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Comment by corygarms | Hacker News Reader

This is nuts. If I'm understanding correctly, the M. ibiricus queen mates with a M. structor male, uses his sperm to create sterile, hybrid female worker ants for her colony, then she (astonishingly) can also lay eggs that develop into fertile M. structor males, which means she has removed her genetic material from the egg and effectively cloned the male she previously mated with.

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The original paper discusses this in more detail. There is a well understood phenomenon called sperm paratism where male sperm will take over the egg, instead of sexually reproducing with the egg.

In sexual reproduction the offspring has 50% of its genetic material from both parents. In sperm parasitism the offspring is 100% related to the male and the female’s genetic material has been destroyed.

These inbericus females are allowing the messor male line to reproduce by sperm parisitism to maintain a domesticated messor line that they can then later sexually reproduce with to create hybrid worker ants.

https://www.nature.com/articles/s41586-025-09425-w

So the messor male line is without sexual reproduction. Essentially a clone army, all with identical genes?

I find it interesting that this has not led to all the clones knocked out by diseases, as happens to eg our banana plantations.

All M. structor's aren't clones, only in that colony.

They are out their in the wild reproducing with other M. structors sexually.

I thought one of the big points was there's no M. structor females on the islands.

They've extended their range, so there's lots of cases where they don't have wild ones around, but there's still overlap, at least according to the article I read.

Yes, in some areas - but not e.g. on Sicily. https://www.youtube.com/watch?v=Z-O4_AwWpfI gives perhaps a better overview

M. Ibericus produces 4 kinds of offsprings.

    1. M. Ibericus queen + no male (unfertilized egg) -> M. Ibericus male.
    2. M. Ibericus queen + M. Ibericus male -> M. Ibericus queen
    3. M. Ibericus queen + M. Structor male -> M. Structor male, no genes from the mother
    4. M. Ibericus queen + M. Structor male -> M. Structor/Ibericus hybrid female, (worker ant, infertile)

The authors tried to find evidence of gene flow between the domesticated messor drones and the wild messor populations, but couldn’t. They sequenced about 100 ants, so it was not an exhaustive search. However if it was common you would have expected to see it.

If you take the idea of genes as the target of evolution seriously, then every possible "bargain" between different genes that moves towards a pareto optimal for those genes, will eventually be discovered through the brute force search.

brute force search can still be limited in the states it can reach. If there's some limitation on the types of moves you can make, which presumably there is, then you're limited to states that have paths between them.

And in Pareto, there's a rule like that built in because you're only allowed to make moves that increase utility. You're not allowed to move backwards, which can lead to getting trapped in a local maximum.

That’s not precisely correct, because there is some “noise” in the system. Also, multiple genes can have competing effects, so one gene’s individual fitness can be suppressed by another.

The mutations are like little nudges to throw you off the local maximum.

And there's junk DNA where mutations can accumulate over time without being subject to selection before getting enabled at random to see if they give you an advantage.

I think both the amount of junk DNA and the mutation rate are themselves subject to evolution for the best trade-off.

Note that most "junk DNA" is just DNA with a purpose we don't understand (originally, all DNA that didn't code for proteins). Some of it is true junk, of course.

That's the exact opposite of random mutation.

Assuming no extinction, climate change, nor heat death of the universe.

Evolution is not a particularly fast optimizer, on the scale of human perception.

Evolution as we know it works on Earth because it is a relatively protected and stable environment, giving genes the time they need to explore a space and adapt to the relatively slow and small changes on Earth.

Imho genes are not what if at all should be attributed aggncy here (that might, if at all, be the decentralised sympoietic process of ecosystems)

Just wanted to add this, i see where the formulation is useful as a figure of speech.

Climate change?

Climate change produces a changed environment. It actually drives more evolution to adapt to that changed environment.

Even very sudden changes such as some that caused mass extinctions, it just changed the direction of evolution. The only species that stopped evolving were those that went extinct. No more dinosaurs (bar birds) but lots more mammals.

This assumes multiple infinite axis which certainly don’t exist and is therefore false.

Any early branch can infinitely on that confined branch. It means the billions of other possible branches may never be explored even given infinite time.

Counter would be the evolutionary fill theory where any branch can become any other branch given an environment.

That’s interesting! What other crazy types of bargains have we seen before or could someone imagine?

I still struggle with the brute force search a bit. Just naively a very small gene has 4^500 possible combinations.

Genetic variation from one generation to the next is incremental - not a matter of tearing it all up and try some something random, not brute force exploring our way through all combinations.

Evolution seems more like building a tree where mostly all you can do is ascend the tree and add finer detail, leaving the trunk and branches (our evolutionary history) in place. It seems unlikely that, say, vertebrates are in the future going to "undo" the major evolutionary developments of the past and lose their skeleton, body symmetry, number of limbs, lungs, alimentary canal, nervous system, brain, etc. We see things like these developing in the evolutionary tree and mostly staying in place once created. Sure some fins turned to limbs, some gills to ears, but once things like that happened they seem to stay in place.

I wonder what evolution would look like if we could see it sped up from the origin of life to billions of years into the future? A building up of complexity to begin with, but those major branches of the evolutionary tree remaining pretty stable it would seem. Continual ongoing change, but of smaller and smaller scope, perhaps - building on what came before.

> It seems unlikely that, say, vertebrates are in the future going to "undo" the major evolutionary developments of the past and lose their skeleton, body symmetry, number of limbs, lungs, alimentary canal, nervous system, brain, etc. We see things like these developing in the evolutionary tree and mostly staying in place once created.

