I was more interested in the differences between J and P, though.
From a biological level, it's a bit more complicated, as it involves the combinations of multiple biological systems. But behaviourally, there's 2 types of self-sustaining behaviours:
1) The person who always gets the job done, but has to be pretty difficult sometimes to get the job done. Their friends and colleagues often overlook the occasional difficulties with the person, because they can be relied upon. But that means that the more extreme their attitude, the more they are reliable, and the more they are valued.
2) The person who is usually easy-going, and would rather not always get the job done, because he'd rather take a more relaxed and easy-going attitude to life. Their friends and colleagues often overlook the occasional screw-up, because they are so easy to get on with. But that means that the more extreme their attitude, the more they are easy to get on with, and the more they are valued.
Both are valued in society, but for different reasons, and consequently for different ways. More importantly, the feedback they get from their behaviours tend to reward being even more and more J or more and more P.