Fun With: Genetic Drift

[Memnon]

Ghetto edit: besides, osmosis is the passive movement of water through a membrane.

[rhoenix]

Well, the mechanism of this particular sort of bacteria is meant to be bad for any actual living beings in the way of a bloom, but there would be a way around it for created structures. Carbon and calcium seemed to fit preliminarily, but I'll admit I hadn't researched that detail just yet.

As for cultural biases (regarding penis size in particular), I can intellectually figure how something like that might occur...but I'll get back to that when I'm revisiting sociology for each of my examples. Thank you for clarifying.

[Memnon]

Well, the mechanism of this particular sort of bacteria is meant to be bad for any actual living beings in the way of a bloom, but there would be a way around it for created structures. Carbon and calcium seemed to fit preliminarily, but I'll admit I hadn't researched that detail just yet.

As for cultural biases (regarding penis size in particular), I can intellectually figure how something like that might occur...but I'll get back to that when I'm revisiting sociology for each of my examples. Thank you for clarifying.

The mechanism - one way you could look at it is ways to kill lots of creatures.
One is to drastically increase or decrease the pH of the surrounding water - some byproduct of their metabolism, or something. I'm not a molecular biologist or chemist, so I couldn't tell you how possible that is. Perhaps these organisms were genetically engineered themselves to perform some pH related function, but accidentally released.
Another way is to increase the heat of the water - though they'd have to be extremophiles which waste tons of energy. Can't think of a reason for that. Another way to do it is to significantly reduce the oxygen in the water, like I said earlier. The problems with all of these is that biological processes can only proceed so quickly.
Note: all of these would probably get energy from photosynthesis. If there's enough CO2, the organisms will survive for an extended period of time after losing sunlight - so it could be a chronic problem, not some sort of uber deadly insta killer.

No problem on the clarification. Remember that, however, there aren't really very many selection pressures on humans as they are now - presumably, if they can colonize space, they will probably have even less. Beyond the engineering, it's likely that they will remain close to H-W equilibrium, which maintains that allele frequencies will remain constant if:
the population is large
there is no sexual selection
there is no mutation
there is no migration
there is no natural selection

Now, these conditions are never met in nature, but you can get pretty damn close. Technology certainly helps that, and I would say that for significant genetic changes to take place in a mere 2000 years there have to be enormous selection pressures beyond the social. Remember that evolution proceeds slowly, with bursts (the punctuated equilibrium hypothesis). Even these bursts are still slow, by our reckoning of time, however.
Example: humans haven't changed much in 10k+ years, with less tech than these people would (presumably) have.

[Memnon]

Okay, time to check out the basic assumptions.

= Technological level should be assumed to be about 200-300 years ahead of our present day, and each colony has the full knowledge of their former homeworld, with the tools needed to be on the same technological level, though adapted for their local environment.
= All three colonies should be assumed as having enough local resources to be completely self-sufficient.

Both of these will mean that significant mutations will likely not occur/spread throughout the civilization unless they engineer themselves. Now, that could certainly be possible if they already engineered themselves, but it's not terribly likely to happen unless they have a very modification-happy culture. In other words: where are the selection pressures? The bacteria seem to be the assigned problem for the water humans, but aren't very convincing yet.

Once you work out the bacteria, I would recommend reading the Pern books by Anne McCaffrey to get an idea of how to treat it (that book has 'thread' which behaves very interestingly. I heartily recommend reading them for social ideas for the water people).

This last question is very interesting. Again, wrt to the water people I would look to the Pern stuff for inspiration. For the earth people - you can draw a bit from the dwarf stereotype. These people will not move very quickly, and that could affect their outlook on life. Probably they would be more pensive than humans are - though it's entirely possible that they could be relatively unaffected by the changes due to technology like their equivalent of automobiles, and speech-to-text. And the other people - it doesn't seem like they would have any significant reason to be culturally different from their founders, since only a few variables are different, and they have fairly advanced tech.