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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?

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    $\begingroup$ 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. jorde-lab.genetics.utah.edu/elibrary/Jorde_2000a.pdf $\endgroup$ – John Mar 23 '17 at 1:42
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    $\begingroup$ 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. $\endgroup$ – Remi.b Mar 23 '17 at 2:49
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    $\begingroup$ The definition of "species" is pretty clear-cut. $\endgroup$ – Carl Witthoft Mar 23 '17 at 15:16
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    $\begingroup$ @CarlWitthoft I disagree. Please have a look at this answer $\endgroup$ – Remi.b Mar 23 '17 at 15:17
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    $\begingroup$ @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. $\endgroup$ – Remi.b Apr 22 '17 at 21:04
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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.

Related posts

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    $\begingroup$ 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. $\endgroup$ – jamesqf Mar 23 '17 at 4:39
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    $\begingroup$ 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? $\endgroup$ – Yordan Yordanov Mar 23 '17 at 4:54
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    $\begingroup$ @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. $\endgroup$ – JMac Mar 23 '17 at 11:41
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    $\begingroup$ 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. $\endgroup$ – Remi.b Mar 23 '17 at 15:15
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    $\begingroup$ 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. $\endgroup$ – Remi.b Mar 23 '17 at 15:15

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