What would be the evolutionary drives of a lush cave environment?

Question

Now, for my D&D-style world, there is an extremely large cave system hidden beneath a mountain. These caverns are illuminated by processes that are a bit too complicated to divulge, but in any case, it's in somewhat of an eternal twilight. The terrain is made up of various terraces surrounding large fields, resulting in a large inverted mountain, if you will. Also, do not ask how the mountain above is supported when there is a cavern of similar shape and equal size beneath it. There is lush vegetation down there, rich in silicon due to various large opal deposits and they also have an unusually high calorie count, for example, talmoss, which is the most abundant vegetation in the caverns and is three times as nutritious than your average grass, once again for complicated reasons. The most nutritious vegetation would be the blue-skinned pataya, a species of dragonfruit with twice the calories than that of a regular one. With this, the average calories of vegetation there would be 25 calories per ounce of plant matter. Since the area varies in plant matter concentration from temperate forest to tropical rainforest, any herbivorous or omnivorous creature would no doubt thrive there. The climate ranges between boreal in the upper reaches, temperate in the middle, and tropical in the lower regions. Animals are almost nonexistent, however. The main danger would be the opal deposits, as they form in such a way as to create spikes, meaning one wrong step would mean death. Also, rarely, strange superstorms form. These thunderstorms are massively deadly, as the lightning created by it is greatly empowered by strange means. Once again, don't ask. Now, for dragons. For clarity, my dragons lay an average of 75 eggs, which hatch into larvae known as wyrms. However, due to the wyrms being highly territorial, and the fact that dragons abandon their young until they pupate, the amount that lives to pupate is 1-3. Wyrms are about a foot long at birth and 2 inches around, and at the stage of "adulthood", wyrms are 8 feet long and 4 feet around, with a sharply tapering end. When a "baby" dragon emerges from the cocoon, they are roughly the size of a shetland pony, being four feet long (not including the tail) and 2 feet tall, with a 5 foot long tail. While the size varies wildly for an adult, the average size is 12 feet tall, with a 21 foot long body and a 32 foot long tail. The dragons are usually lithe, unlike other dragons from fiction. They also use the standard hexapodal body type, with four legs and two wings. The mating ritual involves dragons from several groups congregating about every five years, where both sexes are "tested," if you will. Males are tested for strength, horn size, scale durability, and breath weapon strength. Females are tested for intelligence, "beauty" (let's be honest, dragon beauty standards would be wildly different from ours), and feather beauty (yes, dragons have both scales and feathers in this world). Once the ritual is complete and mating pairs are made, the gestation period is 18 months, the wyrm lifespan is 10-12 months, the pupation period is 4-5 months, the time it takes to reach sexual maturity is 22 years, and the loss of fertility is at 174 years. Since females mate about every 15 years, the amount of time before dragons become independent from their parents, females mate about 11-12 times in their lifetime. They are omnivorous, able to digest pretty much all forms of plant matter, about as intelligent as humans, if not slightly smarter or dumber, and utilize renaissance level technology and agriculture.

Now, why did I tell you all this about dragons and this cave system?

Well, a group of dragons, carrying with them a variety of crops and cattle, had decided to colonize the lush caverns about 3.5 million years ago, enough for major adaptation to occur, and I want to know how these dragons I have presented will adapt to these cave systems i.e. will these conditions make them lose or gain muscle mass over generations. Bonus points for what their society may look like. Also, will they take advantage of the high concentrations of silicon in their food, i.e. opalizing their horns, teeth, and claws?

Oh, and about the whole "3.5 million years ago" thing, it's being discussed in another question of mine.

BTW, here's a link to my chatroom if you're interested in the world.

Answer 1

First Ask, How many Generations are we Dealing With

Human females hit maturity at the age of 13 whereas your dragons hit maturity at an age of 22 with 3.5 million years to work with, you can assume the amount your dragons evolved in this timeframe could be comparable to about 2.4 million years of human evolution at an estimated 160,000 generations. When you look at reconstructions of proto-humans from about this many generations ago like Paranthropus Aethiopicus, Homo habilis, Homo rudolfensis, and all of the various descendants of these species you will notice that they this is enough time that you either may or may not see significant evolutionary changes just depending on what your evolutionary pressures are. You can use how different Humans have been over the past 2.4 million years to set a reasonable baseline for how far your dragons may drift.

One thing to consider is that evolution is never a straight line. At this timescale you will see speciation take hold where different evolutionary paths of dragons will diverge based on the different biomes available in your cave system. No area large enough to sustain a population over millions of years is ever a single homogeneous biome, but rather several, as will be the case with your caverns.

