The debate is always “why feathers unless you could already fly?”
The answer is that feathers are not a half step for flight, but simply one of the many body coverings land and sea creatures luck into.
I find it hard to believe that there were so many cases of birds “losing” the ability to fly.
A better guess would be that flightless birds simply came before the others and diversified early.
Penguin, ostrich, rhea, moa, dodo, kiwi…
Why propose a lot of separate parallel evolutionary “losses” instead of a simple common ancestry. Especially when their habitats are connected by the Southern continent of past eras, and we still have Australia as the home of several missing links in the mammal lineage.
Why is it so hard to believe that there where so many cases of birds losing their ability to fly? It is certainly easier to ‘forget’ how to fly than it is to learn, yet three different vertebrate groups managed to evolve the ability to fly!
The biggest problem with flightlessness among birds is that the fossil record of birds is sparse. The general concensus currently is that most of these birds (the ‘ratites’ - ostriches, emus, cassowaries, moas, elephantbirds, kiwis, etc.) share a common ancestor. Thus, it is only necessary for a single group to have lost the ability to fly, and this characteristic (or, more properly, lack thereof) would then be passed to subsequent lineages. The problem, of course, is that there really is no fossil evidence for the linking of ratite orders to other avian orders. There is also, unfortunately, no strong evidence that they are most closely related to each other than to other birds.
So, the sad truth is, we just don’t know right now. It is certainly possible that the original ratites may have branched off of the main bird line early on. The problem is, ratite fossils are only known from about 5.5 million years ago, indicating a much more recent evolution (the records for flying birds, of course, spans over 144 million years). The simplest explanation right now is that these birds share a common ancestry, and the common ancestor probably lost its flight ability somewhere around the Eocene.
I suggest you read the chapter “To Fly or Not to Fly” in the book The Life of Birds by David Attenborough.
You need to introduce evidence to support your proposal.
People have spent their lives working out these evolutionary relationships. You have spent five minutes typing an electronic bulleten board post. Think about that.
Flight comes at great cost, biologically speaking, and a bird that lost the ability to fly would no longer need to pay those costs. First: Hollow, fragile bones. When a flying bird breaks a bone, it shatters, and never heals. Usually, this is a death sentence for the bird. Presumably, the flightless birds can afford heavier, more durable bones. Second: An extremely fast metabolism, to produce the energy needed for flight. In times of famine, a flying bird will starve to death faster than a grounded bird of the same mass. Third: General complexity. When there are no stong external evolutionary pressures, simpler organisms tend to be favored over more complex ones. As an example, the appendix is steadily becoming shorter among humans, now that it no longer serves a useful purpose: A short appendix is simpler than a long one.
If a bird colonized a new area where there was not a strong evolutionary advantage to flight (due to fewer predators, perhaps), the ability would probably be lost relatively quickly, evolutionarily speaking.
Chronos is mostly correct. The lack of predators is one of the prime contending hypotheses. This is all explained in The Life of Birds by Davis Attenborough in much greater detail that this board can muster.
As usual, Chronos and Wood Thrush have covered it before I got here. Let me just add one thing:
A professor of mine once put it this way: start with a reptile, and modify it the very least amount possible to get efficent flight, and you will have a bird. A feather is nothing more than a scale with a bunch of slits in it, for instance - needed for weight, aerodynamics, and insulation.
I was going to use that to make a point here, but I forgot what it was. Anyway, FWIW.
I must qualify the following by acknowledging that my source is 6 years old, and given the rate of change in avian cladistics, I may now be completely off base. If I am, I trust someone will let me know.
Most of the flightless birds are classified in the Superorder Paleognathae, which includes ostriches, elephantbirds, kiwis, moas, emus, and cassowaries (and others). These birds are characterized by reduced wings, strong & large legs, and solid & bony palates. The habitats of these birds frequently lacked predators that would have been extremely dangerous to birds without the refuge of flight (until humans & our spread of exotic organisms like snakes, dogs, etc.). The Paleognathae still exhibit the broad sternum and vestiges of wings that indicate a flying ancestry.
