Regarding modern medicine, it does seem to have the effect of allowing people to survive today who might not have otherwise. But this means that it increases

Regarding modern medicine, it does seem to have the effect of allowing people to survive today who might not have otherwise. But this means that it increases the diversity of viable genes. Is that really "slowing down" evolution? I'm not sure what a "rate of evolution" might be. Is it the rate of mutation? Has that really changed with medicine? Is it the probability that a mutation turns out to be viable? If so, hasn't that increased at the hands of modern medicine?