mentioned above, all these flightless birds and the several species of tinamous are the surviving members of the infraclass Palaeognathae. These species have since the days of Darwin been recognized as a group that split off early from the other birds. They have features that are more reptile-like than the rest of the birds.
All other birds are in Neognathae. So all the other birds are more closely related to each other than they are to the species in the infraclass Palaeognathae. For example, the hummingbird, robin, chicken, hawk, parrot, and penguin are all more closely related to one another than they are to the ostrich.
Close Relatives Can Infect YouWhy is this important? The more closely related two species are the more communicable diseases they tend to have in common. For example, one of our two closest relatives, the chimpanzee, shares every communicable disease with us, and the other great apes share many. The great apes other than humans are in danger of extinction because the huge human population is fertile ground for new diseases to evolve and those diseases usually infect the great apes also.
Similarly, the closest relatives of the white-tailed deer in North America, the larger deer, for example, the elk and moose, find it difficult or impossible to survive in the white-tailed deer's range because it carries diseases that kill those larger deer, but not the white-tailed deer. (Note to Europeans, what Americans call a moose, you call an elk, and what Americans call an elk is closely related to the red deer.)
Relatives Particularly Dangerous When NumerousSo having close relatives in your environment can be a bad thing, especially when they are numerous. A large population will tend to have more diseases than a small population because it provides more food for the pathogens to live on. For example, humans are a food for the bacteria that causes strep throat.
But the large population has more individuals, any one of which might through mutation gain an immunity that allowed the animal to live with the pathogen. This animal will have more descendants and eventually the whole population will enjoy the immunity. The closely related species with a smaller population suffers from an increasing variety of pathogens but does not have the advantage of more individuals to evolve an immunity. This species will frequently be driven into extinction within the range of their more numerous relative. As mentioned above elk and moose have difficulty surviving in the range of the smaller and more numerous white-tailed deer.
The Danger of Immune CarriersSurvival of the larger species is particularly difficult when the smaller species carries the disease without being seriously harmed by it. In this case, the pathogen can become a symbiont. The animal that is immune but carries the disease gains the advantage that the disease kills competitors for its food supply, and perhaps even predators. The immune carrier can become essentially poisonous to predators.
Opportunity For the More Distantly RelatedThis creates opportunities for groups that are not so closely related to the immune carrier and therefore less likely to share the same diseases. The members of Palaeognathae, ostriches, emus, etc. are by definition not susceptible to diseases that are specific to Neognathae. As this is the case they have an advantage in filling those niches that will support relatively few individuals, for example, large birds. If the birds are large enough they will not be able to fly, so this means that the primitive and therefore rare Palaeognathae have an advantage over Neognathae in filling the large, flightless bird niche.
The Advantage of Losing FlightThis advantage became even greater when the declining Palaeognathae group no longer included long distance flying birds that could reach the isolated land masses: Africa, South America, Australia, Madagascar, and New Zealand. When this happened the birds of the Palaeognathae group on a small land mass were cut off from any Palaeognathae specific diseases circulating among Palaeognathae populations in the larger world. If a disease evolved among the African Ostriches that was specific to the Palaeognathae, it had no way of reaching the Australian emus and cassowaries, the Madagascar elephant birds, the South American rheas and titamous, or the New Zealand moas and kiwis. The various species of titamous do fly, but only for short distances, so it can not carry Palaeognathae specific diseases out of South America.
So the Palaeognathae independently took over and monopolized the large flightless bird niche in Africa, South America, Australia, Madagascar, and New Zealand, because their competition the Neognathae were constantly flying in with new plagues that had evolved around the world. Some of those plagues infected and killed both Palaeognathae and Neognathae, giving little advantage to either group, but others only infected and killed the Neognathae. This was because the birds that carried the diseases were Neognathae. No species of Palaeognathae was capable of flying into the isolated land masses.
Large Species, Few RelativesIsolation from relatives is particularly important for physically large species. Small species frequently have far larger populations, and shorter life cycles, so they can develop immunities to disease more quickly. Many physically large species are living fossils, the last of their kind, at least within their range. There are many examples of this phenomena.
For example, the largest trees Sequoias and coastal redwoods until recently had no relatives in their natural range with a common ancestor more recent than the Mesozoic. Like the ratites, they are part of an ancient lineage that had largely died out. In the ocean, the largest fish are the sharks, which are living fossils. In South America, the largest exclusively freshwater fish is another living fossil, the arapaima. In North America, the largest exclusively freshwater fish is still another living fossil, the alligator gar. Jumping from fish to spiders, the tarantulas, the largest spiders, are the second earliest branch off the spider family tree. This is but a small sampling of the examples that can be drawn from our present flora and fauna and natural history. Similarly, the largest birds, the ratites, have relatively few close relatives in their environment compared to the Neognathae. So it is not surprising that they are the largest.
Using the above analysis based on the spread of communicable diseases the mysterious independent development of large flightless birds of the infraclass Palaeognathae is not so mysterious. It is exactly what we should expect.