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For what biological reasons do we consider that all human beings belong to the same species?

A Thai and a Nigerian share a common ancestor that is 140,000 years old (see Gravel et al. 2010 and this post). Would, for example, a mixed Thai - Nigerian baby suffer from any outbreeding depression?

Remi.b
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    humans have remarkably low genetic diversity especially given our population sizes. On top of that most ~85-92% of our genetic diversity is within population not across populations. Outbreeding depression is not going to happen. http://jorde-lab.genetics.utah.edu/elibrary/Jorde_2000a.pdf – John Mar 23 '17 at 01:42
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    For info: Motivation for the post

    There had been a number of questions on whether it makes sense to classify all humans into a single species. Unfortunately, many of them where poorly phrased and therefore did not get much attention or even got closed. Here is my attempt to open a post that will be referenced to for future questions on the subject.

     – Remi.b Mar 23 '17 at 02:49
  • Are you sure this question is fit for Biology SE? To me it seems like it's more a match for some more "social" SE-e.g. it bear more on anthropology (in all of its dimensions), rather than to biology per se or the tag human-evolution. – Yordan Yordanov Mar 23 '17 at 04:39
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    The definition of "species" is pretty clear-cut. – Carl Witthoft Mar 23 '17 at 15:16
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    @CarlWitthoft I disagree. Please have a look at this answer – Remi.b Mar 23 '17 at 15:17
  • Possible duplicate of: Outbreeding depression in humans – theforestecologist Mar 23 '17 at 17:13
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    @theforestecologist This other post had received very little attention (no upvote, no answer, no comment and only 100 views in 7 months). I marked the other post as a possible duplicate of the current one as the current got a lot more attention in only 16 hours. – Remi.b Mar 23 '17 at 17:16
  • @theforestecologist Another candidate: Do humans have enough biological differences to be grouped into races or subspecies? – fileunderwater Mar 24 '17 at 00:16
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    Neither of those two posts cited make any mention of outbreeding depression. Why then do they support the claim that there could be outbreeding depression between Nigerians and Thais? This seems speculative on the part of the OP. – Chill2Macht Mar 24 '17 at 01:41
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    @Will the reference is here to support the estimate divergence times and the SE post is here to offer a visualization of the tree of human lineages (this SE post is arguably not necessary). Sorry if you misunderstood the purpose of the citations. I reordered the sentence to prevent other misunderstanding. The post does not claim that there is outbreeding depression in humans but ask if there is any. – Remi.b Mar 24 '17 at 02:30
  • This question sounds quite racist.... – altroware Apr 22 '17 at 20:30
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    @altroware While the question may bring up emotional reactions, the question is by its nature purely scientific. You will note by the way that the OP is the same as the one who wrote the top voted answer (and the same who wrote this comment :) ). You will also note that below the post I explained my motivations for asking this question. – Remi.b Apr 22 '17 at 21:04

1 Answers1

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Actually, we not only consider that all human beings belong to the same species (Homo sapiens) but even that we belong to the same subspecies (Homo sapiens sapiens). So, does it really makes sense?

Concept of species

First, please note that the concept of species is more arbitrary than the most layman would think. I wrote my opinion about the concept of species in this post.

Concept of species in sexually reproducing lineages

In sexual lineages, the existence of different species is generally defined by the existence of a reproductive barrier (see reproductive isolation and see UnderstandingEvolution > biological species concept).

A reproductive barrier can be caused by either pre-zygotic or post-zygotic isolation. I will mainly disregard potential case of pre-zygotic reproductive isolation in this answer because such isolation depends highly on the culture and not only on human genetics. I do not mean that pre-zygotic isolation does not matter or are irrelevant, I just decide to focus on post-zygotic isolation because, as being very independent of the culture, it makes its study much easier.

Reproductive isolation might be partial typically leading to cases of outbreeding depression (when it comes to post/zygotic isolation).

If we had many documented cases of outbreeding depression, it would suggest that we should reconsider the decision of considering all human being. So the question is really: Do we have documented cases of outbreeding depression?

Do we have documented cases of outbreeding depression?

Many cases of inbreeding depression have been documented in humans (McQuillan et al. 2012, Strauss et al. 2013, Lettic et al. 2008, Gellera et al. 1990) but cases of outbreeding depression seem much rarer if any!

