When I was studying psychology in college, one of my courses was related to genetics and psychology. Intelligence is largely a heritable characteristic. In identical twin studies, the exception for nominally same IQ of identical twins was always traceable to low birth weight (shared placenta, lighter twin got poorer blood supply) or some kind of disease or accident that damaged the poorer performer. Even when raised apart in a rich/poor adopted out scenario, the poorer performer could be trained to "catch up" with the better performing twin.
In the context of what was known during my college years, the IQ test had a +/-5% repeatability over the lifetime of an individual.
In the current time, there is some evidence that "neural plasticity" can be trained and improved. The BrainHQ program has demonstrated success.
Working on the assumption that the ability to perform a given job has a minimum IQ floor, the higher that threshold, the fewer candidates you will have to fill the position. When you overlay the IQ spread of populations, it explains the number persons from that group that are capable of meeting the minimum. Just because a certain group represents X% of the general population is not a valid reason why that group has the same X% of candidates meeting the IQ minimum to do that job. That might be achieved if the pool of candidates is much greater than the available number of position i.e. 20 positions and 5,000 applicants.
Gimme Dat Job
"This interview is being recorded for quality control purposes. Is that acceptable to you, Mr Jackson?"
"Why, sure. But you can call me Tyrone."
"I'd rather not. What are your educational qualifiers for a position with the National Labor Relations Board, Mr Jackson?"
"I got myself a Bachelor of Arts degree from UNC-Chapel HIll in Afro-American Studies."
"Isn't that the program that didn't have classrooms, instructors, or exams?"
"Yes."
"How could you have learned anything?"
"Well, we hung out on the 'hood mostly, studying the brothers."
"Did you learn anything useful?"
"Oh, hell, yeah. Did you know the CIA introduced crack into the black community? Or that AIDS is just another way to keep the brother down?"
"I see. How do you figure any of this would prepare you for a position on the NLRB?"
"Well, ain't labor unions mostly about preserving jobs that don't exist, paying people to do nothin', and makin' sure no one knows anything different?"
"I think you're going to be happy here, Tyrone."