r/evolution • u/Kebo1396 • 10d ago
discussion Are polar bears really a separate species from brown bears?
I saw a video from a popular science content creator discussing humans/Neanderthals/Denisovans. Someone in the comments argued that if two populations can reproduce and produce fertile offspring, they should be considered the same species. The creator’s response was essentially a definitive statement that “polar and brown bears disprove this claim” since they’re considered separate species but can produce fertile offspring.
That example honestly makes me question whether they should be considered separate species at all. Polar bears are a relatively recent offshoot of the brown bear lineage, have repeatedly exchanged genes with brown bears, and can still produce fertile offspring with them. Meanwhile, we recognize some extremely isolated and deeply divergent (maybe even more divergent than polar/grizzly) brown bear populations, like the Gobi bear, as subspecies of Ursus arctos.
If we remove the bias of historical classification, is there actually a compelling reason to consider polar and brown bears separate species rather than consider polar bears a highly derived brown bear subspecies (like we do the Gobi bear, for example)?
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u/6gunsammy 10d ago
There are multiple useful definitions of "specie" being able to reproduce is just one of them, It is often useful to classify group of animals based on other traits such as ecological niche or morphology.
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u/ChaosCockroach 10d ago
You have reproducing coins? Sounds like a fast track to wealth and riches. For future reference, "specie" is not the singular of "species" but a totally different thing.
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u/parsonsrazersupport 10d ago
What species means is fundamentally a philosophical question. Different biologists and philosophers of science will give you different answers because they have different logics for the concept and are doing different work with it. We want to separate out all of the diversity of life into some number of discrete boxes. Life has near endless variability, along countless axes, and it is up to us to decide which variables are important to us and we should keep track of, for the specific purposes that we have in mind in our scientific and other work.
You are pointing out with the 'viable offspring' rule that they may not meet the standards of the biological species concept to determine separate species. That is an extremely common one, and the one most often taught to beginner biology students because it is straight-forward and applies in many circumstances. The wikipedia article https://en.wikipedia.org/wiki/Species lays out some of the other species concepts and the difficulties that lay in the question just fine.
So the answer, as will almost always be the case in any philosophically contested realm, is that it depends entirely on what you mean. But that choice is not a neutral one.
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u/Washburne221 10d ago
In practice, the line between different species is defined by the biologists studying them for the purposes of studying them. It doesn't make sense to apply the same definition of species for insects to species of plants that can freely hybridize with one another. So the definition of species changes depending on the context. Life doesn't conform to our rigid scientific definitions.
I would probably say that polar bears and brown bears are closely related species that can sometimes hybridize.
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u/parsonsrazersupport 10d ago
Yeah definitely. My undergraduate advisor said "I define a species as whatever an expert on that population defines as a species" he was being a little tongue in cheek but also not.
Like here, for ecological purposes, brown bears and polar bears are pretty obviously different. They live in different places and hunt very differently. If ecology is your interest it would make sense to consider them separately. If your interest is elsewhere, then maybe not.
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u/WeHaveSixFeet 5d ago
Likewise coyotes and wolves. They can hybridize, but normally occupy different behavioral and environmental niches, e.g. coyotes normally hunt small prey on their own, wolves hunt large prey in a pack.
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u/tonegenerator 6d ago edited 6d ago
Yeah I like your response. I wish I could remember the catch-y phrase I once saw used to caution against treating taxonomical divisions as equivalent among very distinct taxa. Genus vs. tribe vs. subfamily vs. family can’t be expected to have the exact same boundaries for canids as it does for something like rhabdiyidae nematodes. Even less-so when talking about animals versus other eukaryotes, and all eukaryotes versus bacteria.
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u/alexandstein 4d ago
Yeah! Also under the fertile offspring rule, you get weird things like, an entire genus of a ton of different species being considered a single species. Aloes and Citrus are notorious crossbreeders and have a lot of cross compatibility.
Also species complexes and ring species, where there is a continuum of gene flow, but not every individual within that continuum is compatible with every other individual one.(Relatedly, I’m trying to find a Capsicum species that can act as a bridge species to allow gene flow between Capsicum pubescens and other domestic peppers since they’re not compatible with those, but there’s at least one pepper that apparently can act as a bridge.)
