Here on the stack exchange we talk about dragons... a lot. We have discussed all the factors that would allow dragons to exist and yet, there is a question we have not asked. What evolutionary pressure would lead to realistic dragons?
A proto-dragon will have to deal with escaping enemies and acquiring food just like every other animal. Below is a rough process for how a dragon would evolve.
Before we get to dragons, let's consider the bat. We start with a quadrupedal animal then end up with two wings and two legs. At every stage, the animal is functional and with slightly higher performance than the previous generation.
So let's look at the Welsh dragon (the image chosen because it forms a basic body plan for a dragon. Of course, there are a thousand different variations on dragons. I've chosen a very basic variety.)
Starting with a basic six legged creature that lives in the trees. Getting from one tree to another is achieved by jumping and those creatures with increased surface area can jump/glide further, thus giving them a survival advantage. As time goes on, the skin between the middle leg and torso stretches to encompass an ever greater area (till the bones in the middle arms become to weak and break frequently enough to cause that sub-species to die off.) At the same time, the tail extends for balance and offers another place to increase surface area.
Claws on the hands and feet offer considerable grip on tree bark and also help with predation of smaller animals or insects. Secondly, being a predator means you only have to eat a few meals a week to get enough energy instead of munching on plants all day like an herbivore.
Scales could evolve from proto-feathers that consolidate to form protective armor during mating rituals. If there are other large predators, these scales will help there too.
After dragons achieve true flight, they can further differentiate to fill different ecological niches. Some will get huge, others will stay smaller.
I've got no answer for how dragons would evolve fire. Maybe I'll come up with something later.
My two cents:
Our starting point is a lizard. Now, there are already arboreal lizards that glide - of course their genus is named Draco - but I'm not using that because I have my doubts they will be able to produce powered flight, much less be able to maintain it at enormous sizes. I also want them to be venomous for reasons I'll get into later, so I think I'll use a monitor lizard. That's quite appropriate because giant monitors are one of the origins of the western dragon myths.
So our venomous monitor lives in a jungle. Pickings seem nice except for a few problems. One: there are bigger dangerous animals on the ground like cats and pigs, which the monitor can fight off but at great danger to itself. Two: Those ground predators keep eating the same stuff the monitor eats. Many individual lizards would leave the jungle, but clever ones would see potential food and safety in the trees. Bird nests and beehives are easy pickings, even if bloody meat is a little more scarce. It's enough to live on.
Our venomous lizard now lives in trees. It has developed a shorter, stronger torso, longer legs and more powerful claws to assist in climbing. Life is good, but there's a problem: falling hurts, a lot. Luckily, some especially long-fingered lizards are born with stretchy skin on their front paws, and use them at first to get an even better grip on the trees, but eventually this webbing expands around the arm and down the torso, allowing the lizard to parachute if it falls. Later they learn to jump from one branch to the other using the flaps to glide.
We now have something comparable to IRL "flying" lizards, but our bois are bigger, stronger, more bat-like and venomous. Don't forget the venom. Gliding is nice, but it's a real pain in the tailbone when you underestimate the jump and land on the manticore-infested jungle floor. Unlike the old gliding lizards whose puny wings come from fragile ribs, our flying monitors can make a little more use out of their muscular arms to pull more lift from the air. They start flapping, and over the generations they get better and better at it. Eventually the flying monitors are capable of fully powered flight.
Our true flying lizards are now much leaner and more elegant than their ancestors, with sleek, muscular bodies they soar away from the jungle where they hunt from an aerial vantage point, much like a bird of prey. They build nests in high places and their scales become many vibrant colors to attract mates and warn off predators. However, competition arises. The lizards, males mostly, must fight over territory, but escalating the conflict with venomous bites is too risky, so they learn to muscle each other out by grappling with their wings, lashing their tails around and butting heads. This style of fighting favors the larger ones, and those whose brow ridges become nubs, then spikes, then full horns.
The giant colorful horned flying lizards are well on their way to being apex predators, but there's still the problem of terrestrial predators being generally stronger, and at their size it's difficult to just fly away from a standing start.
Remember the venom?
Their venom remains a strong deterrent, but it only takes effect when the danger is already in biting range, or the other animal has evolved an instinctive fear of the lizards. So, our lizards start to do what some cobras have already done: they spit the venom to fester in the target's face from a distance.
