If there is any substantial heritability of merit, Henry Harpending notes — where merit is whatever leads to class mobility — then mobility ought to turn classes into hereditary castes surprisingly rapidly:
This figure shows an initial population with normally distributed merit. A new merit based class system is imposed such that the two new classes are of equal size. In this free meritocracy everyone with merit exceeding the population mean moves into the upper class and everyone with merit less than the average moves into the lower class. The second panel of the figure shows the resulting merit distributions by class before reproduction and the bottom panel shows the distributions after endogamous reproduction. This model assumes that the reshuffling of genes during reproduction leads to normal distributions in the next generation within classes.
The process continues for several generations. By analogy with IQ the additive heritability of merit is set to 0.6 so there are substantial random environmental effects. The second figure shows the evolution of class differences over four generations or about 100 years in human terms.
Class mobility after the first generation is 30% while after four generations it has declined to 10% and continues to decline after that. The average merit in the two classes is about -1SD in the lower and +1SD in the upper on the original scale, corresponding to IQs of 85 and 115.