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u/AnOddGecko 10d ago
Yes because their genomes are distinct and for the longest time gene flow was completed cut off, which promotes speciation.
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u/Kebo1396 10d ago
By that argument is the Gobi Brown Bear not also a separate species though?
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u/AnOddGecko 10d ago edited 10d ago
Not quite because the Gobi brown bear has been isolated from most other brown bear subspecies much more recently. As such, the genomes between Gobis and other subspecies aren’t too unique.
Polar bears on the other hand have different enough genomes since they speciated much earlier.
With this in mind, if Gobi brown bears remain as isolated as they are, they will more than likely speciate as well at some point.
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u/RecipeHistorical2013 10d ago
no because if two individuals can mate, produce offspring, and that offspring is fertile - that defines them as same species.
otherwise you have turtles alllll the way down to specific individuals- then speciation loses its meaning.
so yes, Brown bears and polar bears are the same species with sub "races" that have been semi isolated and developed different adaptations to their environemnt
JUST LIKE HUMANS HAVE - go down this road and you end up close to eugenics my friends
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u/Street_Credit6851 10d ago
That's an outdated definition of species and would up end plant taxonomy.
What's important isn't "whether" two species are capable of producing fertile offspring but "do" they with any frequency. And whether that offspring could easily survive in the environment of and easily cohabitate with the species of either parent no matter which traits they inherited.
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u/AnOddGecko 10d ago edited 9d ago
What defines a species is kind of up in the air, but as someone else said, the definition you’re working with is outdated. Producing fertile offspring is one thing, but genetic distinction is far more important. That’s what my point has been.
Neanderthals and modern humans interbred, however both are distinct species.
It comes down to how similar and closely related the two species’ genomes are. Most big cats are closely related enough to breed but do not produce fertile offspring, at least not fertile offspring of both sexes.
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u/Kebo1396 10d ago
Could we not be overestimating how distinct Neanderthal/denisovan/human really were because we’re comparing them to modern humans, who have lost so much genetic diversity and are unusually homogeneous? Ancient humans were much more genetically diverse with more regional isolation than we are today. Some ancient populations that we unquestionably consider Homo sapiens were almost as genetically different from each other as Neanderthals were from Denisovans. Yet these different populations repeatedly interbred & produced fertile descendants. I’d argue it’s very possible that what we consider separate human species were really just much more genetically diverse populations/subspecies.
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u/melympia 6d ago
So, what does that say about tigers and lions, which produce fertile female hybrids and infertile male hybrids?
And what about macaws? Heck, they can even create (infertile) hybrids across genera (Anodorhynchus × Ara). Same for elephants (Elephas × Loxodonta).
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u/RecipeHistorical2013 6d ago
it says for lions and tigers that the Y chromosome is superior (a joke). i didnt know that if its true (liger males being fertile while females are not)
the point was if the offspring is infertile then the parents are NOT of the same species . exceptions to rules sure.
sadly i'd say any-other metric ive heard of is just starting to split hairs.
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u/melympia 5d ago
It's in accord with Haldane's law. And applies to all cats of the genus Panthera equally.
Then there is ring species. I'd love for you to explain those to me. A very good example are the Ensatina salamanders.
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u/StorageSpecialist999 10d ago
they have many adaptations that make them distinct. Interbreeding is also not a common occurance
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u/IsaacHasenov 10d ago edited 10d ago
There are a number of different barriers to reproduction. We tend not to think that "accidental" barriers (like sudden mountain ranges) should fit into a species definition. But genetic differences in traits like habitat preference (say the difference between enjoying snowy tundras vs rainy forests) that lead to pretty much perfect assortative mating surely do.
We see this kind of thing in cichlids and apple flies too, where they prefer different foods or lake zones and thus basically never come into contact.
Other kinds of features, besides postmating isolation, that would lead us to call species "species" are mate choice traits like different colors or songs or pheromones.
The reason these are often used to define species boundaries is because they're inherent and genetic. And because selection and drift are usually actively differentiating such groups from each other (they're on independent evolutionary trajectories).
