I think the implications of adding environmental adaptions even go far beyond player mobility. Let me explain:
Right now, the player species is assumed to be „categorically static“. No matter how much its body plan changes, if the player never changes its name it will always remain Thrivium Primum. The player can change its name, but as far as I remember that doesn‘t result in a split of the species. This is quiet a strange disconnect. How can a species be called by the same name over many milions of years despite its body plan changing completely? As far as I understand, no other species is handled like this. How can one species still carry the name of LUCA despite being something completely different while all others have changed their name (their category as a species) to reflect that they have changed over the millenia?
Let‘s look at another discrepancy for a moment: Evolution doesn‘t work like either of the two solutions suggested here:
It wouldn‘t make sense for 90% of a species to adapt a hurtful mutation in the old for 10% to thrive in a new biome.
Neither would it make sense for a large part of a species to migrate into a new biome simulatneously and getting the fitting mutation at the same time. This might be thought of as the species getting the mutation and as a result thriving in the new biome and dying out in the old biome. But there wouldn‘t be any reason for the mutation to be successful enough in the old biome to replace the specimens who didn‘t get the mutation. If the old body plan still works well in the old biome there is neither a incentive for the mutation to spread there, nor is there a reason for the bulk of the species to migrate to a new patch where the mutation might be more likely to arise.
The only reasonable way for this to work which I see is:
Specieation uponthe passing of an environemtnal threshhold
The way these kinds of dispersals work irl is like this: The bulk of the species remains in the old patch and keeps the old body plan. The mutation arises per chance at the boundary between the old patch and the new one. There the mutation is more of a boon than it is a hinderance. It can therefore proliferate and its bearers can advance into the new patch.
This means that in Thrive, a new species should always be created when the player decides on an adaption which is helpful for the 10% of their population in the new biome but which is very hurtful in the old biome.
If the player does this, he takes control of the newly diverged species and gives up control of the old species which remains in the old patch.
This is logical. The part of the species which remains in the same biome as before and keeps the same phenotype as before keeps the same name. The part which diverges (in space and in its phenotype) gets a new name and is therefore categorized as a new species. But this leaves us with a problem: Now the player controls a species with only 10% of the population of the species they controlled before. Worse still, this species is thrust into a new biome which makes survival more challenging for the player. Does this mean that the player now faces the game over screen after one or at best two deaths?
In the old system, this would be the case. As the game is right now, it‘s game over when the player species goes extinct. But as far as I remember, this isn‘t planned to always remain the case.
If I remember correctly, we are planning for there to eventually be a hardcore and a normal mode. Hardcore would be the way that it is right now: If the player species dies, it‘s game over. Normal mode would be like this: If the player species dies, the player takes control of a species which has most recently split off the player species‘ lineage.
This feature of the player gaining control of a closely related species upon extinction has been discussed in this aforementioned thread: Population System - Species Splitting
Here is a quote from that thread regarding this “planned” feature:
Fourthly we could give the player a certain number of “respawn points”. If your species dies out you have to spend these points to get to another species to play. For example if you are wiped out and you want to switch to a species which has relatedness 4 to you this costs 4 respawn points. If you do not have enough respawn points to switch to a species which is alive then it’s game over. On iron man mode you could get 0 respawn points, on “creative” you could get unlimited (play as any species any time). We could give +1 respawn points every 20 mins of play or something like that.
It‘s also mentioned in this GDD, although I‘m not sure how up-to-date it is: Game Features - Thrive
Rise and Fall
Death, defeat, and extinction are a part of the game. The player will hop from species to species, unless they play exceptionally well and never go extinct. The same will happen with civs that will rise and fall but the player will play a common thread that hops between them.
If this was the case, this would solve our situation: The player takes control of the split-off species in the new biome, but if that species doesn‘t make it, they regain control of the species from which they recently split off.
To reiterate: The player species shouldn’t split if the new mutation is only a minor hinderance to the population in the old patch. It should only happen if the mutation poses a large handicap for the population which remains behind. I know my suggestion of coupling environmental tolerances to a rework of what happens upon player species extinction is a bit drastic. But it’s the only way I see in which these kinds of strong environmental barriers can be implemented in a way which is both realistic and fun to play.