Made this sim to test out a hypothetical gene from The Selfish Gene:
"A gene that gives the instruction 'Body, if you are very much smaller than your litter-mates, give up the struggle and die', could be successful in the gene pool, because it has a 50 per cent chance of being in the body of each brother and sister saved."
My implementation is not "if you are very much smaller than your litter-mates," rather it is "if your litter is going to starve and you are the smallest one," but aside from that I think it is a relatively faithful interpretation.
Often the mutated gene does not survive. It can struggle for a while, flickering at 1-6 individuals, and die out. (Sometimes it dies in the very first generation by sheer dumb bad luck. That's life.) If it reaches a certain critical mass, though, it eventually spreads throughout the entire population.