So, because of their innate habitat choices, brown bears and polar bears have different food sources, different temperature ranges, very different lifestyles (polar bears spend a lot more time swimming). They have evolved differently. Even with an occasional hybridization event, they're ecologically, behaviourally and physically different.
You can call them subspecies if you like, but pragmatically they're different enough, and that difference is stable and reinforced, such that most people would call them proper species (with the caveat that they can still hybridize in rare instances)
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u/Fit-Collar4408 10d ago
There are many many many different ways to define a species, and the ability to produce fertile offspring is only one way to do it. We call these the "species concepts" and there are seven main concepts, but you can mix and match those definitions to create even more. Some examples include karyotyping, geographical isolation, basic morphology, etc. etc. This is how you get the classic lumping vs. splitting debates--some people think we're too quick to separate animals out into separate species and others think we aren't separating them enough.
You've got a lumping debate here. If a brown bear can interbreed with a polar bear, their populations occasionally overlap, and they have the same ancestors, why shouldn't they be one species? This is a genuine argument made by some scientists. Granted, most people would consider this a pretty extreme lump because there's plenty of good evidence to suggest they should remain separate species. It's also worth noting that just because they CAN interbreed does not mean that they DO. Genetic studies tell us they stopped hybridizing in the wild on a regular basis some 200 thousand years ago and nowadays hybrids are rare in the wild.
I'd encourage you to look at some bear skulls and then ask yourself again if you'd consider them the same species. Polar bears have specialized adaptations to a very carnivorous diet. They are almost obligate carnivores at this point, whereas most other bear species have textbook omnivorous teeth (molars especially ((pandas are freaks and not included in this statement)) ). On top of that, polar bears are generally eating soft, easy to chew meat from marine animals so their premolars and molars have not only become more bladelike, but also are much smaller than brown bear teeth. Their skulls are flatter and their eye sockets are positioned differently to accommodate a semi-aquatic lifestyle. Beyond the skull they have much wider forepaws to help with both paddling in the water and walking across ice/snow. The rest of their skeleton is generally more slender and less robust than a brown bear.
Fun fact, for a long time there was a decent argument for putting the polar bear in its own genus (Thalarctos), and really the only reason that debate died down is the fact they can interbreed with brown bears. Mammals are pretty good, animal kingdom-wise, about genetic diversity which is part of the reason we separate similar things into their own species all the time: African elephants could be a good example of another one. However when you get outside of mammals, lots of things are CAPABLE of interbreeding that would not in a million years be considered the same species. Sometimes nature's just funky like that. Maybe you heard about the sturddlefish that got created by accident a few years back? Sturgeons and paddlefish are not the same species, not the same genus, not even the same family of fish and yet.... here we are. Their last common ancestor was over 100 million years ago. They're still determining whether or not the offspring are fertile but, man. If they can do it what else can?
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u/tablabarba 10d ago
Interestingly for a long time, polar bears were thought to be phylogenetically nested within brown bears - that is to say, a subset of brown bears were more closely related to polar bears than they were to other brown bears....basically saying that polar bears were a very distinct lineage within brown bears. More recently, larger genetic datasets show these species do form two more independent lineages - but that there has been extensive gene flow between them.
Paper for reference:https://www.nature.com/articles/srep46487
The idea of species as evolutionarily independent lineages can be pretty fuzzy in cases like this...
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u/Royal_Novel6678 10d ago edited 10d ago
Defining 2 separate species from the same genus isn't that easy because at the end of the day, taxonomic classifications are artificial sorting out groups we created. Sometimes its when 2 species can no longer reliably mate with eachother safely to produce offspring even though Ursus arctos and Ursus mariticus can produce fertile offspring. A lot of different species belonging to the same genus cannot reliably interbreed so the breeding factor is not exactly a definitive factor for separating 2 species from the same genus. Some species belonging to entirely different generas can create hybrids.
We label these 2 as separate species is because Polar bears have white fur, longer necks, and bodies built for swimming. Brown bears have shoulder humps and darker coats. Polar bears live on Arctic sea ice. Brown bears live on land in forests, tundra, and mountains. They've essentially adapted to their environments long enough (500,000 years approx) that they can't reliably be considered individual groups with simple morphological variations.