I spent some time screening through the literature searching for potential evidence of outbreeding depression. The only paper I found is Udry et al. 2003.

Udry et al. 2003:

  • First, note that they do not place their results in the context of our present discussion and do not talk about outbreeding depression.

  • They report that children of "mixed-race" in US colleges report having more behavioral troubles. One would obviously note that there could well be true that "mixed-race" children experience a different environment (incl. social environment) than "non-mixed-race" children. Such results, therefore, does not suggest the existence of any outbreeding depression.

  • They also report more skin problem in mixed-race children. Unfortunately, 1) their p.value is only slightly significant and 2) they track tens of variables without correcting for multiple comparisons and only one came out significant causing the expected family-wise false positive rate to be higher than the observed family-wise positive rates (see false positive and related concept in statistics).

So, in short: No, there is little to no evidence of outbreeding depression in humans

Potential publication bias and taboo

It is not impossible that there is a publication bias in which researchers looked for inbreeding depression more than for outbreeding depression. One would note also that it would probably be quite taboo to attempt to suggest that there are several species of humans for social reasons and such taboo could cause a publication bias too.

I personally doubt it would be the case that such publication bias exists. Note also that if the evidence of outbreeding depression were obvious and common, then we would not have a problem telling that there are several species of humans. In essence, if there is a publication bias that is causing us to fail to see existing outbreeding depression, it must mean that existing outbreeding depression would be relatively rare and with small effect anyway.

Other definitions of species and taboo

Well, the issue is that there is no other commonly accepted definition of species. Typically, above I only considered outbreeding depression and I did not consider a potential pre-zygotic barrier that would result from geographic isolation or from cultural reasons (e.g. maybe on the ethnic group does not like the appearance of people of another ethnic group and vice versa). I don't doubt that for social reasons, we tend to be quite happy that our only definition of species support the idea that all humans belong to the same species.

It is hard (for me at least) to tell whether we might tend to be more permissive in other lineages and naming two sister lineages as belonging to different species.

Why same subspecies and not only species?

Under the biological species concept, H. sapiens and H. neanderthalis should belong to the same species. This is why we now rather call them H. sapiens sapiens and H. sapien neanderthalis. I suppose it would feel painful to many to not highlight our differences with H. neanderthalis and so, we decided to discriminate.

Again, it is hard (for me at least) to appreciate how this subspecies discrimination in the Homo lineage compares to discrimination among potential subspecies in other lineages.

