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To be clear, I'm not doubting that Homo sapiens and Homo neanderthalensis did interbreed: of that much I'm convinced.

Within the past few years I've seen an upcropping of pop-sci articles discussing the interbreeding between pre-historic species of humans. In everything that I see in these articles, as well as in scientific literature (my college Bio textbook, among others), I see these different human groups being referred to as separate species.

This conflicts with my understanding of a species. Given the following definition, wouldn't Homo sapiens and Homo neanderthalensis be the same species?

A species is often defined as the largest group of organisms where two hybrids are capable of reproducing fertile offspring, typically using sexual reproduction. ~Wikipedia

  • Is this definition incorrect?
  • Are the publications using "species" colloquially, as opposed to scientifically?
  • Is "species" still a poorly defined concept? (see Ring Species)

Thanks!

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Short answer

The concept of species is poorly defined and is often misleading. The concepts of lineage and clade / monophyletic group are much more helpful. IMO, the only usefulness of this poorly defined concept that is the "species" is to have a common vocabulary for naming lineages.

Note that Homo neanderthalis is sometimes (although it is rare) called H. sapiens neanderthalis though highlighting that some would consider neanderthals and modern humans as being part of the same species.

Long answer

Are neanderthals and modern humans really considered different species?

Often, yes they are considered as different species, neanderthals being called Homo neanderthalis and modern humans are being called Homo sapiens. However, some authors prefer to call neanderthals Homo sapiens neanderthalis and modern humans Homo sapiens sapiens, putting both lineages in the same species (but different subspecies).

How common were interbreeding between H. sapiens and H. neanderthalis

Please, have a look at @iayork's answer.

The rest of the post is here to highlight that whether you consider H. sapiens and H. neanderthalis to be the same species or not is mainly a matter of personal preference given that the concept of species is mainly arbitrary.

Short history of the concept of species

To my knowledge, the concept of species has first been used in the antiquity. At this time, most people viewed species as fixed entities, unable to change through time and without within-population variance (see Aristotle and Plato's thoughts). For some reason, we stuck to this concept even though it sometimes appears to not be very useful.

Charles Darwin already understood that as he says in On the Origin of Species (see here)

Certainly no clear line of demarcation has as yet been drawn between species and sub-species- that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at the rank of species; or, again, between sub-species and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other in an insensible series; and a series impresses the mind with the idea of an actual passage.

You might also want to have a look at the post Why are there species instead of a continuum of various animals?

Several definitions of species

There are several definitions of species that yield me once again to argue that we should rather forget about this concept and just use the term lineage and use an accurate description of the reproductive barriers or genetic/functional divergence between lineage rather than using this made-up word that is "species".

I will below discuss the most commonly used definition (the one you cite) that is called the Biological species concept.

Problems with the definition you cite

A species is often defined as the largest group of organisms where two hybrids are capable of reproducing fertile offspring, typically using sexual reproduction.

Only applies to species that reproduce sexually

Of course, this definition only applies to lineages that use sexual reproduction. If we were to use this definition for asexual lineages, then every single individual would be its own species.

In practice

In general, everybody refers to this definition when talking about sexual lineages but IMO few people are correctly applying for practical reasons of communicating effectively.

How low the fitness of the hybrids need to be?

One has to arbitrarily define a limit of the minimal fitness (or maximal outbreeding depression) to get an accurate definition. Such boundary can be defined in absolute terms or in relative terms (relative to the fitness of the "parent lineages"). If, the hybrid has a fitness that is 100 times lower than any of the two parent lineages, then would you consider the two parent lineages to belong to the same species?

Type of reproductive isolation

We generally categorize the types of reproductive isolation into post-zygotic and pre-zygotic reproductive isolation (see wiki). There is a lot to say on this subject but let's just focus on two interesting hypothetical cases:

  • Let's consider two lineages of birds. One lineage has blue feathers while the other has red feathers. They absolutely never interbreed because the blue birds don't like the red and the red birds don't like the blue. But if you artificially fuse their gametes, then you get a viable and fertile offspring. Are they of the same species?

  • Let's imagine we have two lineages of mosquitoes living in the same geographic region. One flying between 6 pm and 8 pm while the other is flying between 1 am and 3 am. They never see each other. But if they were to meet while flying they would mate together and have viable and fertile offsprings. Are they of the same species?

