If you hover over a pop type in your system screen there will be a number that represents growth points. The base value for each pop type is +50 growth points per turn. Lets say you are getting a pop in 10 turns. You have 1 craver and 1 minor pop and no growth boosters are applied (I mean luxuries). Then those two pops will grow equaly and at turn 9 they will have 9 times 50 = 450 growth points each. Turn 10 they will have 500 each BUT cravers are you primary pop and you will get 1 craver instead of 1 minor pop all the time even though they are equal. After you get that craver at turn 10, growth points of your craver pop are reduced to 0 (I think? mb not to 0 but they are reduced for sure) and your minor pop growth points are still 500 and keep growing, so most likely your next pop will be your minor pop cause its growth points will be bigger than your cravers' by the time you get a new pop once again.

Any luxury booster makes a pop to get 100 growth points per turn instead of 50. So if we look at the same example but your minor pop will be boosted by a luxury, then at turn 9 Cravers will have 450 growth points and minor pop will have 900 and will be your next pop.

So having a lot of different pops doesn't matter if you don't boost them all but it does matter if you have a lot of pops and you boost them all the time, while your cravers are not affected by the luxury.

loading up a lategame craver game , hovering over homeworld system:

chance to increment

haroshem 25 ( +25 per T) .   [ 0 + 25 + 25 + 25 ..] 

horatio 100 (+50 per T) [ 0 + 50 + 50 ..]  // booster on

yuusho 75  (+25 per T)   [ 0 + 25 + 25 + 25 ..] 

sefaloros 0 (+25 per T)   [ 0 + 25 + 25 + 25 ..] 

kalgeros 250 (+50 per T) [ 0 + 50 + 50 ..]  // booster on   // this is the one listed as the next pop in 1 turn [way more than 300 food per turn in system]

then there's 3 cravers and 2 tikanan lumped under a 5 since the interface ran out of space.

OK looking on another planet: 

Horatio 100 50/T // boost on

Sefaloros 25 25/T

Haroshem 175 25/T   // this one will grow next

Craver  0 50/T   // CRAVER GET 50/T without lux booster  (went and buffed them for testing went to 100/T  WOW)

Yuusho  75 25/T 

So probably I'll get horatio next here and then craver ? 

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so before more testing

main pop types gets +50 per T / +100 per T  to growth bucket   ( values after / are bosted via lux)

other pop types get +25 per / +50 per T  to growth bucket   

One needs to buff the other pop types and not buff the main one for them to have 'equal' growth speed.

After a pop is born - the pop type with the biggest accumulated growth [at that time final decision or re-evaluates every turn ?] gets picked to grow next. 

Will keep a eye out to see how this goes on a turn by turn basis .

This expains why when getting a new pop type in an system that already has N pop types it takes FOREVER to grow one.  Since all the N rezidents will already have stuff in their growth buckets and all will grow before then new guy..  Max Growth rate for prefered pop would be 1 every 2 turns in a system with it boosted by lux and cravers non-boosted by lux. 

And yes, ammasing multiple pop types in a system will lower craver growth rate /waste but in the same time make it harder to grow the 'desired' minor races quickly .. [still in doubt if when seeding new colonies sending just 1 desirable to grow it quickly or multiple pop types to lower craver waste is better. Perhaps a compromise 2-3 desirables per new system ]

What happens in case of equal growth bucket amounts ? Major pop wins every time ? What about equality bewteen the others ? 

OK loaded up a horatio end game and surprise: 

 horatio main pop +100/T booster on

kal'Tik'Ma +100/T booster on. 

So unless other fuckery is afoot (should check Turn 1 saves) it might mean default is +50 to growth bucket for all 'species' in most empires BUT slave races in the craver empire get halved growth TO MAKE IT HARDER FOR THEM TO OUTNUMBER CRAVER / LESS EFFICIENT TO GENOCIDE CRAVERS since the non-cravers grow much slower..

THE UI IS MISLEADING (chance to increment) - IT SEEMS TO BE AN ACCUMULATED GROWTH BUCKET - HIGHEST NUMBER WINS - NO 'CHANCES' OR PERCENTAGES ABOUT IT .

Also if you have small enough number of population of a type in a system that all of them can fit in the spaceport slots IT (removes them as next pop to grow if they where the next to grows) AND RESETS THEIR GROWTH BUCKET TO ZERO. [ONLY WORKS FOR NON PRIMARY RACE/SPECIES]  !!! This is part of an exploit to keep only one race growing despite a lot of others with low enough numbers. (move the 3 haroshems to the spaceport and back; move the 2 tikanan to the spaceport and back;  move the 1 remnant to the spaceport and back YAY WE'RE ONLY GROWING  HORATIO EVERY TURN .. )