This is better, although still not ideal, as to an oblivious predator it would at first feel like saliva and that might not be enough to safe the lizard. FIRE on the other hand, is immediately recognizable to any animal, as extremely dangerous. Like the bombardier beetle, the spitting lizard evolves two different venom types, that when mixed as when sprayed from the mouth, cause an intense chemical reaction, extreme heat and possibly even full-on combustion. The beast uses this new fire-venom to fend off other species, fight rivals of its own kind, flush prey out of hiding, and set wildfires so it can return to eat the charred corpses.
Now there is a giant, shiny, powerful, spiky, flying lizard that spits fire. And that's a godam dragon if I ever saw one.
I think one of the best evolutionary incentives for dragon development would have been competition with other dragons. In a dragon's case, it's not so much a fight against prey as it is a fight against other predators; most likely the only cause of a dragon's death before humans would be because another dragon stole their food (or killed them for it). This competition could lead to bigger dragons who can bully the smaller ones, or flying dragons that can get to the prey faster. Fire breath may have been an early anti-dragon development, for which fire-proofing was evolved (thus, dragons aren't fireproof to defend against their own fire, but to defend against the fire of competitors).
After this competition is established, you can probably come up with many different evolutionary paths depending on where you want to split dragons off from the real evolutionary tree. I would suggest that they started out with the ability to fly, as that's a hard one to develop after you've grown to the size of a small mountain. So perhaps they were flying dinosaurs that survived the extinction event and didn't turn into birds.
Well, this is all dependent on what a "realistic" dragon is. It sounds like you are referring to some other Q&A on the SE. I am going to base my answer around a dragon being an extremely large lizard that breathes fire. I suppose I might want to consider wings as well, as that is a common things dragons are depicted with.
So how can we explain these evolutionary traits? With SCIENCE!
How do we get an extremely large lizard? Well, thats easy. A predator's size is determined by the prey they eat. Small lizards eat bugs. Medium size lizards eat small mammals. Large lizards, like crocodiles and komodo dragons, eat buffalo. As their plant eating prey grow larger, so do their predators. And we know from dinosaurs, land predators can get very large.
Now on to breathing fire, what would cause that to evolve? Again, thats an easy one. What other reptile spits something out of its mouth? If you guessed a spitting cobra, you were right! Why do venomous snakes use venom? Many reasons, but often to incapacitate their prey and defend themselves. BTW, this is a great documentary on the evolution of venom. A chemical based fire would evolve in the exact same way, for the same reason.
The last evolutionary trait is wings. That one is too obvious to go into.
Just answering the flying dragon aspect.
I believe that evolutionarily, learning to glide and not plummet straight to your death when jumping off a high surface is a precursor to flight. It's also useful, as mentioned above, in helping get away from predators.
In reverse, some birds have actually evolved with wings and have lost the ability of flight. Eg ostrich and penguins.
If you are wanting you dragons to have evolved from the existing evolutionary tree you can have them gain and lose the ability to fly several times over time. This way, you can incorporate several differing traits from non flying beasts into your various dragon species.
Just seen this video of the 'rare' footage of a peacock flying titled 'the mysterious flight of the peacock' (I think I got my phone to copy the correct link)
That! Right there! Is the beginnings of a magnificent dragon! I actually think they could be halfway there already! Just got to work on fireproofing those feathers!
First off. It's not rare or technically flying. Apparently peacocks somehow get up in tree branches and on top of roofs (especially at night) and then glide off.
With what I've read and heard of peacocks, they are vicious, screeching, argumentative creatures (especially at 4 in the morning) and neighbours hate those birdlovers that rear them in the backyard. Sound like a dragon to you?
And to top it all off, in that video you have the basis of all the assumptions that dragons are vain.
When planning a plausible explanation of evolution of an imaginary creature, what I do is think in the evolution path in reverse direction. I mean: What is a probable ancestor, and the ancestor of that one, and so on. Because evolution usually takes little steps, something that was useful for one thing, maybe evolves in something useful for another thing.
Some thoughts about dragons:
- They could produce the gas, ot they could have some microbiome. That opens two different paths, everyone with interesting options.
- If they produce the gas: it could be that they fly and they have some gas exchange mechanism related with flotation (I use this in a Universe), like a submarine. Also it could be gas expelling to suffocate prays/predators.
- If you take the microbiome path: they develope wings before or after the first "fires".
- How gas turns into fire is related to how gas expelling evolves to fire expelling: They have some catalist in their teeth? Or in the throats? Or it's catalysed only by high pressure and temperatures? Every possibility arrise more questions that show possible evolutionary paths.
- At first, they expelled big amount of fire, or it was just a little amount? It was related to food digestion?