Under what condition is the hybrids survival and fertility measured

Modern biology can do great stuff! Does it count if the hybrid can't develop in the mother's uterus (let's assume we are talking about mammals) but can develop in some other environment and then become a healthy adult?

Ring species in space

As you said in your question, ring species is another good example as to why the concept of species is not very helpful (see the post Transitivity of Species Definitions). Ensatina eschscholtzii (a salamander; see DeVitt et al. 2011 and other articles from the same group) is a classic example of ring species.

Species transition through time

Many modern lineages cannot interbreed with their ancestors. So, then people might be asking, when exactly did the species change occurred? What generation of parent where part of species A and offspring where part of species B. Of course, there is no such clearly defined time in which transition occurred. It is more a smooth transition from being clearly reproductively isolated (if they were placed to each other) from being clearly the same species.

Practical issue - Renaming lineages

How boring it would be if every time we discover the two species can in some circumstances interbreed, we had to rename them! That would be a mess.

Time

Of course, when we talk about a species we refer to a group of individuals at a given time. However, we don't want to rename the group of individuals of interest every time a single individual die and get born. This notion yield to the question of how long in time can a single species exist. Consider a lineage that has not split for 60,000 years. Was the population 60,000 years ago the same species as the one today? The two groups may differ a lot phenotypically and may actually be reproductively isolated if they were to exist at the same time.

Special cases

When considering a few special cases, the concept of species become even harder to apply.

The Amazon molly (a fish) is a "species" that have "sexual intercourse" without having "sexual reproduction" and there are no males in the species! How is it possible? The females have to seek for sperm in a sister species in order to activate the development of the eggs but the genes of the father from the sister species are not used (Kokko et al. (2008)).

In an ant "species", males and females can both reproduce by parthenogenesis (some kind of cloning but with meiosis and cross-over) and don't need each other to reproduce. In this respect, males could actually be called females. But they still meet to reproduce together. The offsprings of a male and a female (via sexual reproduction) are sterile workers. So males and females are just like two sister species that reproduce sexually to create a sterile army to protect and feed them (Fournier et al. (2005)).

Bias

It often brings fame to discover a large new species. In consequence, scientists might tend to apply a definition of species that allow them to tell that their species is a new one. A typical example of such eventual bias concern dinosaurs where many new fossils are abusively called a new species while they sometimes are just the same species but at a different stage of development (according to this TED).

So why do we still use the concept of species?

Naming

IMO, its only usefulness is that it allows us to name lineages. And it is very important that we have the appropriate vocabulary to name different lineages even if this brings us to make a few mistakes and use some bad definitions.

The alternative use of the concept of lineage

It is important though that we are aware that the concept of species is poorly defined and that if we need to be accurate that we can talk in terms of lineages. The main issue with the term lineage is not semantic and comes about the fact that gene lineages may well differ considerably from what one would consider being the "species lineage" as defined by the "lineages of most sequences"... but this is a story for another time.

In consequence

In consequence to the above issues, we often call two lineages that can interbreed to some extent by different species names. On the other hand, two lineages that can hardly interbreed are sometimes called by the same species name but I would expect this case to be rarer (as discussed by @DarrelHoffman and @AMR in the comments).

Homo lineages

I hope it makes sense from the above that the question is really not related to the special case of the interbreeding between the Homo sapiens and the Homo neanderthalis lineages. The issue is a matter of the definition of species.

Video and podcast

SciShow made a video on the subject: What Makes a Species a Species?

For the French speakers, you will find an interesting (one hour long) podcast on the consequence of the false belief that the concept of species is an objective concept on conservation science at podcast.unil.ch > La biodiversité - plus qu'une simple question de conservation > Pierre-Henry Gouyon


