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Currently coming up with a race that lives on floating islands. They are a still primarily stuck to the ground and I intend for them to be able to fly. Or else they are really gonna be stuck on those islands forever.

However, I can't come up with a good reason for them to need nor evolve to fly. They are essentially entirely self-sufficiently surviving off the ecology of the island so they have no pressing need to get off of it nor are they in peril from predators that might wish to make a snack off them. Other than some godly intervention[Forcibly making them grow wings], I have been baffled for some time now.

So what might cause them to evolve the ability to fly?

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You are getting it wrong. It is not that change is caused by environmental pressure; change just happens (genetic recombination/mutations) and, if it is effective, it sticks into the population.

So imagine that each generation, a few individuals are more able than others to glide and control their descent. The difference with the rest of the species will be not big, so there are two scenarios:

  • that ability gives them some advantage in the island (for example, gliding from trees, or the underground caverns are big enough that gliding is useful there) so at each generation a bigger % of the offspring comes from parent with enhanced gliding.

  • that difference means that they have a higher chance of surviving a hop to another island. Each x time, another island comes near and your beings can see how full of fruits that island is and try to hop in (nobody said they were intelligent), with the "enhanced" individuals becoming most successful. In the new islands (but not in the original ones), the average individual will have an enchanced gliding skill because of his ancestors will be more likely to have had it.

    Repeat it 1000x times and maybe you can get an island with flying creatures.

    Of course, this will not cause the population of the original island to change (they feel no pressure) unless some of the population of the new islands "colonizes it" again and (their differences giving them an advantage) force the old population to mix in (if the genetic differences between species are still not big enough) or become extinct.

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  • $\begingroup$ "that difference means that they have a higher chance of surviving a hop to another island". I was thinking that too until I realised that during the intermediate stage those most likely to glide some distance (but not the entire distance) would be the one falling to their deaths and reinforcing the group mentality...'gliding bad'. They would effectively drive their own evolutionary extinction before they evolved fully! $\endgroup$ – EveryBitHelps Sep 4 '16 at 16:13
  • $\begingroup$ @EveryBitHelps That is why I said (nobody said they were intelligent). An intelligent race that sees a success rate of 1% will probably stop trying, if we are talking only about animals they can be fooled more easily. $\endgroup$ – SJuan76 Sep 4 '16 at 16:30
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There's a timescale problem in this question. We tend to think about sapient races on timescales of tens to hundreds of thousands of years, because that's how long humanity has been around, but evolving really major changes in morphology takes much longer than that.

So if you want a flying sapient species, have them evolve from flying non-sapient ancestors. There are plenty of reasons why those species could have gained competitive advantages from the ability to glide, soar and gradually fly in an environment consisting of flying islands.

Those include more food sources, larger gene pools through mixing the populations of different islands, seasonal migration by hopping onto islands going the right way, ability to keep their young safe in places ground-bound predators can't reach, and all the other ways that birds and bats exploit their ability to fly.

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  • $\begingroup$ The question never mentions sapience. In Darwin’s work etc. races is a generalization of breeds, species, populations isolated physically, etc. $\endgroup$ – JDługosz Sep 3 '16 at 10:33
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Curiosity

Why not make them a species of curious little creatures that have turns to gliders or other similar technology in order to explore their world? Maybe they had too because of massive nasty creatures in the sea or other environmental reasons. Just think of the Polynesian expansion but instead of boats they turn to air planes, hot air balloons, gliders, etc. It could give you an awesome steampunk feel.

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Simple - because your floating island has an underside. Stuff is not just going to be living and growing on the top of it. Plants will be growing in cracks in the rocks underneath, or dangling off the side like a curtain. It won't be quite as diverse as up top, because of the tendency for soil and water to plummet to the ground! Might be a water shortage, since it has the island above acting as an umbrella to keep off the rain. But there WILL be life there.

Any creature which can climb or fly can exploit this underside resources. Birds will be nesting on ledges on the underside (like seabirds on cliffs) or gluing their nests to it (like swallows and swifts), bats will be roosting in caves.

If a climber heads down to gather bird eggs and slips and falls, they are dead. Even if they have a soft landing, they fall off the island and are never seen again. So being able to fly is a lifesaver. There will be a big evolutionary pressure towards flight (and also towards 'not falling off' adaptations, like spiders making safety lines with their silk).

Alternatively... your species has no predators, so can only die of accident, old age, disease or murder. Unless disease and murder are rife they (or their animal ancestors) are going to breed and breed and breed until they overpopulate the island. So there will always be pressure for some of the population to disperse to new islands.

If the other islands conveniently 'dock' with the original now and then, no problem. But if the other islands never touch, they are going to have to jump. Again natural selection will favour creatures who arrive in their new home without broken bones or as a messy splat. Jumping ability, then gliding ability then true flying ability.

Once they can fly, they can colonise islands that never come within jumping or gliding distance - for instance one which is floating far above their current home.

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Flight has evolved multiple times in totally different species, so it happens.

However, without pressure, you get an unchanging species. If the environment is unchanging and the species is well suited, it will remain the same.

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