Question about speculative future human evolution?

Question

I am wanting to make a story where humans are seperated in two groups, a colony in space and those on earth. and there was a nuclear war induced ice age also. I discovered that an ice age can be at least 100,000 years long, I want to be somewhat accurate but not so much it hinders my main premise.

how long would two groups of humans have to be seperated for them to meet and be genetically incompatible?

Answer 1

The same rules don't apply to humans who can selectively breed and modify the genetics artificially.  Minus that, the lowest number I have heard from from a geneticist was something like 40,000 years. It seems way to short of a time to me.  Some animals can produce hybrids or viable offspring when they are separated by a few million years.

Answer 2

That's a huge question so let me throw just one possibility at you that haven't considered yet:

If Hi-Tech civilization holds together long enough to both build a large space colony and see the next ice age, then for better and for worse, it will  develop the abilty to consciously engineer the human genome.  In which case, there will not be just two types of humans, but many.....

Not to mention intelligent animals, intelligent chimeras (completely new lifeforms), and sentient computers....

Any number of Science-Fiction novels have dealt with these possibilities, far too many in fact to go into here.  One SF movie that dealt with this subject very well was "Gattacca" with Ethan Hawke and Uma Thurman; if you haven't seen it, drop what you're doing and get a copy.  

On the non-fiction front, a book well worth reading would be:

"Redesigning Humans: Our Inevitable Genetic Future" by Gregory Stock  (Amazon link below)

I hope all this helps....

Nimadan

Answer 3

To live at a certain gravitational level requires a certain type of body to survive alone in the environment.  I think our genes immediatly start to determine changes to acommodate the situation.  Space beings probably have the same trouble getting to us as we do getting to the lives that live at the bottom of the sea.  They need special aparatus to survive at these depths and to survive at these depths of human existence on the planet too can be troublesome for those who live on higher more subtle levels.

Answer 4

There would have to be a significant genetic mutations and a species shift for that to happen. Time isnt the issue accept in terms of a mutation...

Answer 5

It depends on how extreme the differences in environment are.  If the space environment requires some trait that is essential for survival that wouldn't be required or helpful on earth then the separation would happen faster.  On the other hand, you also have to account for culture.  Humans evolve slower biologically now than we do culturally.  Maybe your separation could be a cultural separation instead of a biological one.

Answer 6

Humans are 200,000 years old, but we kept inter-mingling. So I am pretty sure that 100K would be enough if you have separated groups in different environment.