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cell0
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Remi.b
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    I may possibly be prejudiced, but casual observation suggests that humans experience the opposite of outbreeding depression. Don't have a clue as to where or how you'd find actual evidence, though. – jamesqf Mar 23 '17 at 04:39
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    The entire issue of inbreeding vs. outbreeding depression makes no sense in the description of the human population since it lives under very diverse natural conditions so something that can be classified as "fit" for some conditions (for example, better thermoregulation good for hotter climate) may actually be disadvantageous for other conditions. This makes the "fitness" of human rather an arbitrary category too broad to define out of particular social and/or geographic context and with the lack of such definition for fitness how can you say depression means anything,too? – Yordan Yordanov Mar 23 '17 at 04:54
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    @YordanYordanov Great points. I was going to say that humans are highly adaptive to their environments, but I don't think that's the best way to describe it. Humans have gotten very good at adapting our environment to us. In many places in the world "natural conditions" are just the comfort standard we've adopted for day to day activities. It's only if you're extremely unfit for conditions that you would really be able to tell. – JMac Mar 23 '17 at 11:41
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    You'll find in any lineage environmental variance for fitness and even eventually environmental variance for fitness in outbred individuals. It does not prevent the scientific community to come with arguments as to whether or not whether the lineage should be considered as a single species of as several. – Remi.b Mar 23 '17 at 15:15
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    Of course, the concept of species is largely undefined (see linked post in my answer), this is why I restricted the question to the existence of outbreeding depression. Any evidence of outbreeding depression (under any environment) would already suggest there are more than one species of humans. Failure to find such evidence would suggest that there is only one species of humans. – Remi.b Mar 23 '17 at 15:15
  • I'm a bit annoyed that you place your personal pet-peeve of species definitions/concepts that upfront in an answer that is meant as a 'standard' reference for future use (on mostly another issue). I get that you take issue with the species concept (you have expressed this several times at BioSE), but this is hardly an uncontroversial point of view. For more info, you also link to your own answer on the issue, which is (in my opinion) quite unbalanced and biased. This answer would be better and more to the point without this section. – fileunderwater Mar 23 '17 at 20:44
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    @fileunderwater Yes, I agree with the feeling it may give. However, in this answer (unlike in the comment response to CarlWitthoft), the link to my other answer is to further ones understanding. The answer does not rely on the other post. To clarify the independence of the current answer to my other answer, I added a link to Understanding Evolution that define the so-called biological concept of species to justify why I am focusing on evidence of outbreeding depression. Does it sound better now? – Remi.b Mar 23 '17 at 20:51
  • Also, outbreeding depression does not necessarily imply several species (which you seem to say in the comments and in the answer). If this is indeed what you claim, this is definitely not the standard definition of outbreeding depression, and your use of the term would differ drastically from standard use. – fileunderwater Mar 23 '17 at 20:52
  • A reference to the Biological species concept is probably useful. What I take issue with is the description (and need for) the "concept of species" section. – fileunderwater Mar 23 '17 at 21:20
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    I am not sure I fully understand the issue. I added "my opinion" somewhere in the "concept of species" section. – Remi.b Mar 23 '17 at 21:27
  • Coming from a layman (sorry), what about species inter-breeding? There's a theory that we may be something like 2% Neanderthal. – Jack R. Woods Mar 31 '17 at 00:14
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    @JackR.Woods It is a very good remark. The answer is found in the section Concept of species and especially in the linked post (How could humans have interbred with Neanderthals if we're a different species?). – Remi.b Mar 31 '17 at 02:02
  • @Remi.b "I will mainly disregard potential case of pre-zygotic reproductive isolation in this answer because such isolation depends highly on the culture and not only on human genetics." I'm not sure how to interpret this, are you saying that you think that reproductive isolation only should be measured by the resulting general genetical fitness of cross-breeding? Isn't the variance resulting from all kind of barriers, even if they are purely cultural or geographical, highly relevant to be a part of a classification system that describes the artifacts of reproductive isolation? – Alex Oct 14 '18 at 07:36
  • Like what about the enforced reproductive barriers that are created by conscious selective breeding? Especially with pets like for example dogs, where the concept of outbreeding depression is quite irrelevant. Do the breeds of contemporary dogs have a place on a description of the tree of life? Should we only talk about wolfs, and say that the different "variations" of wolf, like bulldogs or poodles, are of no interest when we classify that branch of life? – Alex Oct 14 '18 at 07:47
  • @Alex That's a good point you are highlighting in your first comment. I added I do not mean that pre-zygotic isolation do not matter or are irrelevant, I just decide to focus on post-zygotic isolation because, as being very independent of the culture, it makes its study much easier. While I would happily down play the conceptual importance of geographical barrier in defining species, I would clearly not do so for other factors causing pre-zygotic isolation (such as not finding each others sexy) – Remi.b Oct 14 '18 at 15:21
  • @Alex I don't understand how dog breeds relate to the first comment. I do consider that domesticated species do belong to the tree of life and semantic difficulties of species definition also apply to them. – Remi.b Oct 14 '18 at 15:23
  • @Remi.b The comment about dogs was intended as an example to clarify what I perceived to be a problem with the approach of basing the classification of populations on evolutionary fitness. Dog breeds have clearly been under pre-zygotic reproductive isolation, and it's highly useful to categorize the different isolated populations regardless of the fact that they all could interbreed and it's arguably not relevant to talk about evolutionary fitness. Even when you don't care about statistical phenotypical traits. – Alex Oct 14 '18 at 16:24
  • I find it meaningful that a classification of populations would include significant historical de facto reproductive isolation, whatever the isolation is caused by, that may or may not have resulted in a divergent statistical genotype that you may or may not know about. I get the feeling that you find it more relevant to base the classification on future potential reproductive isolation based on genetical compatability? Not finding each others sexy could be both cultural and genetical, would you distinguish the hypothetical cases where it's 100% genetical from where it's 100% cultural? – Alex Oct 14 '18 at 16:25
  • Oh I have to apologize, I read this question when refreshing on the human races issue, and somehow fooled myself into thinking that this question and answer were concerned about the subspecies/race level. Seeing that this is actually about the species-level, I agree with you that is is reasonable to focus on genetic compatibility as you describe. – Alex Oct 14 '18 at 18:27