Human ability generally seems to be power-law distributed - the "80-20" rule often hold in research. I'm just checking out Murray's "Human Accomplishment", and this is the impression I get from his data - whether it is valid data remains to be seen. But this might have many other causes, from Matthew effects where widely cited people become even more cited (and maybe get great research environments) to multiplicator effects where productivity is due to multiplicative effects of more or less random factors - only a few gets a lot of them, and the result is a lognormal distribution.

IQ, as ability to make rational inferences in new domains, may be just one of these factors. Low IQ certainly precludes much scientific achievement. There are also selection effects where getting into the right schools or professions require overcoming IQ-loaded hurdles. The real benefits of IQ among geniuses might be smaller than other factors - but having more people with high IQ will certainly not decrease the pool of potential geniuses.