My understanding of biology is that all living organisms adapt to the environment around them. I would even say the environment shapes the very essence of an organism.
Gravity alone is a huge factor and will be the most imminent one. It is what I am most interested in this question but I believe there will be a number of other strong factors.
Example - Due to fact the food is being manufactured it could be an effective way to feed a population by only liquid or gel food (maybe a few pills or something). Intravenal food would be the next step. That will greatly reduce the amount of energy needed for digestion... changing our bodies in the process.
This doesn't only involve humans. The genetic information in Escherichia coli can be modified to suit human needs better on Mars...
Eyes can be adapted to low light vison.
Space perception will change a lot due to the fact that nobody on Mars will be able to experience open space... at least not in the early generations.
Our sound perception will change. Martians will only hear the sounds they or their machines create... other than that maybe howling wind.
This all will have an impact on the brain structure as well. In very close connection with modern technology...
The list goes on and on... it leads me to one conclusion. It is way more effective to a adapt human body to the new conditions than to artificially create conditions that exist only on Earth and can never exist on Mars. So... Is there any SpaceX vision what adaptation process will the colonists go through?