Here is a related answer

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    $\begingroup$ Might also be worth mentioning the flip-side of this, that just because two animals are considered the same species does not guarantee they are capable of producing offspring. For example, all domestic dogs are generally considered a single species, but a female Chihuahua would be unlikely to be able to produce a viable litter with a male Great Dane. $\endgroup$ Oct 18, 2015 at 15:12
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    $\begingroup$ @DarrelHoffman however a female Great Dane could produce a litter with a male Chihuahua. On a chromosomal level, they are pretty much the same. Pure bred French Bulldogs have to be delivered via Caesarean, as even the females of the breed are not able to birth the heads of their offspring. $\endgroup$
    – AMR
    Oct 18, 2015 at 16:00
  • $\begingroup$ In reaction to your comments I added two sections: In consequence and The bias of Humans. Let me know if this seems good enough and whether I should develop it further. Thanks $\endgroup$
    – Remi.b
    Oct 18, 2015 at 17:50
  • $\begingroup$ Although the discussion on problems with the species concept is interesting, I find this answer to be quite strongly worded/controversial (against "species", for "lineages"), but still basically lacking references (except a couple of tangential links). I would like to see a clearer separation between personal opinion and scientific consensus and/or ongoing scientific discussions. $\endgroup$ Dec 19, 2016 at 11:20
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    $\begingroup$ @fileunderwater I agree with you! I have discussed a good part of these ideas with colleagues and also with Pierre-Henri Gouyon after a talk he made about the detrimental effect of the concept of species in conservation biology. If you speak french, you can find a podcast here under "La biodiversité - plus qu'une simple question de conservation" Sorry, I can't cite any papers. $\endgroup$
    – Remi.b
    Dec 19, 2016 at 11:30
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The definition of species is open for debate, and this is especially the case when you try to define it from a paleontology perspective.

Homo neanderthalensis was first discovered and defined in the 1860's, long before we were able to sequence their genome, which was published in 2010. There genome was different enough that most scientists would still say that they are distinct from modern humans, but that doesn't necessarily mean that those distinctions were enough that it affected the ability to produce fertile offspring between sapiens/neanderthalensis matings.

In fact on the wikipedia article I linked to the alternative Homo sapiens neanderthalensis is suggested as a synonym making us Homo sapiens sapiens for anatomically modern humans. This is similar to distinctions made in the wolf lineages where you have Canis lupus for grey wolves, Canis lupus familiaris for Domestic dogs, Canis lupus dingo for the wild dogs of Australia, and on and on for many other sub species of Canis lupus. Wolf/Dog hybrids are known to be fertile.

Female Horses and Male Donkeys can breed to create sterile offspring, Mules. The sterility is likely due to the fact that there is a greater phylogenetic distance between Horses and Donkey than there is for sapiens/neanderthalensis and also due to the fact that Horses have a different number of chromosomes to Donkey's , where as sapiens/neanderthalensis have the same number.

Another thing you have to remember is that the discovery of Homo neanderthalensis occurred about 5 years after Darwin created an uproar about Humans being evolved primates. Now you were faced with a skeleton that clearly was different from modern human but was clearly hominid. The closely held view that humans were special and therefore held dominion over all other creatures made it difficult to accept the fact that we may actually have evolved.

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  • $\begingroup$ I am confused by your last paragraph. Was Homo neanderthalensis not regarded as part of human evolution? $\endgroup$
    – Carsten S
    Oct 19, 2015 at 9:45
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In addition to @Remi.b's answer on the species concept, and the perils of using human definitions to try to encompass biological reality, you need to understand what "interbreeding" meant to humans and neanderthals. Fertile crosses between sapiens and neandertalis were very rare, probably less than one successful cross per generation, and there's some evidence that male hybrids were almost all sterile (or that this was a fatal condition). This is probably less successful interbreeding than horse-donkey crosses, where the two partners are unambiguously different species by just about any definition (they have different chromosome numbers!). So it's not like sapiens and neandertalis could effortlessly hybridize; it was very rare and generally unsuccessful.

References:

We find that observed low levels of Neanderthal ancestry in Eurasians are compatible with a very low rate of interbreeding (<2%), potentially attributable to a very strong avoidance of interspecific matings, a low fitness of hybrids, or both. These results suggesting the presence of very effective barriers to gene flow between the two species are robust to uncertainties about the exact demography of the Paleolithic populations, and they are also found to be compatible with the observed lack of mtDNA introgression. Our model additionally suggests that similarly low levels of introgression in Europe and Asia may result from distinct admixture events having occurred beyond the Middle East, after the split of Europeans and Asians.

--Strong reproductive isolation between humans and Neanderthals inferred from observed patterns of introgression. Currat M, Excoffier L. Proc Natl Acad Sci U S A. 2011 Sep 13;108(37):15129-34

Our results indicate that the amount of Neanderthal DNA in living non-Africans can be explained with maximum probability by the exchange of a single pair of individuals between the subpopulations at each 77 generations, but larger exchange frequencies are also allowed with sizeable probability.

