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My planet is a tidally locked rocky planet (90% Earth surface gravity) orbiting an M-type star. This M-type star orbits a G-type star at roughly 1.5AU.

This means that it has a kind of day/night cycle determined by how fast the planet orbits the M-type. These "days" (both the day and night cycle together) last about 48 hours. On the side facing the red dwarf, days would be near Earth luminance, and "nights" would be lit by the red dwarf at approximately 60% Earth luminance. On the side facing away from the red dwarf, days would be lit by the yellow sun at approximately 40% Earth luminance (like days on Mars), and nights would be lit by whatever light refracts through the atmosphere from the other side of the planet.

Assume that the planet had liquid water oceans before terraforming and has since been terraformed sufficiently enough to have a breathable atmosphere, bacteria, plankton, algae, fungi, lichens, grasses, some trees/ferns, etc. (you know, the basics).

The planet has a band of habitability around its terminator (mostly on the warmer side of the planet) which is broadly suitable for humans (survivable, at least, if not exactly ideal). The only land masses in this habitable area are a series of large, volcanic islands (Iceland to Greenland in size). The general idea for this planet is that it is habitable in the broad sense, but harsh. Probably very windy, lots of storms, and broad temperature variation.

Once this base is established, animals are introduced. Building off of others' posts/answers that I have read, let's assume our terraformers sent the bare basics and intentionally didn't send any creatures harmful to humans unless they play an essential role in building the new ecosystem (i.e., nothing poisonous, venomous, crop-destroying, or disease-carrying, and no large apex predators).

I'll leave aside aquatic fauna for now because that's a whole 'nother barrel of fish (ha), but assume the aquatic ecosystem has been functionally stocked with fauna.

Turning to terrestrial fauna, our terraformers have introduced various forms of insects (especially pollinators), insectivore birds and rodents, pollinating birds, herbivorous rodents, small-to-midsize predatory birds, small-to-midsize carnivora, some small-to-midsize prey animals for hunting that may not fall into the above categories, and hardy/wild forms of domesticated animals (boars, goats/sheep, etc.).

Our terraformers give the animals time to adapt to this new environment so that any unfit animals have died off by the time humans arrive. What kind of new or surprising adaptations might we expect to see by the time humans arrive, and how quickly might we expect those adaptations to evolve?

To give some starting points for answers, maybe consider the following points in time for potential human arrival:

  • 1000 years after animals arrive
  • 25,000 years after animals arrive (this is the time frame I was originally thinking if enough interesting evolutionary changes would be plausible)
  • 100,000 years after animals arrive
  • 1 million years after animals arrive

To give some further starting points for answers, consider some potential adaptations that I have brainstormed and address them or respond with your own:

  • Animals getting larger due to slightly lower surface gravity (especially birds??)
  • Changes in the grazing or social habits of some animals due to environmental changes
  • Heightened sensitivity to infrared light due to overall lower luminance and higher infrared radiance from the red dwarf
  • small-to-midsize carnivora rapidly becoming larger to fill an apex predator niche
  • along the same lines, general effects of both "island gigantism" and "island dwarfism" (i.e., the tendency for large animals introduced to an island to grow smaller, and small animals to grow larger)
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I think there is an important line on evolution and variation here. The starting animals would vary 'rapidly' to fit their ecological niche. That would mean that within 1,000 years you'd find colouration, wooliness, size and so on starting to shift about.

After a million years you may have had something like a Cambrian explosion, but it would depend on how complete an ecology you imported to the planet. For all that time you would likely have a very unstable ecosystem due to small environments being destabilised by rapid changes. Some islands would probably lose all their animals, or all their animals that rely on X, and be recolonised by neighbours. But that wouldn't give enough time for the recolonised islands to get interesting bird led ecologies with cool evolutions form that, you'd need much longer time spans to get dodos or penguins to appear.

Really interesting things take enormously long time spans to evolve. From the first emergence of veined feathers to wings was about 10m years, and to then evolve into something that looks like a bird a further 50m. In a million years you would really be putting the animals we have today in front of a fair ground mirror - some funny looking things fitting to their ecosystem, but recognisably the same underlying creature.

On the bird size point, I think you would see them get bigger. But interestingly one of the major constraints on flying bird size is the rate at which a bird can replace its feathers as they get worn and lose quality. So if there is less UV and fewer parasites there may be less need to replace feathers and therefore no real upper limit imposed by gravity alone.

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  • $\begingroup$ Really great answer, thank you! I kind of knew that really large changes would take a long time, but you have some good examples of the kinds of changes that could possibly happen on a (relatively) short timescale. $\endgroup$
    – DMacc1917
    Mar 22, 2023 at 17:12
  • $\begingroup$ Now I want to think some more about how humans would react to an ecosystem where the dominant non-aquatic apex predators are probably all giant birds. The closest historical analogue is probably the Maori interactions with the Haast's eagle, but unfortunately they didn't have written language at that point. :( $\endgroup$
    – DMacc1917
    Mar 22, 2023 at 17:15
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    $\begingroup$ This Wikipedia paragraph on Haast's Eagle is useful for timescale: H. moorei is estimated to have diverged from these smaller eagles as recently as 1.8 million to 700,000 years ago. If this estimate is correct, its increase in weight by ten to fifteen times is an exceptionally rapid weight increase. The suggested increase in the average weight of Haast's eagle over that period would therefore represent the largest, fastest evolutionary increase in average weight of any known vertebrate species. $\endgroup$
    – John McD
    Mar 22, 2023 at 17:23
  • $\begingroup$ Not correct. The terraformed planet will have complex organism from the beginning. The Cambrian explosion will start immediately, not after 1 million years. Check on the wikipedia page of the Cambrian explosion the Hox genes theory. $\endgroup$
    – FluidCode
    Mar 22, 2023 at 21:47
  • $\begingroup$ I said after a million you may have had something like the Cambrian Explosion - past tense. The 'may' is because everything might be extinct or there may equally be complex creates well suited to their niches. The explosion was possible because of the rich range of environments to exploit with no incumbents. It's not clear that the seeded animals will not, in fact, dominate their environment and prevent the wild experimentation of the explosion by alread being pretty good. $\endgroup$
    – John McD
    Mar 22, 2023 at 21:57
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Genetics studies carried over last thirty years have shown that the genome of most creature more sophisticated than simple unicellular organisms has a high degree of variability.

Leaving on the ground only few selected species would remove a lot of environmental pressure. A less selective environment would result in a Cambrian explosion, the sudden emergence of a huge number of new species, many of them will exploit unoccupied niches.

Probably 1000 years are not enough to create a stable biosphere where the evolution is slowed down by a well balanced system. 100,000 years I don't know, but I suspect that new settlers would find a very changeable environment.

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