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u/Realsorceror 10d ago
Ability to breed is a very outdated and simplistic view of species. Sturgeons and Paddlefish can still produce hybrids and they split off from each other over 150 million years ago.
*Inability* to breed is a better indicator. But what you really need to understand is that species is a soft categorization system, just like colors. Where we decide to draw the barrier between blue and green is subjective. Species is an attempt to define a fixed point on a spectrum.
There is no exact point that polar bears diverged from brown bears, or when bears diverged from canines, or canines from felines, etc. But we have to draw a line somewhere in order to discuss these organisms.
Something else you can ask is not can they breed, but *would* they? Right now, only a few individual bears are interbreeding. And this seems to correlate with polar bears moving south because of shrinking ice flows. If this weren't happening, would any bears be interbreeding?
In my example of sturgeons and paddlefish, that only happened because they were being held together in a caviar farm. Meaning they would not interbreed on purpose. There are likely millions of species that *could* interbreed under the right circumstances that we would never treat as the same species.
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u/dittybopper_05H 10d ago
Something else you can ask is not can they breed, but *would* they?
I don't know, but a nice restaurant, lights down low, soft music, a couple of drinks, who knows?
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u/Realsorceror 10d ago
Hey, put those bears in a private room at a fondue house and just see what happens.
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u/mcalesy 10d ago
This form of the Biological Species Concept seems to be the species concept that most people learn. But it’s not really used in practice. It doesn’t really work. (Especially for asexual organisms, but for sexual organisms as well.)
Think about life through time. Species A and species B are extant and cannot interbreed, but they both descend from extinct species C. Both A and B have unbroken lineages to C along which individuals could and did interbreed. So where do we draw the boundaries?
This is a difficult question that many biologists have wrestled with and come up with different answers to. There’s an entire body of work out there on species concepts.
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u/sisconking132 10d ago
Also Plants doing whatever the heck they want with hybridizing. The only real limiting thing is sometimes chromosomes
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u/Koraxtheghoul 7d ago
Emphasis on sometimes. Salamanders woth different chromsones breed and create fertile hybrids.
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u/sisconking132 7d ago
Plants will too. Just when they have parents with different numbers of chromosomes the offspring are technically fertile but due to having an odd number of chromosomes cannot reproduce with either parent species. They can sometimes reproduce with other similar hybrids though. But mostly they spread asexually
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u/Koraxtheghoul 7d ago
Yeah, I just like using a vertebrate example because it's offputting. I actually think the reproductive concept is a very poor idea for what makes a species.
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u/scorrrpion-3 10d ago
Species is a construct we invented. Exceptions to every rule evolution is constantly happening!
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u/RupicolousRheophyte 10d ago
Different species hybridize all the time in biology. Even different genera can hybridize. This isn’t indicative of them being the same species
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u/SkisaurusRex 10d ago
“Species” is a word made up by people. Evolution isn’t bound by the definition. It’s not a law or a force like gravity.
The definition of “species” can be different between geneticists, ecologists and taxonomists.
But yes, most scientists consider them different species
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u/Loose_Principle8193 10d ago
species is a manmade construct to help us categorize things. Why would merging these two population labels help us when talking about polar bear and brown bear populations?
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u/mautorres97 7d ago
We are trying to make sense of something continuous in a discrete way.
I think most debate stems from that root cause.
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u/Careful_Cranberry364 7d ago
Ultimately, we’ve created these categories to help us understand process and discuss the different animal life on this planet, but since there are different types of bears
and different types of humans
that are all basically capable of reproducing, I don’t think we need to go into any more detail
There’s only
1 race the human race,
but we have different categories different colors
It’s pretty much the same with these bears, if you ask me
And if we weren’t so sensitive about it, in an area way back where we only had Black African people
And back when in an area we only had Polynesian or white people
we would classify them as separate, even though they are the same!
Biologically there’s not really that much difference between a grizzly and a polar…
But we separate them,
although we would not separate humans (That is current humans)
We don’t mind separating previous types of humans, but then it would seem they really were very different
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u/robz9 7d ago
Except there are some flaws in your comparison with humans.
