Primarily a biological question, but I'd also love to dig into social implications.

Here's the story -

Humans have colonized at least one other planet and trade / communication remained open for some time before a massive and catastrophic event took place that effectively isolated the planets completely. Note: haven't nailed down an exact cause yet, though I'm leaning towards hostile alien occupation that cripples the systems and regresses technological advancement to pre-historic times, or simultaneous world wars that lead to a similar outcome.

Here are the questions -

(1) How many generations would the isolated populations have to go through before their genomes were incompatible for reproduction? Is there any evidence for a timeline of separation of species like this? What kind of time frame am I looking at? I'm thinking Darwin's island birds, though I don't know of any generation estimates. Clearly something that can happen within a relatively short time-span (millions of years or less).

(2) Assuming both populations were able to recover and re-advance technologically, what would the social implications be once "first" contact was made? If one system was able to win the space race in a land-slide, would they look down on the other as inferior (animals / barbarians) and be left alone as some sort of nature preserve? What if advancement was at a similar pace and apparent evolution was minimal, with the exception of the inability to procreate?

  • $\begingroup$ ncbi.nlm.nih.gov/pmc/articles/PMC2605086 says 20-200 generations for a kind of fish. $\endgroup$
    – DKNguyen
    Commented Jul 8, 2020 at 23:51
  • $\begingroup$ @DKNguyen I suppose with that information my focus would be more on Part 2. Part 1 of the story sets the scene, Part 2 grapples with the implications and thus the meat of the World I am Building. $\endgroup$ Commented Jul 8, 2020 at 23:55
  • $\begingroup$ Although that article talks a lot about gene flow and to me, at least, muddles speciation with gene flow. $\endgroup$
    – DKNguyen
    Commented Jul 8, 2020 at 23:57
  • $\begingroup$ @DKNguyen so this scenario could potentially take place in as little as 1000 years? Seems like warp speed 0.o From what I understand, human genetics have very little variation across the globe and are - in a way - somewhat resistant to change. $\endgroup$ Commented Jul 8, 2020 at 23:57
  • 1
    $\begingroup$ I haven't been around for a thousand years so I really couldn't tell you. But maybe in complete isolation and wildly different selective pressures, maybe? I have no idea. I'd multiply 10...just to be safe. Heh. $\endgroup$
    – DKNguyen
    Commented Jul 8, 2020 at 23:58

4 Answers 4


If there was a driving force, then very quickly; otherwise it's extremely variable

If you WANT your two groups to be unable to interbreed, then you can have the colonists use some sort of genetic manipulation as their technology starts declining to make them more compatible with their new native world. I imagine them living in domes, and realizing that without advanced tech that they can't get replacement parts for, they'll all die unless they (tolerate extreme temps/are immune to arsenic/can use stereoisomers present in native organisms).

A lot of changes, or a few big ones, would move critical genes to spots and regulatory sites (or even new chromosomes) that mean the resulting offspring are non-viable or sterile mules. Some might even render the populations unable to eat the same foods, or they could even be allergic to the other population! The allergy thing could even be non-genetic if you wanted (everyone on planet X has traces of compound X, and everyone on planet Y naturally reacts to compound X because it resembles the protein of a common parasite).

Without this preemptive change, speciation can take centuries or millennia - it's up to you and mutation + evolutionary pressure. Some hominids were able to hybridize with H. sapiens (like the neanderthals) while others probably couldn't. I love biology because it CAN do almost anything, even if it usually doesn't.

  • $\begingroup$ Ohh the forced genetic manipulation is an interesting twist that I like. Wouldn't have thought of that. They may be able to maintain minimal contact in this scenario which would set up quite the conundrum when the two groups are re-introduced to each other on a massive scale; after maintaining somewhat of a shared history. $\endgroup$ Commented Jul 9, 2020 at 0:21

If you are also open to a social answer, your populations could be genetically compatible, yet still not mate between themselves.

From a racial perspective, the two groups could diverge quite easily, having a different skin tone / eye color / hairstyle ... They would deem the other people to be disgusting (just look, they are so barbarian they don't even have a clan tattoo!).

This may feel not disconnected enough to our world, though, so you could have other reasons. Maybe the Spacers will always wear space suits, with the Earth counterparts unwilling to mate with someone on a space suit. Or quite the contrary, the Spacers always wearing special suits on their atmosphere, they will only intimate when being completely naked, which was abhorrent for the mentality of the other group.

Another strategy for species differentiation would be that they mating cycles diverged. Perhaps, even before, separation, the human species slowly changes and end ups only being fertile during a full moon. Later, when the groups gets separated, the colony slowly adapts to the gravity and moon cycle of their new world. When they meet again, they have different cycles which precludes them from having an offspring (or perhaps they will only converge once every thousand years).


You can manipulate your mutation rate.

If two isolated human groups mantain the same selective pressures and the same environment and the same ecosistem, you will have to wait a lot (maybe thousands of generations) until the changes are signifficant to make their genetic code incompatible.

But, you can also include in your story a slightly increased mutation rate (for any reason) in one of the human groups, and create an ecosistem that could give some competitive advantage to some specific mutations (perhaps very short humans could need less calories, or could eat grass or other things available in their environment, or they could evolve a thicker skin due to the atmospheric conditions) In that case, you could achieve the genetic difference you need in just a few generations (perhaps 20 or 30 could do the trick). You have good real life examples of the influence in the animals of an increased mutation rates in Fukushima, Chernobyl, and Three Mile Island.

And about the second question: Both isolated human groups will evolve thinking "they" are the humans (no matter if they evolved green skin and horns), and historically, humans tend to dislike other humans that are very different to them (different to their own "normal human" concept), so... yes, I suppose some conflict could be unavoidable, how much and what type of conflict? Well... that is totally opinion based :) But I suppose it may depend on how "racist" or "mature" any of the two groups are.


It would take as long as the two colonies take to adapt to their environments. As a group evolves to their location, their genome changes. Wait long enough, they can make an infertile hybrid. Wait even longer, and incompatibility finally arises.


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