--Extremely Rare Interbreeding Events Can Explain Neanderthal DNA in Living Humans. Neves AGM, Serva M PLoS ONE 2012 7(10): e47076

These results suggest that part of the explanation for genomic regions of reduced Neanderthal ancestry is Neanderthal alleles that caused decreased fertility in males when moved to a modern human genetic background.

--The genomic landscape of Neanderthal ancestry in present-day humans Sankararaman et al Nature 2014 507:354–357

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    $\begingroup$ "Fertile crosses between sapiens and neandertalis were very rare, probably less than one successful cross per generation, and there's some evidence that male hybrids were almost all sterile (or that this was a fatal condition" This is interesting. I would like to read more about it... could you provide references to this? $\endgroup$
    – AMR
    Nov 17, 2015 at 2:33
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    $\begingroup$ Among others - Strong reproductive isolation between humans and Neanderthals inferred from observed patterns of introgression. Currat M, Excoffier L. Proc Natl Acad Sci U S A. 2011 Sep 13;108(37):15129-34; Neves AGM, Serva M (2012) Extremely Rare Interbreeding Events Can Explain Neanderthal DNA in Living Humans. PLoS ONE 7(10): e47076; The genomic landscape of Neanderthal ancestry in present-day humans Sankararaman et al Nature 507, 354–357 (2014) $\endgroup$
    – iayork
    Nov 17, 2015 at 2:48
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    $\begingroup$ @Will - it's not true that there are no recorded instances of fertile donkey/horse offspring. This article and this one list several well-documented examples in the past century, quite possibly more instances than all successful sapiens/neandertal crosses over the thousands of years they co-existed. $\endgroup$
    – iayork
    Mar 24, 2017 at 9:44
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    $\begingroup$ @iayork Oh cool I never knew about that, thanks! I guess to be fair though the descendants of human-neanderthal hybrids seem to have been able to make some contribution to the human gene pool, while even the rare fertile mules don't seem able to produce offspring capable of living long enough to contribute to either the donkey or horse gene pools. But I might be misunderstanding. $\endgroup$ Mar 24, 2017 at 12:18
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    $\begingroup$ @Will Human/Neandertal hybrids almost certainly had some selective advantages that counterbalanced their strong disadvantages and led to some of the Neandertal genome being maintained -- speculation includes immunological advantages from Neandertal genes. But also, it's likely that some Neandertal genes were carried along simply because of the general expansion of H. sapiens, even where those genes were neutral or even weakly harmful. $\endgroup$
    – iayork
    Mar 24, 2017 at 12:21
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The same way lions and tiger are different species but can interbreed. They do so poorly, not very often, and the male hybrids are infertile, although the female hybrids retain some fertility. Gene flow between the two species is possible but very limited. We call lions and tigers two species because gene flow between the two is limited. Free flow of genes between the two species is not possible.

If we look at the papers coming out it is obvious that the flow of genes between Homo Sapiens and Neanderthals was limited. It was so limited that we can say that two species were on the verge of cutting all ties between each other.

Firstly, there are no Neanderthal mitochrondria, meaning successful mating involving Neanderthals was mainly between human females and male Neanderthals. Next there are no Neanderthal Y chromosomes, meaning male hybrids were sterile.

Although there are Neanderthal gene in autosomes, there are no Neanderthal genes on X chromosomes, which house many of the genes regulating intelligence and fertility. So natural selection has selectively weeded out Neanderthal genes in those two areas, probably because they don't play well with their human counterparts (i.e. they cause a fitness disadvantage).

Also, the Neanderthal gene segments, although found in all non-Africans, are in small pieces. Individual have less than 2% (although as much as 35-60% of the Neanderthal genome remains in the non-African human population). So it means that the hybridization event occurred rarely and far in the past — probably when the first Homo sapiens left Africa.

