Evolution of Flight

Factual Questions
1

How did flight evolve?

It seems to me rather obvious that it didn’t happen in one generation, and it happened multiple times (once for insects, once for birds, as I can’t see how birds would possibly evolve out of insects O_o).

So, how did the middle steps progress? What “advantages” did the beginnings of flight provide to their mutated owners that allowed them to succeed as a new species?

As I understand the thoery, first they (“they” being another debate) got feathers that provided them with warmth… but feathers does not flight make. They also had to develop other avian features and biology, including turning their forelimbs into true wings and lightening their load by devloping avian bones and such, correct?

But since critters can’t “select” what they will evolve, was it just chance over millions of years that had creatures mutate to have feathers, survive and thrive, then mutate to have lighter bones, and somehow thrive even being landbound, then wings? Were there steps in between that have “died off” evolutionarily? Or did they develop starting out as small critters leaping around the ground, getting selected for being lighter and more efficient, and their forearms slowly mutating as they used them to glide more and more?

:confused:

2

Imagine, if you will, a land of reptiles, among which are a biped hunter. As did some of his contemporary, he had quills, which had allowed his kind to spread to cooler, and dryer climates. His front arms were most useful to him as balance, as was his tail. It was running that made him so successful. Among the many variations of his near relatives, his own was a large flap of skin between his arms and his body.

It was not a wing.

But, when he ran hard and fast, it allowed his body to loose heat much more efficiently, and when he stood still, it allowed his quill coat to insulate him more than a naked reptile might. It was so useful, in fact, he, and his near relatives increased in number for millions of years.

Among his descendants were some individuals whose proto wings began to actually benefit their life in a an odd way. By leaning into the onrush of air as they ran, they were able to take weight off their hind legs, and so run for greater distances. This adaptation caused its individuals to flourish.

It still wasn’t a wing.

An another million years passed, and the many species of this genus of reptiles were able to survive mutations that gave them greater complexity of quills, that had better insulating qualities. Their bodies could maintain heat with less body mass. And with less body mass, a funny thing happened. Those silly flaps turned out to be useful in stopping, and turning quickly as they ran. The ability to “grab some air” as they turned made them very agile runners.

But they still weren’t wings.

More time passes, and those with the lightest frames, and bones gained the most benefit from their now developing arm flaps. One day, a very lucky little feathered lizard ran down a hill to get away from a predator, and he actually glided down the hill. He survived.

And they were wings.

Tris

3

Flight’s actually evolved at least 4 times, I believe. Insects, pterosaurs, birds, and bats. And gliding capacity has evolved in several other instances. Useful abilities, shouldn’t be any surprise.

4

You missed reptiles (all extinct) and mammals (bats).

Everything about this is pretty speculative, since we had no witnesses, but there are bits and pieces of evidence.

On preview, I see that Triskadecamus beat me to the bird story. I’ll just point out that the pre-birds might have hunted by climbing up on something and pouncing on passers-by. The same sort of thing, only with flaps of skin, probably happened with reptiles and mammals.

Insects are different. It’s fairly common for adult insects to end up with extra legs, often malformed. A couple of entomologists (sorry, no cite) were studying how butterfly wings can catch the sun to help them warm up in the morning. (Naturalists had been watching them for years, turning at right angles to the sun first thing in the morning. Warm butterflies are more active, so those who warm up first get first crack at the flowers, and are more likely to make that early bird go eat worms.) They tried models, and found that wings of increasing size, up to a certain limit, helped warm up quicker. That limit happened to be just about the same as the smallest size that stabilized their models while falling through the air. If that holds for real butterflies, and they move those wings, they’re well on the way to flight.

5

Looking at birds, feathers are just a modification of scales. Cladistically, birds are within the dinosaur clade.

If I were to hazard a guess, it flight probably evolved from a running->running and jumping->running, jumping, and flapping to extend the jump->short flight->long flight road. While there are plenty of intermediate extinct animals, one can look at animals in the world now, like the flying squirrel or the chicken, and see plenty of examples where gliding or assistance with jumping or running is necessary for survival. Looking at the end point – a mosquito, a swallow, or a bat – disregards all of those animals to whom incomplete flight is an advantage.

Evolution works by winnowing genetic variability in a large population. There was a large population of dinosaurs in a (presumably) cold climate that developed feathers for warmth. Perhaps at the same time, they took to the trees or developed a running/jumping escape strategy to avoid predation. Only the members of the population that were the fastest or could glide the longest survived. This population expanded and genetic variability increased by spontaneous mutation. Repeat a few times and you have birds.

I am relatively familiar with the evolutionary history of insects. Wings are not leg anlages in insects (unlike the antennae, mouthparts, and genitals). They probably are derived from a duplication with divergence of the ancestral gill, which became the tracheal system and the wing in Pterygota, the winged insect division of insects (i.e. all insects except silverfish, firebrats, and bristetails). Some also attribute the gill to the evolutionary precursor for the spinneret of spiders.

Flight arose once in insects, probably around 300 million years ago. It has subsequently been lost in many insects, many of which do not retain any remnants of wings. In Dipterans, there are the primary wings and the halteres (balance organs) which are anlages. They move in counterstroke, with each driven by different analogous muscles. Other insects have true duplications of the wings – Odontata (dragonflies) come to mind.

