Concerning long term effects of zero gravity on humans living in space compared with those living on super earths with 1.5x to 2x G, how could the gravity differences cause reproductive failure among the two groups? What biologic traits might be selected for (or against) that would start to cause infertility between humans living in zero gravity, and those living on planets with more gravity than earth?
What you're referring to is known as speciation. In the context of two planets between which there is minimal gene flow, this is probably going to be allopatric speciation. Speciation is the process of a single species breaking into multiple species, which are generally defined as populations which cannot create viable offspring when breeding with one another.
Allopatric speciation is speciation that occurs due to geographic separation. This is seen on earth when something like a river valley separates a population of animals into two populations that can no longer reach each other. Chimpanzees and bonobos are one possible example. They do not differ significantly in terms of their environment, but their separation on either side of the Congo river has led to the formation of two different species.
In order for allopatric speciation to occur, there do not need to be any specific population pressures that force evolution of the two groups. Rather, it has been hypothesized that genetic drift is sufficient to cause the two groups to become reproductively incompatible given sufficient time.
A long period of separation is the critical factor, here. Over shorter periods of time, the two populations are likely to remain capable of producing healthy children, even if population pressures have changed the physiological traits of one of the populations. Corgis and German Shepards, for example, can still breed, despite the fact that they've evolved (with human help) to thrive in very different roles.
The first obstacle would be the ability to physically mate with each other. Modern astronauts face fairly serious muscle loss and atrophy after weeks in space, so humans living in space would be that much weaker. I'm doubtful that they would even survive visiting the 2G planet, let alone build the necessary blood pressure to perform the act.
The 'super-earth' humans would also have problems, if they were to meet the others in space. Not surprisingly, there has been quite a bit of speculation (and very little actual testing) about zero-G sex, and the consensus seems to be that it's very difficult. Basically, without gravity there is nothing to keep the participants together. One thrust and you're tumbling across the room. There is also apparently a blood flow issue, in which the human body, adapted for Earth's gravity, has trouble sustaining physical arousal. It would presumably be even harder (or ...not) for people adapted to higher gravity.
This also leads to a physical adaptation that could cause speciation, as natural selection would lead to the space-people to be better able to copulate in zero-G, which could eventually lead to substantial changes in the reproductive organs.
It could be that our reproductive system doesn't work too well in zero gravity. That would impose a strong selection pressure to adapt the reproductive system to zero gravity, which then in turn might it incompatible with those living in a gravitational field.
The inability of cross-breeding would on one hand be a physical incompatibility, but the divergent evolution might also create a genetic incompatibility (for example, if the genes for the different reproductive organs are incompatible with each other).
In any case, something like this can only happen if the groups are not cross-breeding for a sufficiently long time (which may be reinforced by other adaptations to the respective environment which might make it impossible for the space dwellers people to even visit the planet surface, and at least inconvenient for planet dwellers to be in space).
If we imagine the adaptations for the different lifestyles are based on genetic engineering ( which seems quite logical in a interplanetary civilisation ) then we could expect rapid changes which may be sufficient to cause effective speciation. Not being a biologist I don't know exactly where you reach the point that offspring are infertile, but if peoples are engineering themselves in the interests of fitting better in specific environments then they are likely to hit that boundary sooner or later.
Also people who engineer themselves to be able to live in space might take things to a much greater extent than one might expect, like the Ousters in Dan Simmons' Hyperion novels - the lack of gravity meaning that our limbs designed for ground travel might be significantly changed or discarded to make it easier to get around.
There could be an interesting debate among colonists at the speciation point, where you have a political question of whether they want to become a new species or not and that becomes a decision for all the colonists of the new world. A speciation referendum is a great starting point for interesting stories.
A scenario that might lead to this case might be genetic engineering followed by a loss of the technology.
The rationale is if you took a group of colonists and dropped them on a 2G world, they wouldn't survive. Our circulatory system, bones, and joints couldn't handle it. Our bodies would break. The fittest might survive for a while, but eventually the high gravity would kill everyone.
So to survive on the planet you'd probably need one of these:
- Anti-Gravity technology to lower the gravity around the colony
Other advanced technology, perhaps mechanical exoskeletons and implanted devices that support the heart and lungs.
Genetic engineering, to modify the human body to make it more sturdy and to re-enforce the circulatory system. Perhaps supplemental organic hearts.
If you have the first or second, then infertility between the two branches of humanity wouldn't be an issue because there'd be no evolutionary pressure for humans to change. At least not for hundreds of thousands or millions of years.
If you have the 3rd, interbreeding would still be possible because of the high level of attainment in genetic engineering. They could just alter the genes as necessary.
So you'd have to have the genetic engineering that would make interbreeding impossible... but then lose that technology to prevent further engineering to make it possible.
On the flip side, the zero-G humans really have no reason to ever need to be zero-G humans. Rotating a space habitat can create 'artificial' gravity. And there's plenty of places in the universe where, even if at low G, there's enough gravity to prevent major health problems.
Another aspect that no one has touched on is radiation: You will be exposed to radiation that people on Earth aren’t.
We can assume shielding and whatnot, and so you’ll still be able to live mostly cancer-free and make babies, but you will see mutations that Earth-people will simply not have.
Like others, keeping the population separate, having properties naturally selected for based on environmental changes, and difference in goals for genetic engineering will drive these changes. But the continuous difference in amount and composition of the radiation these space-people will endure will be the driving force behind the actual genome shuffling required to make Homo Aéronomie and Homo Sapiens incompatible.