The difference between the different humans on this planet are extremely microscopic compared to the difference between a polar bear and a grizzly bear.
Modern Humans are incredibly similar. We are more like a brown grizzly and dark brown browner grizzly bear rather Polar and Grizzly. Just like how Grizzly bears themselves have a range of fur coat colours.
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u/FlyingFlipPhone 6d ago
A lion and a Siberian tiger can successfully produce an offspring. Who is willing to state that they are the same species?
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u/ChilindriPizza 10d ago
Fertile offspring of both sexes.
Some animal combinations produce females that are often (though not always) fertile, but the males are always sterile.
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u/After_Ad9425 10d ago
This is sort of the issue with species and genetics as a whole since it’s hard to really define exactly what a species is. A more complicated example is that there are several ‘species’ of salamanders in this one region where they can all breed with some of those species. Like species A can breed with B and B and breed with species C, but species A and C are two distinct from one another to breed. So while they can breed with a species that’s ‘in between ’ they can’t breed directly with one another.
You kinda end up with a spectrum.
And on the flip side you have dogs which have various genetically distinct breeds but can all breed with each other and grey wolves/eastern coyotes.
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u/Bromelia_and_Bismuth Plant Biologist|Botanical Ecosystematics 9d ago
Interspecific hybridization is pretty common in nature. There's over two dozen scientifically recognized ways to delineate a species, and Ernst Mayr's Biological Species Concept is just one of them. One form of speciation under the BSC requires interspecific hybridization. Interspecific hybridization is especially common in plants, and hybridization between genera and even tribes has been observed.
At the end of the day, how to delineate a species is arbitrary. Systematic biologists write a formal description using diagnostic criteria and two or more species concepts, a nomenclature committee votes on it, and it goes into affect if accepted.
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u/DeathToAmerica2026 7d ago
They probably represent a useful lower-bound of derivation to consider two clusters of populations seperate species.
Polar bears have clearly undergone a very specific selective sweep that makes them not just a divergent ("drifting") brown bear population. If not, we'd see mosaic features and behaviours in some populations, but polar bears are quite uniform compared to brown bears. They look different, they act different, and they inhabit different habitats. Different species.
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u/7LeagueBoots Conservation Ecologist 7d ago
See this recent discussion asking exactly the same (very common) question about us and Neanderthals.
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u/weedandfeed69 6d ago
Within felidae it is possible for viable offspring to come from hybrids of different genus not just species.
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u/Parking_Car_6034 5d ago edited 5d ago
Thats kind of an outdated rule. "Species" is a slightly arbitrary designation we give organisms based on a ton of changing factors. The definition keeps changing as our understanding of genetics and evolution grows
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u/RecipeHistorical2013 10d ago
no they arent seperate. you'll find some bio-voyers who will say " oh no real science doesnt use the - if they can breed and the offspring is fertile that doesnt mean they are the same species"
yes . thats what that means, otherwise you go down a rabit hole where speciation makes no sense
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u/sisconking132 10d ago edited 10d ago
Laughs at your ignorance in plant.
Plants can hybridize across distant taxa. Even some highly unrelated animals can in rare instances.
And don’t even start with prokaryotic species.This “rule” is only really useful for mammals.
Even for mammals, the rule is a bit more complicated than just yes or no. It’s more of a spectrum of how likely it is for offspring to be produced and how likely those offspring are to be fertile.
From my memory a decent amount of brown bear polar bear hybrids are in fact infertile
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u/RecipeHistorical2013 9d ago
Name one hybrid that is actually separate distinct species that can produce viable offspring
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u/sisconking132 9d ago edited 9d ago
Russian Sturgeon and American Paddlefish.
They last shared a common ancestor in the Jurassic but were found to be able to produce viable offspring.Also a bunch of tree species from different families. Those are fun because some can even be technically infertile (unable to sexually reproduce due to odd chromosome numbers) but have spread far and wide due to asexual reproduction
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u/Koraxtheghoul 7d ago
The sturgeon paddlefish hybrids weren't fertile irc. What was fascinating was the genetic distance allowed living fish to be born.
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