Each tribal polity can only occupy one hex at a time (not a strictly realistic assumption, but we can simply say that there are no other polities of note in the area). Carrying capacity $K$ decreases as resources in the hex are used up. Once $K$ converges with the population size, the tribe must either move or split to maintain balance. When a tribe leaves, the area begins to recover.
There are certain hexes with a high desirability which serve as generators: once a tribe reaches one, there's never any incentive to leave, and so more and more tribes are spun off as a result. When I add discoveries, these will become probable Ur-city locations.
The desirability issue is particularly obvious when a saturation point is reached. I've not yet added oceanic migration, so the tribes bounce around from place to place like so many gas molecules in a closed container.
(I feel compelled to point out that 1500 years is a pretty long time. In 519 AD, the Kingdom of Wessex was founded. In fantasy, we typically deal with deep-time related to people groups - thousand year reigns are not unusual. In real life, empires are generally much more temporary.)
This simulation progresses much faster than the "civilized" simulation, due to code rewrites and the fact that these tribes aren't doing much other than running around and eating all the wildlife.
In these first runs, I noticed that the raw desirability, which is the main selection factor, is far too high for areas that humans (the only race I'm working with for exploratory purposes) would normally avoid, such as deep tundra. So that will need to be fixed. I also want to add a mechanism for tribal warfare, if there is a some greener grass on the other side but it's occupied already.
