The idea of divergence due to certain kinds of people choosing to go to Mars is interesting.

On the other hand, natural selection will not play a role. The times scales are not long enough. Natural selection requires hundreds or thousands of years, and it presumes that people mate and that people selectively survive due to environmental factors. On Mars, humans will have the support of technology, so people will die from accidents rather than from being eaten by an animal because their vision is not as well adapted to Mars.

We also will have to deal with the immunity problem, because people will come and go between Mars and Earth: a Mars settlement cannot afford to risk a pandemic. But we are developing a better understanding of our genetics and immunity every day. Presumably by the time there is a substantial colony on Mars, we will have figured out how to control our immune systems, and deal with pandemics using biotech.

What might happen is that humans will intentionally engineer themselves to be more compatible with the Mars environment. They can choose to do this in a way that will preserve their ability to return to Earth. For example, humans might alter their genes so that their bone density stays high, despite the low Martian gravity.

It is hard to predict what will occur, but things will not play out the way they did during the evolution of primitive humans. Technology changes things.