In principle you can imagine a group experiencing selection for some trait by having members leave. However, it takes special circumstances, unusual circumstances. First, the people who defect have to be uncharacteristic: they have to differ from the group in the trait, having higher (or lower) than average values of that trait. The trait has to be heritable, but that’s not so much of an obstacle. Most traits are.
The next point is hard to arrange: in order to select strongly, as large as fraction as possible have to leave (and they should be be as different as possible), but at the same time, the per-generation fraction defecting has to be smaller than the rate of population growth, else the group simply goes extinct before anything interesting has time to happen. Since the long-run rate of population growth has been close to zero for most of human history, this mechanism usually doesn’t work.
There are exceptions. The Amish are the clearest case. They have very high rates of population growth, and have had for a long time, possible only in a really non-Malthusian society like the US. They also have had a high defection rate, but that has decreased quite a bit over the last century. In one Ohio Amish community, it seems to have dropped from 30% down to about 5%. Over times, high rates of defections may have selected for lower values of the ‘bright lights, big city’ trait. It’s possible that the Amish are genetically ‘plainer’ than they originally were. In fact, this almost has to be the case. It’s not clear how much of the decline in defection is explained by selection: the Amish may also have developed social techniques that make defection less likely, or it might be that the outside world is less attractive than it used to be. Or all three things may have happened.
I call selection involving differential defection “boiling off” – the lighter molecules escape more easily than the heavy ones, and the soup gets thicker.