Based on your description, I suspect you will have at least 4 distinct biomes in your cave system, though you may decide that you have slightly more or less depending on your story. Consider the following as a guideline for creating your biomes and how evolution diverges based on them and you can tweak the specifics of additional biomes to meet your needs following this pattern.

The Core Biome: Your terraced floor means that you will have low lands with big open spaces where water will naturally gather. Even with all light being homogeneous, these area will be able to support the most biomass because of water availability.

The Wall biome: As you move up to higher terraces, water will flow away from here meaning you will have less biomass, but still plenty of open air where flying will remain relevant.

The Tunnel Biome: As you get even further into the perimeter of the caverns, you will stop having open spaces and instead start having narrow tunnels. Some just being where your system of terraces pinches off and others being caused by erosion from underground rivers or lava flows that lead into the main chamber system. Either way these cave systems will be too small to fly in and in many cases too small for most dragons in general. Water will pool into low areas of these caves resulting in a sprinkling of oasises of water and food, depending on where your light is coming from, some of these places might be removed from your light source; so, these places could also be darker in areas, thus not necessarily supporting plant growth everywhere.

The Island Biome: Lastly, as the ground changes shape over this kind of time scale, chambers will open up and close off; so, you may wind up will small pockets of isolated or hard to get too chambers that are smaller, but otherwise have the potential for open or closed in spaces.

How Evolution will Affect your Dragons

Qualities that all of your underground dragons will evolve towards: Your eternal twilight and underground environment will mean that certain protective adaptations will dwindle over time. Surface Dragons need more melanin in their skin to protect against the direct light of a mid-day sun whereas your underground dragons just have a constant barrage of gentile light. So instead of the Deep Greens, Reds, and/or Blacks of surface dragons,these dragons will likely adopt a pastel color spectrum if not favor out-right albinism depending on how hard it is to make enough vitamin D from the available not-sunlight.

They will also not need to endure the cold of night and/or winter or seasonal scarcity. This means they will produce less body fat and downy feathers since a low BFI will be more beneficial for flying and climbing and feathers can hide parasites which is why animals evolve less dense fur/feathers when cold is not an issue.

Since they are evolving to be less fatty they could in theory also become more muscular, larger, and/or well armored to account for the extra weight they could now support.

Biome Specific Adaptations

The Core Biome: These Dragons will have wide open spaces, and tons of food. The only real challenge they will have to their selective fitness will be the threat of other dragons pushing them out of their ideal territory. As such, these dragons will evolve to be quite large and powerful. They could be as much as 50% larger than their surface dragon ancestors, and much tougher since they are literally surrounded by lots of competition living in a small place. They will also tend towards being more aggressive than other dragons since they will be forced to compete so directly to maintain their good territories.

The Wall Biome: These dragons will likely remain about the size of their surface world ancestors. There would still be plenty of food to survive off of, but not enough to encourage gigantism or overpopulation. Sufficiently high ceilings means flying will still be favored by them. Encounters with the larger core area dragons will be common because of overlapping territories; so, speed and perception will become key attributes for these dragons. Since they will not be big enough to survive a fight with a core dragon, only the fastest and most observant wall dragons will be able to survive these unwanted encounters.

The Tunnel Biome: Dragons in the tunnel biomes will benefit from smaller size. The smaller a dragon is, the more tunnels they have the option of inhabiting. These dragons could easily evolve to be about 1/2 the size of their surface dwelling relatives in this time period. Wings also interfere with tight tunnel dwelling so they would probably have much smaller wings than other dragons for their size as well. I don't know if 3.5 million years is quite enough to become completely flightless under this much evolutionary pressure, but I'd estimate at most they would be limited to wings just barely good enough for a short burst of flight kind of like a domestic chicken.

The Island Biomes: These populations would generally be the result of population bottlenecks meaning what makes these dragons unique is as much a matter of random selection as it is of evolution. So much the same way we see different variations of the same animals on one island vs another, each island chamber would have its own breed of dragons. Some bigger, some smaller, some with unique color patterns, some with unique behavioral patterns, etc. For world building purposes, you probably don't need to define every exact kind of island biome dragon, just know that these places will give you lots of room for diversity.

As For Society:

The Stone Age is the only human epoch that rivals this sort of time frame. Knowledge only begets more knowledge; so, the only way to keep these dragons from stampeding through technological ages and becoming space aged in this sort of time period is to hold them so far back that they barely understand the concept of technology it self. Yes, they will have tools that they pass down from one generation to the next, but once they start realizing things like specialization of labor, writing, etc. You won't be able to expect their technological state to just stagnate.