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There're over 7 billion people in the world and every one of them is different from everyone else. Is it possible that there are two people so different that they belong to different species (in the sense that they cannot reproduce to produce fertile offspring)?

This doesn't have to mean that there's someone in the world who's completely unable to reproduce with everyone else, just that these two individuals are so different that they can't reproduce with each other (they can still reproduce with other people who can reproduce with the other individual - a ring species system).

I'm particularly interested in an answer based on how much the genome varies between people vs. how much they vary against our closest relatives (such as Neanderthals). For example if humans and Neanderthals share 99% of their genes, and the largest variation among current humans is 0.001% of genes, then the answer to this question would be "no".

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    $\begingroup$ though not an answer, you might be interested in ring species $\endgroup$ – theforestecologist♦ Nov 16 '18 at 3:14
  • $\begingroup$ Yes, I was originally going to phrase the question in terms of that but then forget about it. Let me edit. $\endgroup$ – Allure Nov 16 '18 at 3:31
  • $\begingroup$ I have edited your question to change genetic code to genome. If you do not understand why, please read e.g. the Wikipedia entry on the genetic code. $\endgroup$ – David Nov 16 '18 at 9:23
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    $\begingroup$ Your question is basically interesting, although the idea that people can belong to a different species and that mere % difference in the genome will determine the fertility of offspring seems to me naive. I think a more scientifically productive approach would be to consider known examples of interspecies crosses like that between the horse and the donkey. Then you can ask how different the genomes have to be for viable offspring and what causes offspring to be sterile. I don't know the answer but would encourage anyone who does to respond. $\endgroup$ – David Nov 16 '18 at 9:29
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At the beginning, the population of the parents who we are talking about should be made more specific. Let's limit it to adult, healthy individuals (let's skip pathologies, various kinds of random body injuries, personal sexual preferences and close relatives).

From the descendants group, let's also exclude children who would die from random causes, body injuries (unintentional :-( and intended :-((( ), genetic diseases, etc.In addition to the genetic variation (A global reference for human genetic variation), epigenetic variability (Worldwide patterns of human epigenetic variation) should also be taken into account.

If we give the most popular definition of a species (and there are very many of them), as individuals capable of giving fertile offspring, of course all humans belong to one species. Contrary to popular opinion, we are similar to one another like drops of water.

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  • $\begingroup$ I'm not sure that I think the first two paragraphs of your answer are particularly pertinent, but at least you provide some sort of references. However your final sentence is obviously untrue: "Contrary to popular opinion, we are similar to one another like drops of water." Like it or not, some populations are very different from one another in appearance, a difference that has a genetic basis. Asserting otherwise betrays a political agenda and discredits your argument. In the context of the question all that is important is that different populations can mate and produce fertile offspring. $\endgroup$ – David Nov 16 '18 at 15:44
  • $\begingroup$ It is obvious that every population has a certain variation, but our impression of human diversity is illusory. It results from our perspective. Identical to us other individuals (eg penguins) are perfectly able to recognize each other. From their point of view, each penguin is different and unique. A similar situation occurs when a resident of Europe, Asia or Africa goes to another place where a different type of appearance prevails. At the beginning, everyone seems the same and only after some time begins to notice differences. $\endgroup$ – Olga Świder Nov 16 '18 at 16:25
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    $\begingroup$ I happen to believe that we should live in racial harmony, but my points are (1) that this is a biology list, so that we should be careful to restrict ourselves to a discussion of biological facts in answer to a question that may or may not be loaded, (2) by making well-meaning but weak assertions you risk undermining the credibility of a position that may be correct/just. An example is the "like two drops of water". It would be easy for a racist to reply "No, like oil and water, which don't mix". So it is really best if social, religious and political opinions are kept off the list. $\endgroup$ – David Nov 16 '18 at 17:11
  • $\begingroup$ I assume that we are conducting a substantive discussion, not a worldview dispute. Hence the works cited earlier and now: How Humans Differ from Other Animals in Their Levels of Morphological Variation. I maintain my opinion: people are more similar to one another (less changeable) than most animals. $\endgroup$ – Olga Świder Nov 19 '18 at 8:28
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The question boils down to the following things:

  • Whether arbitrary mutations are possible. Yes, they are. (A simple proof: any rearrangements of matter that follow physical laws are possible - that's a direct consequence of physics and probability theory.) But most mutations have very low probability.

  • Whether further speciation is possible for humans. Yes, it is. (The proof is an unlikely and trivial example: mutating (see above) to become any other species that has existed, for example the ancestor species. We know that these species can exist. And probably an enormous number of new species are conceivable.)

From the above it follows that mutations towards speciation are possible (but improbable). So the answer is: yes, there is a very small statistical probability. (Even though some people like to claim that very small probability equals zero, but that is not correct.)

And by the way, we don't know what mutations are out there. Even if we would, we still wouldn't know much, because some speciation-relevant properties are not obvious just by looking at genotypes.

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    $\begingroup$ At SE we're looking for answers that provide some explanation and context. You need to explain why your answer is right, ideally with citations. Otherwise how can the poster or anyone else know whether or not you are correct? Assertions of the type you have made just don't cut it. See the Help for "how do I write a good answer?". $\endgroup$ – David Nov 16 '18 at 15:36
  • $\begingroup$ Edited to add proofs based on mathematics and physics. $\endgroup$ – root Nov 17 '18 at 15:24
  • $\begingroup$ OK. You support your premises now. I just do not agree that it boils down to the two things you mention. I think it boils down to the what determines separate species as defined by the poster "cannot reproduce to produce fertile offspring". It is my gut feeling that the variation in modern humans is less than that between humans and neanderthals or denisovians, and so the answer to his question is no. But I don't have the numbers. You might also argue that the neanderthals were different enough from us to be regarded as a separate species, but that's another question. $\endgroup$ – David Nov 17 '18 at 15:44
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    $\begingroup$ You are talking about how low the probability is. Yes, it is very low. But note that very low probability is not the same as zero probability. Poster asks "is it possible", i.e. is probability above zero. My proof works for any definition of "species". $\endgroup$ – root Nov 17 '18 at 16:20

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