Notably, penguins are classified in the Superorder Neognathae and are believed to be far more closely related to loons and grebes than to other flightless birds. Penguins really do fly; it’s just that they do so in a medium far more dense and viscous than air. I’m not aware of any evidence that would place penguins in the Paleognathae.
I don’t have much to contribute on the timing of divergence between these groups, but the other superorder in the Neornithes is the Odontognathae (various Cretaceous genera that retained teeth)… perhaps the divergence could then be traced to the Cretaceous (145-65 mya)?
Sources:
Walker, W. and Liem, K. 1994. Functional Anatomy of the Vertebrates. Saunders College Publishing. p. 93-98.
Skelton, P. (Ed.) 1993. Evolution: A Biological and Palaeontological Approach. The Open University
Hmmm… According to one of my sources (which is a bit older than 6 years old ;)), all flightless birds, in fact, most birds belong to Superorder Neognathae. The ‘Paleognathes’, so-called because of a shared palate formation, known, strangely enough, as a paleognathus palate. Essentially, this palate type is less mobile than the neognathus palate of most birds. While it is true that all ratites share a paleognathus palate, it is considered a primitive characteristic, and primitive characteristics are insufficient to establish monophyletic groups (a group that is known to be descended from a common ancestor is monophyletic, and such a group is characterized by shared, derived characters).
Another source, however, agrees with the sources cited by wevets, even though it is only two years more recent - I guess a lot had changed between 1988 and 1990 (admittedly, however, neither of these focus on birds).
Incidently, it is not so much lack of predation that drives loss of flight, but a selection for a heavier body type. However, it is true that on isolated islands, where a given bird species may be free from predators, flight may be lost as it is no longer necessary.
However, I reiterate what I said in my earlier post - that the fossil records of these flightless birds only date back to about the Eocene, possibly as far back as the Late Cretaceous. Assuming we are not missing a large chunk of ratite evolution, they must have evolved after flying birds where already established.
Sources:
Carroll, Robert L., 1988. Vertebrate Paleontology and Evolution.
I don’t think we’ve spread snakes much - Australia (the usual example of an ecology endangered by human imported animals) has its own very varied range of snakes totally distinct from other land masses.
We tend to import as standard dogs, cats, rats & people (not necessarily in order of damage!) But it is the rats & people that did for the dodo. Rabbits are commonly introduced for food too (Romans to Britain, British to Australia - see it’s all the romans’ fault!)
Fi.
This very point was made to me recently, when my wife and I went to the Smithsonian Museum of Natural History to see the IMAX movie on the Galapagos Islands. Galapagos cormorants, descendants of birds who flew over from the Central American mainland, have lost the ability to fly, thanks to the lack of predators. They now have stubby, almost vestigial, wings.
Importing snakes may be rare to continents, because most (the only exception is Antarctica) have indigenous snakes. On islands, it’s a different story: Guam, for example. It is no coincidence that many of these flightless birds inhabit islands also.
MauveDog:
I’m not entirely surprised by the conflict between sources in this case; I’d guess that the major evolutionary relationships between bird groups are in a state of flux and we probably won’t really have them figured out for decades, if ever. I’m disturbed by the possibility that the Paleognathae may be a paraphyletic group, though. Did your source mention other characteristics that define the Paleognathae? Perhaps they’ve got shared derived characteristics, but their group is still named after an ancestral character through some quirk? Must find out if Prfsrs. Walker and Liem are true cladists; and if not, appropriately punish their heresy.
You’re right about the development of flightlessness. Even the Paleognathae have characteristics that indicate that the common ancestor with other avians flew (wings, sternum, etc.). These birds have definitely lost flight secondarily, which I would guess is not all that hard given that other bird groups have done so more recently (Penguins, pldennison’s cormorants).
wevets:
Actually, the Benton source did mention two synapomorphies (:)): “the extensive vomer-pterygoid joint, [and] the elongate basipterygoid processes that meet the pterygoid”, and mentioned vaguely others “in other parts of the skull.”
Not terribly helpful, but a reference for those shared characters was provided:
Cracraft, J. 1988. The major clades of birds. In The phylogeny and classification of the tetrapods, volume 1: amphibians, reptiles, birds, M. J. Benton (ed.), Syst. Ass. Spec. Vol.35A, 339-61. Oxford: Clarendon Press
