How do we know that something isn’t happening? Perhaps that 0.004% is “explosively taking over” on an evolutionary timescale which means that (no other factors interfering) it might be the dominant situation in another few hundred thousand or some millions of years.
Of course what seems a lot more likely is that it might be “small beer” compared to a lot of other environmental changes that happen meanwhile, necessitating much larger adaptations for reproductive / survival fitness.