6

I saw a documentary once, or read an article, that suggested this about insects:

Some insects lived in or on water. They may have developed “flippers” to help them move faster to evade predators. Since the insects with larger flippers could move faster, they tended to survive predation better than those of their kin without. Other mutations made the flippers larger, and the insects could leave the surface for short intervals; I guess like a flying fish. Finally, they mutated into actual wings that allowed the insects to fly greater distances.

Not being an entymolgist or any other sort of scientist, I don’t know how accurate that is; but it sounds logical.

7

While the “ground up” theory has already been covered (and is currently the front runner for how birds evolved), there is also the “tree down” theory, which posits that tree gliders eventually developed full flight. And it’s entirely possible that one group (birds) used one method while another (bats) used the other.

8

I tend to go with the “tree-downers” personally. Elongated proto-feathers on the forelimb seem to me to be the morphological equivalent of the skin flaps of a gliding mammal. The longer pre-feathers would have been easier for a gliding dinosaur to develope than the skin flaps, especially given that the hip structure of the saurischian theropods did not allow the leg to swing out sideways to serve as an aft suport for the skin “wing”.

9

Ohy, and could someone explain to me how an adaptation that keeps the animal’s feet from contacting the ground somehow helps it run faster? When we designed an analogous system for, say, racecars, the airfoil serves to keep the wheels in contact with the track, not lift it off for tiny little hops…

10

It might not run faster, but it might hop higher and farther with a proto-wing to better escape predators. And better running/gliding/hopping might be just the ticket to stay alive long enough to pass those genes along to offspring.

11

Okay, let’s not think about hopping higher or running faster. A proto-wing could help them change direction in mid-hop, which could help them evade predation or more effectively capture their own prey. A proto-wing used to stop quickly would also generate the familiar flapping motion.

But I don’t buy it. All other flying vertebrates are clearly tree-downers. A ground-up origin for birds would have to answer a whole bucketload of questions we have hardly even formulated yet for me to accept it…

12

Why not? Wouldn’t that provide an evolutionary advantage under some circumstances?

Consider turkeys or chickens. They fly; not very well, not very far. Are they just learning, not yet having reached their evolutionary pinnacle, or did they once fly better and lost the ability? Or is quality flying/hopping not a major factor in their species’ survival?

13

One of those would be flying fish, and here the sequence is tolerably obvious: A species of fish sought by predators evolves the habit of escaping them by leaping from the water. The predators get better at catching them. The fish’s fins can be used to extend the leap by aerodynamic effects. The predators catch on to this. The fish evolve longer and better-shaped fins, and eventually are gliding a hundred feet or more. It’s not impossible that in a few thousand generations, they’ll be truly flying.

14

Actually, I said run for longer, not run faster. Endurance could benefit. Most animals don’t run fast enough to have much difficulty getting traction.

It was just an imaginary example, though. I have no evidence that ground up is more likely than tree down.

15

There have been experiments done on modern chickens which prove that the wings function admirably as spoilers and increase traction. It’s all a matter of angles.

16

Well, modern domestic chicken and turkeys are clearly degenerate former flyers, just as ostrichs and penguins are. Hardly a good example to argue either way about the original evolution of flight.

Lumpy – I’m curious, do you have any good sources for current thinking on the ground-up versus trees-down hypotheses?
(to me, tree-down seems more plausible, but wondering what the experts have found out)

17

Darn it I can’t remember the name of this creature or enough about it it coax info out of Google, or even which continent it lives on. . . but anyway.

There is out there in the modern world a non-extinct (obviously) sort of half-evolved throwback of a bird. It looks like a chicken with a lizard’s head, it may even have teeth. It flies - sort of - it can flap its way into low branches but it certainly can’t fly in the traditional ‘like a bird’ manner.

Please someone help me out and find a cite for this critter.

As to the OP I would point (as others have) to modern animals that sort of fly, like flying - squirrels/lizards/fish/frogs/snakes(!) It’s easy to see how these guys benefit from being good at falling. I for one would like to see what the flying frog is going to evolve into. I expect the flying snake is more of a dead end.

18

Small Clanger, you’re thinking of the Hoatzin. It can fly about as well as a chicken. It does retain several primitive characteristics, including claws on its wing when juvenile.

While true flight only evolved four times, there are plenty of gliding animals out there that could be models for the pre-flying ancestors of birds, bats, and pterosaurs. Flying lemurs, flying fish, gliding possums, flying squirrels, Draco volans (the gliding lizard), gliding frogs…pretty much any climbing creature can benefit from some way to slow or direct a jump or fall.

There are dozens of gliding species, but the only pre-flying ground species we have are birds, who clearly aren’t on their way to flying, but have lost flying secondarily. That’s why the running–>flying path doesn’t make sense to me, but the climing–>gliding–>flying path does.

19

Scientific American and Discover magazine mainly. Couldn’t tell you exactly which issues.

20

Yup, that’s the birdie! Thanks. I notice at least two of the sites Google brings up describle it as ‘prehistoric’ looking. What I remember from a nature documentary that featured these things was just how not very good they were at flying, after having presumably having had just as long (if not longer) as any other bird to evolve the requisite muscles/bones whatever. It’s like they just couldn’t be bothered.

Now I’m off to investigate Draco volans and flying lemurs.