When I first saw someone say this I thought they were exaggerating, but I wanted to do an egalitarian, xenophile, psychic ascension run. Everything's going well for the first 50ish years, I have a Fanatic Pacifist advanced start on one border (free influence since he can't declare war) and some other fun empires around. I befriend a megacorp neighbor we get all the treaties (except research because that wouldn't benefit me at all), I was sitting at around 60-70 pops, all of them my species. 100 years later I am at ~600ish pops and my primary species is at 90, the majority of those came from colonizing new worlds using them. So after I transcend I've got less than 20% of my population actually ascending, all because I had a migration treaty with ONE empire. That empire probably had treaties with other empires (it was a Megacorp afterall), but even so my primary species basically stopped growing all together, at least in my empire. When I looked at the galaxy species tab I discovered there were 130 members of my species in other empires. So by signing one migration treaty I basically ended all pop growth of my primary species in my empire and somehow transferred it all out into other empires.
I think the problem is the current species growth formula is basically just:
What species is most underrepresented in your empire? Lets grow that one on every world!
I think a far better formula would be something along the lines of:
If Species has no representation = 0.33
If Species is underrepresented
Weight = current species representation / (1 - (ideal representation - current representation))
If species if over or equal represented
Weight = Ideal species representation + 0.25 * (current representation - ideal representation)
Where ideal representation is equal representation modified by habitability (so 3 species, with 80%, 80%, and 65% habitabilities will have an ideal rep of 35.56%, 35.56%, & 28.88%).
Example:
On planet Alpha species A currently has 70% of the population (7 pops) and species B has 30% (3 pops). Both species have the same climate preference and so the ideal ratio is 50:50.
Species A has weight = 0.5 + 0.25 * (0.7 - 0.5) = .55
Because the species is over represented it gets no bonus weight, but there is no penalty for over representation.
Species B has weight = 0.3 / (1 - (0.5 - 0.3)) = .375
So species B has a boost to its chance for being under represented, but still has a lower chance than the primary species.
So lets look at how the weight changes over the next few pops. Assume the next pop is species A.
Now we have A with 8 pops and B with 3.
Weight A = 0.5 + 0.25 (.7272 - .5) = .5568
Weight B = .2727 * (1 - (0.5 - 0.2727)) = .353
Species B gets a bit less likely as it becomes more of a minority (as it should be), but the decrease is mitigated by the increase in the delta between ideal and current representation.
Lets assume the next pop goes to B, so A = 8, B = 4
Weight A = 0.5 + 0.25 ( 0.66-0.5) = .54
Weight B = 0.33 / (1 - (0.5 - 0.33)) = 0.398
B's weight now goes up while A's remains
One thing this will do is ensure core worlds remain predominantly their own species, while allowing colonies that are developed after migration treaties are signed to be truly multicultural. There is no factor for empire wide representation (which is just ridiculous in general, pops growing on planets should be influenced by whats on the planet, not by whats in your entire empire) so that's a plus.
A simple way to include habitability into this equation is to just multiply the weight by the habitability (limit of x 1, habitability over 100% shouldn't contribute extra).
I think this would far better represent actual pop demographic while still allowing minorities to eventually grow on developed planets (it will just take quite a while).
