if we differentiate between species by not being able to provide fertile descendents. Why do we say neanderthals were a different species from us but also say we got some of their genes via cross reproduction?
In: Biology
Speciation is a gradual process that occurs as gene pools slowly drift apart. There’s no single point in time where two groups abruptly stop being able to interbreed; they just become less and less likely to produce viable offspring. The definition of species is fuzzy around the edges because gene pools themselves have no distinct edge.
Neanderthals and Humans are both the same species. We are both Homo-sapiens. We are different sub-species Neanderthals being Homo-sapiens Neanderthalensis and modern Humans being Homo-sapiens Sapiens. It is akin to Dogs and Wolves with Dogs being Canus Familaris and wolves being Canus Lupis. They are different subspecies but are still able to reproduce.
There are several different criteria to differentiate species. There are scientist who advocate for stricter criteria, and there are scientist who advocate for looser criteria. I’ll name just a few that come to mind.
* Fertile offspring: Some animals can create offspring, but they’re not fertile. The classic example is the mule. They are not considered the same species.
* Lock and key: Some insects can produce fertile offspring, but their sexual organs don’t fit together. They can never mate in the wild and are so considered different species.
* Breeding habbits: Some animals can produce fertile offspring, but their mating rituals differ so much that they will never mate on their own naturally.
I think you should see a pattern here. The fact that they *could* in theory produce fertile offspring isn’t as important as if they naturally do so. If their gene pools aren’t mixing naturally, most scientist would argue they shouldn’t be considered the same species. You could make the argument that neanderthals were the same species as early humans, but you could also make the argument they were not. They had such significantly different traits than us that it generally makes more sense to talk about them as a different species. Our gene pools did mix a bit, apparently not to a great extent. Otherwise we would have more or less the same traits.
“Species” isn’t an especially scientific term precisely because its usage varies and contains many exceptions. The procreation argument is a rule of thumb but one we know has exceptions (for example there are documented cases of Ligers procreating).
The reality is that species don’t sit in discrete boxes neatly, and so rules of thumb shorthand is the best we can get when it comes to common vocabulary.
Humans and Neanderthals could not freely and easily reproduce, which is a major support to the faction of scientists who consider them to be a different species. It looks like there is no male Neanderthal DNA in the human genome. Only female Neanderthal DNA. It appears that human males and female Neanderthals could mate and produce fertile offspring, but human females and male Neanderthals could not produce fertile offspring.
1. Neanderthals were homo-sapian, same species, but different sub-species, like grey wolf vs Weiner dog (we’re the Weiner dog). They were homo-sapiens neanderthalis, we’re homo-sapien sapiens (the species so nice we named ourselves twice).
2. The species line is where they *start* to have trouble breeding. Genus (the “homo” in “homo-sapien”) is where remaining ability to breed ends.
3. Taxonomy (putting creatures into categories like kingdom, family, genus, species, etc.) is technically ‘non-scientific’. It’s not a system that always makes sense nor has perfect rules, and there’s a lot of winging-it. For example, thanks to viral horizontal gene transfer (that’s when viruses inject DNA into you and it becomes permanently part of you along with any kids you have later), any species can technically end up sharing genes with another. The only reason we keep taxonomy around is it’s because it’s “good enough” and easier to use than the actually scientific way: genetic distance (which instead compares creatures by how different their DNA is… the downside? No convenient categories, so they still borrow from taxonomy to talk about it, just willing to admit where it gets fuzzy.
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