Welp, things went a bit off the rail this weekend while trying to control tribal expansion:

As you can see, tribes (or rather, the one and only tribe) end up expanding uncontrollably and at a ridiculous fast pace. What you see above occurs in less than a 100 years of simulation. 

This is the result of me fixing a bunch of minor migration issues that either made regular migration events fail more or less frequently, or generated invalid migration pressure values. Those issues appear to be what truly limited tribal expansion in the past. Once those "minor" things where fixed, the migration algorithm went to town migrating tribal populations. I'll have to redouble my efforts this next weekend to figure out what makes tribal populations expand so aggressively...