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Coming from an earlier asked question, I was wondering how long it would take (potentially a range) for two human populations that are separated from each other to speciate enough that they would no longer be able to interbreed.

A scenario for such a case would be:

Far in the future, humans have developed the technology to commit to interstellar travel and colonize new planets. Different groups set off to new parts of the universe, and can sometimes get lost/cut off from the rest of the original group of human civilization that they derived from. At what point would speciation prevent one segregate popoulation be unable to breed with another?

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    $\begingroup$ Welcome to the site, Andrew! Does this speciation question answer your query? $\endgroup$ – Frostfyre Jan 22 at 18:19
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    $\begingroup$ It appears to have an adequate answer, thought the answer provided was never accepted by the original asker. $\endgroup$ – Andrew Su Jan 22 at 18:23
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    $\begingroup$ South America was populated not less than 11,000 years ago, most likely closer to 15,000 years ago, and afterwards had basically no interaction with the rest of the world. So we are quite certain that 11,000 years is not enough. For comparison, the entire history, from the oldest clay Sumerian tablets which we can read to our very days, spans less than 5,000 years. $\endgroup$ – AlexP Jan 22 at 18:36
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    $\begingroup$ How are you defining ability to breed? There may be a period of time where the two populations are genetically close enough to breed; i.e. no chromosome mismatch problems, but still unable to breed in practice. A current example is mastiffs and teacup pomeranians. A male pomeranian isn't tall enough to mount a female mastiff, and a male mastiff would crush a female pomeranian. But if artificially inseminated, either pair could produce viable offspring. $\endgroup$ – Ryan_L Jan 22 at 19:00
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    $\begingroup$ Does this answer your question? How long would a modern human population need to be separated in order to suffer allopatric speciation? $\endgroup$ – Dewi Morgan Jan 22 at 19:29
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The factor you're looking for isn't time. That sounds funny, doesn't it? But it's true. You're looking for a divergence in the human gene pool to the point where it would be incapable of reproducing with the previous point. And to that end, time is one factor among many, and not even the most important one. There are species on this planet which have basically remained stagnant and unchanged for ten of thousands of years, and species that have radically altered between the same set of ten of thousands to produce a dozen species that can't interbreed.

What you're looking for is environmental pressures. Evolution is always in response to a stimulus, and time isn't that stimulus. Predators growing better, new and different ecology, a random mutation which makes the animal far more suited to it's current predicament and thus possesses a massive advantage towards reproduction - those are all things which affect evolution, and time only affects the last one. True, the current model suggests that we need the last step (random mutation) for evolution to occur, but the time it takes to mutate seems like a limiting factor, i.e. will not happen in less that this level of time if all the other criteria are met.

In other words, take your example. The humans gene pool will alter based on whatever climate that the planet contains - if it's a warm planet, it would develop to be similar to humans in warm places on earth, and so on and so forth with things like cold climates and damp climates. To truly have a radical shift in the human genome, you need them to live unaided in an environment that human aren't capable of living in now unaided, and them having adapted to it, presumably having sped up the process with genetic engineering of some form.

It's possible that, regardless, there will be a buildup of thousand upon thousands of small random mutations within a gene pool that will eventually lock everyone else out, but that's a time scale of hundred of thousands of years, if that's even possible to occur without some form of outside stimulus. (This is based on the fact that there doesn't seem to be a problem with interacting with uncontacted tribes, and some of these have possibly been in isolation since the dawn of man.)

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    $\begingroup$ Is there a way to reword the original question to further exploration of the topic of what factors would need to be in place in order to bring about such a change in the evolutionary process. Considering the scenario where a colonizing population is completely isolated in an new frontier, sometimes with the inability to hand-pick the ideal planet to settle in. $\endgroup$ – Andrew Su Jan 22 at 18:55
  • $\begingroup$ Asking what factors should be on a planet to induce evolution seems is likely to get shot down as too opinion based - there are just too many ways to go, though if you want something specific, like how a mostly water world would affect evolution, that's perfectly fine. You should also make sure to do something about the technology level - civilizations capable of space travel can build self-contained environments in harsh conditions, so have them reduced back to the Stone Age or something. Finally, it's poor etiquette to change a question drastically, so don't reword, ask a new question. $\endgroup$ – Halfthawed Jan 22 at 19:08
  • $\begingroup$ Wouldn't the adaption also have to be directly related in some way to reproduction, in order to make the two strains incompatible? We've adapted to various environmental factors on earth, but that hasn't introduced any issues, I think? And unless the reproductive method changed, isn't there a limit to how much everything else could change? $\endgroup$ – MartinArrJay Jan 23 at 9:01
  • $\begingroup$ @MartinArrJay I'm guessing that random mutations could eventually move the biology of a population to be incompatible with the reproductive cycle of that which it was derived from. You wouldn't need to change the reproductive method per say, but the chemical composition of different parts of the process could also be a factor e.g. the fluids carrying gametes and the tissue holding them. The chemical composition could be influenced by the environment then. Not an expert on the topic, but this explanation seems to be adequate from a cursory glance. $\endgroup$ – Andrew Su Jan 23 at 13:28
  • $\begingroup$ That or instead of modifying the physical reproductive process from an organism's perspective, but the expression of certain chromosomes during the zygote's initial replication. $\endgroup$ – Andrew Su Jan 23 at 13:36
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This can't really be answered in any definitive terms as it relies way to much on a ton of specific factors. Speciation in the past happened by selective pressure from environmental conditions over long periods of time. This isn't as applicable in modern or future civilizations where we have more control over our local environment.

Modern humans have been living in wildly difference climates for around 200,000 years and to my knowledge we haven't developed any non-reproductively viable groupings. Homo-sapiens and Neanderthals deviated approximately 400,000 years ago and could probably interbreed up until they disappeared 40,000 years ago.

If your story requires severe speciation in a short amount of time, you can explain it with genetic engineering to adapt a colony to their new biosphere. You could also blame the harsh cosmic radiation mixed with an inbreeding that occurred on the colony ship's voyage.

With an extreme amount of genetic engineering this would possibly be achievable in a few generations, but would come with a large amount of ethical gray area and risks of unforeseen consequences.

If we can accept the ability for two populations to be able to breed, but produce sterile offspring, then I would give a ballpark of 100 years of intensive genetic modification or 1000 years of moderate genetic modification.

You could also concoct an artificial divide between the populations, say if one genetically engineered a resistance to deadly pathogens and thus became passive carriers for the disease,they could be restricted from intimately interacting with a more vulnerable group.

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