"The population of America . . ." sounds as if it should be followed by "is 300,000,000" or "was 200,000,000 in 1913," or "has increased blank percent in the last 50 years." The preposition of seems to be go best with the population as a whole, not with the various segments within that population.
On the other hand, "The population in America . . ." sounds as if it should be followed by any of the following: 1) "is comprised of 20 percent Black; 53 percent white; 15 percent Hispanic," etc.; 2) "has grown by 128 percent since 1913"; 3) "[comma] compared to the next-most populous nation in the world, is only 20 million fewer people"; or 4) "in the over-85 age-group has grown significantly in the last decade, from 12 percent of the total population to 18 percent of the total population."
The preposition in seems to work best when addressing the various segments (plural) within the population (see number 1 above), or a segment--as in a rate, percent or percentage--that represents an increase or decrease (see numbers 2, 3, and 4 above).
There may be a few exceptions to the above, but the information on the whole is likely accurate.