Only if the bigger brain gives said person an advantage in the mating game. Anyway, I doubt we would know a new mutation that increased brain size in humans if it happened. There's so much variation among humans today, and only quite small changes are going to be survivable, such that it seems manifestly unlikely that any such change would be detectable. At least, not at the present time.
I think it'd be more fruitful, then, to look at past mutations, instead of current ones. Mutations that separate us from chimpanzees. Many geneticists have been looking rather hard at the human genome for areas in our genes that have been positively selected for. These regions are known as HAR's, short for "human accelerated regions," and we have found a number of these, and yes, some of the genes that changed significantly between us and chimpanzees are active in the brain. This is an area of current research, though, and I suspect the arguments are still ongoing and the evidence to be rather subtle.
But for evidence that some of our genes give us bigger brains over some other potential gene combinations, one need look no further than microcephaly:
http://en.wikipedia.org/wiki/Microcephaly
Microcephaly is a disorder where there is a particular mutation in one of the genes that is active in brain development. A destructive mutation in this gene causes the person's brain to be much, much smaller than normal.
Ok, thanks
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