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Let's suppose that a homo species divides in two separate species (like H. sapiens and H. neanderthalensis) and they cannot breed. Could their descendants evolve to be capable of reproducing together again?

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    $\begingroup$ H. sapiens and H. neanderthalensis actually did produce fertile offspring. All modern humans who are not of pure sub-Saharan African descent have Neanderthal blood. People of Asian descent have not only Neanderthal but also Denisovan blood. (And in some genera promiscuity reigns supreme; for example, wolves, dogs, jackals and coyotes are all interfertile despite being different species.) $\endgroup$
    – AlexP
    Jan 22, 2020 at 10:06
  • $\begingroup$ Yes, I know. I use them as an example of two related species that can breed and I presuppose that in the future they will not can (if the H. neanderthalensis were still alive). $\endgroup$ Jan 22, 2020 at 11:02
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    $\begingroup$ Reproductive isolation between species can have different mechanisms. There can be geographical barriers. Or the two species can be sympatric but never try to mate with each other, because they don't consider members of the other species to be potential mates. See the reproductive isolation article on Wikipedia for a sample (far from complete) of diverse mechanisms. Which means that you need to specify why your two species cannot breed; some reproductive isolation mechanisms are much easier to overcome than others. $\endgroup$
    – AlexP
    Jan 22, 2020 at 11:13
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    $\begingroup$ There are many cases of pairs of related species that can and occasionally do interbreed. Dogs and wolves and coyotes, Horses and donkeys. Lions and tigers (but not bears, oh my! -- but yes grizzly, kodiak, and polar bears). House cats and multiple species of wild cats. Bison and domestic cattle. Some even produce fertile offspring (dogs and all their relatives, some of the cat hybrids). $\endgroup$
    – Zeiss Ikon
    Jan 22, 2020 at 14:34

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It depends on your definition of "cannot breed".

  • If it is completely impossible to have a hybrid (like human and chimps) - then no. It needs a statistical miracle or genetic engineering to make it happen.
  • If species can have a hybrid, but it is infertile - then (partially) yes it is theoretically possible. For example, mules are infertile, but there are known cases when a female mule got pregnant from a horse. So if mating is happening quite often, donkeys would "inbreed" into horses and might introduce new mule-like species that can breed with other horses.
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  • $\begingroup$ Yes the process of swapping genes in this way is possible and is called introgression. The possibility of a human chimp hybrid might not be completely impossible as the two species are closely related. Although they differ in chromosome number so do donkeys and horses. Also some humans actually do have 48 chromosomes instead of 46. $\endgroup$
    – Slarty
    Jan 22, 2020 at 13:54
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Yes, it is possible.

There is a known event where two separate specialized species of fish inhabited some African lake, one was pelagic and other benthic, they did not cross-bred due to pre-zygotal isolation (not trying to mate each other even if biologically capable).

But then, pollution to the lake led to disruption of their habitat, and they merged into a single species by starting to breed again.

Just as possible with hominids and likely also. I can also imagine partial post-zygotal isolation reversed (low inter-species fertility becoming greater as they mix).

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  • $\begingroup$ But were they incapable of breed at some point? Not due to isolation but biological incapability. Maybe the answer is implicit in the terms you use, but i'm non native speaker and I have no idea about biology either. Looking an answer "for dummies". $\endgroup$ Jan 22, 2020 at 10:58
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    $\begingroup$ @MiguelNoTeimporta: Reproductive isolation can have different mechanisms. In the case of the cyclids mentioned in the answer, reproductive isolation was behavioral -- the two species of fish lived at different depths, and even if they encountered one another accidentally they did not consider fish of the other species to be potential mates. $\endgroup$
    – AlexP
    Jan 22, 2020 at 11:08
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It is important to be very clear about the definition of the word species in this question. Many definitions count separated groups of animals that have evolved different characteristics (and either do not meet each other or do not choose to breed if they do) as different species for example polar bears and brown bears, however by other definitions polar bears and brown bears are the same species because they can mate to produce viable off spring and have recently been found to have done so in the wild.

So ignoring geographical isolation / morphology and purely looking at the genetic possibility of breeding the answer is probably not if there are just two groups from one common ancestor and the genetic separation is large. Over time the fertility of cross breeds would decline and at some point it would not be possible. From that point on the two groups are separate species and will not be able to merge again because of the random nature of mutations and the difficulty in unscrambling an egg.

However there is another possibility, if there were multiple groups descended from a common ancestor it is possible that group A could breed with group B who could breed with group C, but where group A and C could not breed. Repeated cross breeding between A-B and B-C might provide the possibility for an A-B hybrid to breed successfully with a B-C hybrid and mix the genetic lines again, but it would depend on the detailed circumstances if this were possible or not.

This is the situation described as a ring species and several examples have been claimed to exist.

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A common definition of species is "a group that interbreeds in the wild to produce viable offspring."

If two groups do not meet in the wild, or if they meet in the wild but choose not to mate, then they are different species even if they can produce viable offspring in captivity. There are numerous examples of such divisions. For the cases where the groups don't meet due to a physical barrier (e.g. mountains or oceans), all it may take to reunite them into a single species is to remove said barrier.

Not producing viable offspring is a bigger challenge. Once the groups have been separated and face different selection pressures for long enough, their genomes will diverge. They might still produce non-viable offspring in captivity, though, and those offspring may be able to interbreed with one or both parent species. Do this for many generations in just the right combinations, and you may be able to make their genomes converge enough that offspring are viable again.

If their genomes have diverged enough they can't even produce non-viable offspring, you will have to resort to genetic engineering at a level far beyond what we have available today.

The specific example you gave, Homo sapiens and Homo neanderthalensis, is another matter. These are in fact two subspecies, more correctly called Homo sapiens sapiens and Homo sapiens neanderthalensis. They can and did interbreed in the wild to produce viable offspring, which is why they are members of a single species, Homo sapiens. The Neanderthals didn't really disappear; they were just assimilated into the Cro-Magnons, and we are the result you propose.

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