How long would it take for two human populations to become a distinct species?

Say in the far future a population of 1000 humans apiece were somehow transported to the planets Zig and Zog (both of which have a mysteriously Earth-like environment and are on opposite sides of a galaxy) and lost communication with each other.

How many generations would it take before either planet’s inhabitants can be considered a separate species from the other (assuming the environment is identical on both and both have the same level of technology)? How would it be decided that they were if the two groups were to make contact with each other again?

The longest time, historically, that a group of humans has been isolated from the rest of the species is about 10,000 years in the case of the Tasmanian Aborigines. So the time would be significantly more than 10,000 years.

It’s actually trickier than that.

A new “species” is a subjective term.

If you’re talking separate enough to no longer be interbreedable, there are too many factors to give a hard, solid answer. As Giles mentioned, we know we can separate people for 10,000 years and still have modern homo sapiens.

But evolution is quirky. There might be a new species of hominids in an isolated valley in North Dakota, or the house next door. Chances are, there are thousands of new species of humans on the earth as we speak, even restricting the definition to those that are not genetically compatible with other humans. The problem is that none of these new species are likely to take hold because they’re probably not breeding amongst themselves. Incest is very frowned upon these days pretty much globally.

Now take a hundred couples and isolate them so that their offspring must mate with eachother and you increase the chances of finding a viable new species in only a few generations.

But two suffiently large breeding pools will never produce a new species as those belonging to a new species would not produce offspring as they try to mate with the general population.

In the scenario given here, it’s that the overall population would change over time (both planets are seperated from each other, after all).

Except that the gene pool of the entire population could drift, very slowly such that each generation is still able to breed with the previous generation, but adding up over time such that eventually members of one population can’t breed with the other.

Back to the OP, the question is somewhat complicated by the fact that “species” isn’t just defined based on whether two populations can breed together, but whether they do, when left to their own devices. This can result in artificial splitting of what, biologically speaking, would otherwise be considered a single species. For instance, chimpanzees and bonobos never interbreed in nature, and so are considered separate species, but the primary reason they never interbreed in nature is that they never get the chance: They live on opposite sides of a river that neither is able to cross. But if the Congo River dried up, or changed course, such that chimps and bonobos could cross, who knows, maybe they would interbreed.

Of course, your across-the-Galaxy isolation is even more extreme, so just based on that, you could say that they’re separate species immediately, since no Zigian will ever breed with a Zogian, even if they wanted to.

Gene pools do not drift. At least there is no evidence of such thing occuring on this planet. Every member of any given species can trace their genetics to a common single ancestor. It’s much easier to spread a mutation (evolution) from one point if there is little competition. Hence - Incest is best for evolution. Think about how long it would take for your genes to spread through the entire population or even half of it, given normal breeding practices in human culture.

While its possible for two unrelated (or remotely related) creatures to simultaneously develop the same mutation, the odds are astronomically against it.

Environment is key.

How radically different are the environments the two groups reside in?

Which one mandates rapid change to adapt or die?

Genetic drift occurs. The smaller the population the faster it occurs. With 6+ billion people on the planet it is a very slow process but it happens.

That said I’d be surprised if humans 5 million years from now could breed successfully with us.

especially as I will be dead

A valley? In the flatland of North Dakota? That would be a discovery to make the news!

The received wisdom in the biological community is that for large mammals like humans, about 500k years minimum. Maybe closer to 2M years.

Of course, it all depends on how much environmental pressure there is driving genetic changes in the respective environments.

What do you mean by this? What’s a gene pool, and what would it mean for one to drift?

Probably longer for humans than for other mammals our size, given our long lifespan and maturation time. A reasonable approximation would probably be that it takes a certain number of generations, and human generations are pretty long.

Speciation will happen when the two populations are unable to interbreed. But that doesn’t happen on a fixed timetable.

Isolation can be postzygotic–meaning that even if the species mate a zygote cannot form. Or it can be prezygotic–meaning, some change that makes it unlikely for members of the two populations to interbreed.

Prezygotic isolation could be something as simple as a geographic barrier. Or different mating seasons, or different secondary sexual charateristics, or changes in size. See Reproductive isolation - Wikipedia

Sorry, this is largely nonsense. There seems to be a fundamental problem here in your understanding of what a species is, and how speciation generally occurs. Speciation is not something that happens to individuals, it is something that happens to populations. It’s very unlikely that there are “thousands” of rare genotypes around who are able to produce offspring only with others of the same genotype, but not with any other humans. Given different selection regimes and sufficient time, even large breeding pools will produce different species as genetic differences accumulate between them.

You seem to be imagining that speciation happens when mutations occur that immediately prevent breeding with the ancestral population. While this may happen in very rare instances, most speciation occurs when populations become isolated and many small genetic differences accumulate between them over time.

From this I must assume that you don’t know what genetic drift even is. As per the link provided by Lemur866, genetic drift is a very important evolutionary principle. It changes gene frequencies in populations, and may eliminate some alleles entirely. It occurs in pretty much every species on the planet.

Hijack–it’s so rare they named a town after it! http://www.hellovalley.com/

Given that viruse are currently being used as delivery agents to modify DNA (like fixing colourblind Capuchin monkeys), it should soon be possible to radically modifya population within zero generations. I have a feeling Jules Verne suggested this :slight_smile:

No, I simplified. Usually at the time of species emergence, there is a hybrid period where there are members of a species that can mate and produce offspring with both the old species and the new one. Perhaps this is where “Genetic Drift” comes into play…

Maybe the allele involved in the “drift” is the protomutation stage. But it still takes a single member of a new species to start a new species. At Generation N, the whole population doesn’t miraculously simultaneously find just the right genetic combination to be a new species. One member of the new species is born with a genetic difference to make them a new species. The proto stage would and could be the “hybrid” members of the prior species - those that are genetically compatible with both the old and the new.

For a long period of time that new species will likely exist intermingled with the old. Breeding with those that it can. But at some point (okay here the concepts of genetic drift kind of apply) one or the other species will emerge. It takes an advantage of somekind for the new species to be the most likely one to flourish.

The Hybrid would technically still be genetically the old species, they just wouldn’t reject the embryo of a new species. In other words for best results - old species hybrid is female, new species is male. This explains why Mitochondrial Eve exists tens of thousands of years before the emergences of the modern Human species.

Yes, a virus could modify DNA.

I don’t know whether the modified creature counts as a new species. I am also unsure if the modfication becomes heriditary.

I grew up in ND. To call the valley that Valley City is in a ‘Valley’ is to make a mockery of the term ‘Valley’. :slight_smile: