It's not a rupture of the rubric of natural selection. It's a break in the point of natural selection. If the kid dies we eliminate of chain of people that are highly susceptible to that disease, by saving the kid's life we are perpetuating that weakness. There's a lot of bad side effects to that, my wife's family is a great example, every generation has lower and lower standard health, has to take more and more prescription medicines just to reach adulthood, and then they pass those weaknesses on to the next generation. Eventually there's got to be a meltdown point.
So since we've been tinkering with genetics on accident for centuries there could be a good reason to tinker deliberately. The problem is when to say when. Using genetic modification to get rid of things like sickle cell and asthma are probably good. Cosmetic alterations would definitely be bad. Somewhere in between there is the line, not sure where though.
But those kinds of questions pop up with any technological advance. "Who gets the advance & who doesn't?" "How do 'we' decide?"
In general the best answer has always been to let the free market decide. Then people's actual values get to come into play in an organic way to achieve the (presumably) most ideal result that the real world can produce.
In an authoritarian, fascist, overregulated society a new technology such as this could easily turn into a horror. But in the free world? The best (or at least the most benign) scenarios are most likely.
That's why I'm basically an optimist WRT these new technologies. Because I'm basically an optimist WRT the future of freedom generally.