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    $\begingroup$ There is a lot of conjecture in your answer and no evidence at all. Even though the notion of your answer might be correct, your conclusions are - given the data - way too strong and some of the information are even wrong (there is introgressed Neandertal DNA on the X, just about 5 times less then on autosomes). You will not find references to support most of your claims. -1 $\endgroup$ Dec 19, 2016 at 11:10
  • $\begingroup$ Really? I would think anybody would has read the literature would have said the same. All i am doing is repeating what has already been said. I am not adding anything new. Even my conclusion are just repeating the what has been concluded. The hottest thing is how many other species have been added to our genome. When and where. Latest news indicate that the how Neanderthal gene got into the human population is far more complicate that a single one off event. $\endgroup$
    – JayCkat
    Dec 19, 2016 at 14:32
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    $\begingroup$ No, you are putting it way too strongly. There is indicative evidence to support some of your claims, but nothing shows that the admixture events occurred only once or only far back in the past or even that male hybrids in fact were sterile. And just because we have not found Neandertal mitochondria or Y-chromosomes does not imply that there are none - absence of evidence is not evidence of absence. Remember that a mitochondrial genome is just one locus and how efficient drift is in populations with small effective sizes. The evidence is far from decisive. $\endgroup$ Dec 19, 2016 at 16:19
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I'm not really educated into the scientific community's mindset very much, but as a person who is interested in taxonomy to one degree or another, as well as plant breeding, I can give you some perspective to help you decide on an answer.

Lots of different species can hybridize (interspecies hybrids): e.g. some different pepper species, some different tomato species, some different toad species, cows and bison, lions and tigers, different species of strawberries, etc. Interspecies hybrids sometimes even become invasive species. For instance, collared doves invaded my area years ago, and hybridized with the native doves (which native doves were quite a bit different: much smaller, much different behavior; they made different sounds, and had no collars); we've had the hybrids ever since (although they took some years to stabilize much--which was a fascinating process to observe), and the other two species appear to be gone, for the most part. In my personal opinion, interspecies hybrids (perhaps^ driven by changes in habitat: for instance, if the magnetic fields of the earth change, maybe because of a disturbance caused by a solar storm, animals might migrate different places than they used to, because some animals migrate via their ability to detect magnetic fields; if they migrate different places than usual, they might find new animals to hybridize with) are a very significant driver of new forms of life, which often gets overlooked.

^That's a hypothesis of mine, anyway. I don't claim it's a proven fact.

There are even hybrids between different genera, although they're pretty rare (sheep and goats have been known to cross, for instance; hybrids can also supposedly be made between goji and tomatoes--although I have yet to see plants or seeds of the hybrid for sale).

My impression from things I've studied isn't that species differentiate sexually incompatible groups, but rather that they differentiate sexually compatible groups that tend to stick together. (You're probably not going to see llamas and camels hanging out very often, or horses and zebras, or donkeys and zebras, but that doesn't mean they can't reproduce. They are not the same species, however, and some--not all--interspecies hybrids have infertile offspring^.) It seems that other traits are also important for defining species: such as structure and lifestyle, but it's a work in progress (and they move things around taxonomically sometimes).

^It should be noted that you can have diploids and tetraploids (or greater) of the same species of plant (thanks, in part, to chromosome doubling chemicals), and their offspring aren't fertile (but if you doubled the chromosomes of the offspring, they would be).

None of the higher ranks guarantee sexual compatibility (even if it sometimes can happen). However, chromosome doubling and such aside, you can generally expect members of a species to be compatible with confidence.

In summary, it sounds like members of the same species tend to reproduce with each other over another species when given the chance (but there's no guarantee they can't and won't breed with other species, generally speaking; you can evaluate that on a case-by-case basis, however). Each species tends to have a certain set of traits (and habits) that set it apart. But, we're still figuring everything out.

That's what I gather, anyway.

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A microbiology professor once told us "The Russians have about 14 times more species than the English"

The word species (Greek εἶδος¹) begins in greece. According to St. Thomas Aquinas (Summa Theologica, ca. 1265-1274 A.D.), used as the scientific way of referring to different types of living things.

It's a notable buzzword in english owing to it's rhyming syllables, it focuses attention, and my... It isn't more precise than referring to colours... Espèces in French doesn't warrant the same obsessive buzz. Genetics is a spectrum and we divide them as best we can a bit like we divide colours of the rainbow.

Neanderthals came about only half a million years ago, and we diverge from them perhaps a million years ago.

Dinosaur species usually live for about 3 to 10 million years before changing two new species according to scientists, and their generations are closer together than humans...

So they are closer than bonobos and classical chimpanzees, perhaps something like the northern and the southern white rhino.

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