The class of a person in the 1984 world is determined according to the class one is born into and by ability as defined by the Party, according to the discussion in Part II Chapter IX. There are three classes, the proles, the Outer Party, and the Inner Party. The proles make up about 85% of the total population, the Outer Party about 13%, and the Inner Party about 2%.
If a person is born as a prole, they cannot enter any other class. Extremely intelligent proles who appear to have the potential to make trouble are eliminated by the Thought Police as a routine matter, to ensure social stability. The rest are simply left alone.
Party membership is open to and relatively equallly distributed among persons of all races and regional origins. Persons born into the Outer or Inner Parties usually remain in the class they were born into. In principle, admission to either branch of the Party is governed by examinations taken at the age of 16. In practice, the only function of these examinations is to allow especially stupid members of the Inner Party to be demoted to the Outer Party, and especially clever and ambitious members of the Outer Party to rise to the Inner Party, again to guarantee social stability. The number of those who move in either direction is, however, quite small.
The result is an oligarchy whose membership is extremely stable and relatively unchanging. Nevertheless, if it were pragmatically desirable to change recruitment patterns -- to take in talent from the proles, for instance -- this could also be allowed. The overwhelming concern of the system is to produce a stable and enduring ruling class that shares a common ideology and allegiance. All else is secondary.