|MadSci Network: Evolution|
Your question is a very interesting and complex one, and I am not sure that I can answer it fully. This is because if ever we get to the point of mating and making children in space or on the planets, there may be may details of the whole process which do not meet our expectations!
One may, however, expect something like this:
People are not very suited for space travel. Even in relatively short duration flights (say to the moon, or on a space station) there are many effects on the body. One receives a lot more particulate radiation (cosmic rays) then at the bottom of the atmosphere, and there are losses in muscle strength and bone calcium levels which occur rather quickly. (These and other effects have been studied extensively, and as you point out there will be many experiments done on the new space station).
The first generations of any animal (human or otherwise) will not automatically be adapted to their environment, for the simple reason that offspring are normally very like their parents (with variations). Darwin correctly hypothesized that it is the interaction of the variations with the environment which produces species change, and that "Lamarkian" processes do not occur. (If they did, for example, then we should expect that amputees would have limbless children lots of the time).
If living on Mars (for example) required a very different body type than we have on Earth, then it would take many generations for this to come about. Those who are more adapted to the environment would live longer and have more children than do the critters that are not as well adapted. Those who have more children would eventually dominate the population.
On the other hand, humans are very good at creating artificial environments which remove the selection pressures. We would bring our air and water with us (or make them locally) and learn how to get the right exercises or diet or whatever to minimize the effects of living in low gravity or no gravity.
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