Natural selection happens just like it used to, it cannot really be changed. Whether an
organism survives or not as part of natural selection is determined by that organisms adaptation
to it's environment. Thus
fitness is relative to environment.
Crocodiles are well adapted to swamps, thus they are "fit" in them and survive, in the arctic they are not "
fit" and do not survive.
So we come to humans who, at this point are able to create an environment that fits our needs, thus making us fit without changing phsyically. If we lose this
ability to customize our environment (
nuclear war,
asteroid), we become correspondingly less fit and therefore less prone to survival. In our current state the selection pressures have simply changed form, instead of being
predators and pointy rocks, they are now
high-cholesterol foods and
terrorists.