Try looking at whale skeletons over time. What isn't beneficial gets undone.

Try looking at the giraffe's recurrent laryngeal nerve. What isn't beneficial is sometimes retained as long as the cost isn't bad enough to impair reproduction.

https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve#Evid...

What doesn't affect fitness, or has relatively little cost, can and does propagate over time. By definition, nothing is selecting against them.

Similarly, beneficial and complex traits, like eyes, can "regress" if nothing selects against that trait. Plenty of species have lost their sight, making them less generally fit for many environments, because in a certain place and time those species could reproduce even without perfect vision, or just as the result of genetic drift.

Yes, but it still seems that large scale structure tends to be preserved and it's more localized things like limbs/eyes/ears/teeth that may adapt. A chicken may have no teeth, but it's still basically a therapod.

I'm guessing there may be at least a couple of reasons for this:

1) Large scale structures evolved over long periods of time, involving layer upon layer of genetic change. This isn't going to be undone quickly or by any localized change, and those rare cases where a genetic change/defect does impact some fundamental aspect of the body plan (e.g. a frog with six legs) are very unlikely to be successful.

2) It seems possible that evolution acts to preserve large scale structure that has proved itself over time, and changes to which tend to be detrimental. In the same way that sexual reproduction seems like an evolution hack to evolve faster, then perhaps animals have also evolved genetic hacks to preserve/stabilize large scale structures that are critical to survival.

>>> It seems unlikely that, say, vertebrates are in the future going to "undo" the major evolutionary developments of the past and lose their skeleton, body symmetry, number of limbs

> but it still seems that large scale structure tends to be preserved and it's more localized things like limbs/eyes/ears/teeth that may adapt

Make up your mind.

Large scale vs localized. They are not the same thing.

"Unlikely" events happen nonetheless. Viruses and barnacles are examples of discarding structures, and it could possibly happen in some vertebrates.

In a billion years, the sun's intensity would have increased such that life and the Earth itself will look very different, assuming life can adapt to living on basically a different planet. There might not be oceans left by then.

When the environment changes sufficiently to wipe out whole branches of the evolutionary tree, I'd still expect those branches still alive to evolve in incremental fashion. Even if most lineages were wiped out, leaving only extremophiles, then those would still be building upon their own evolutionary history.

I assume we will have sterilizing temperatures and pressures on Earth if all of the water ends up in the atmosphere, and heat releases even more greenhouse gases + subterranean water, leading to a runaway situation like Venus or worse.

Maybe eventually, but I'm not sure of the relevance to the discussion, which is about the incremental nature of evolution.

Point is the Earth might reach a state that even incremental evolution can't overcome, my response is to your question about what life might look like if we sped up by billions of years. It might not look like anything.

It's not brute force search, more like stochastic sampling of random variants. It's a directed search where feedback from the environment prunes and weights the search space, and reproduction is a stochastic process biased by fitness. And fitness is defined by the survival and chances of successful mating, aka reproductive success.

Basically, your sex drive is the main search optimization :p

Edit: This is essentially how genetic algorithms in computer science work. They’re often remarkably effective at finding good solutions without needing brute force.

It is possible that not all combinations are equally likely.

Yeah it's working so something's going on .

The actual space is a lot smaller than it looks. Many amino acids have multiple codons that encode for them. You can also exclude cases where you have repeating stop codons (which detatch the RNA from the ribosome).

There's lots of processes that favor certain patterns over others, only considering the biochemistry of the cell, not even the fitness of the animal.

> Many amino acids have multiple codons that encode for them.

I didn't know this. I suspect this evolved because some amino acids are more useful than others, and increasing the probability of encoding for them was beneficial.

Have a read (rather than guessing) - it’s fascinating! Your other reply has a good insight but on more of a related topic but the primary reason this exists is for error correction. So approx one third of single nucleotide mutations have no change on the expression of the DNA or protein. And some of those that do change the amino acid are actually conservative; ie changing a basic amino acid to another basic amino acid which may still end up folding in the same way

There was a paper a while ago documenting that "synonymous" codons took different amounts of time during assembly, causing differences in the folded structure of the protein.

I've spent hours watching Drew Berry/WEHI movies and that whole process just seems like straight up alien technology. Blows me away to think about the scale that it's operating at within my body as I type this.

I think that maybe (with perhaps a very small probability) it actually could be alien technology.

Yeah, I came here to say the same thing. I'm really confused how the female can produce a clone of the male of another species. Wouldn't the other males sperm contain only half the genetic material needed to reproduce? But apparently ant DNA doesn't work that way for sex:

https://press.uni-mainz.de/determining-sex-in-ants/

somehow a male ant has one set of chromosomes while the female ant has two sets of chromosomes. So a male ant sperm must contain enough information to make a complete male? Then when they mate with the female of the other species, the females egg actually gets blanked out so to speak, containing none of the females own genetic material. Then the male sperm fertilizes the egg with one set of chromosomes producing a male offspring that is a clone?

> I'm really confused how the female can produce a clone of the male of another species.

In normal ants, the queen can produce haploid (single set of chromosomes) unfertilized eggs that hatch into males. Normal ant males are haploid. They don't have a father, they can not have sons (but the do have a grandfather, and their daughters will make them grandsons). When the ant queen decides to produce sons, she will make haploid eggs via meiosis as normal, and just won't fertilize them with male sperm.

Ants don't have sex chromosomes. An individual with a single set of chromosomes (haploid) is a male, an individual with double set of chromosomes (diploid) is a female. Ant males are almost like sperm cells that grew into multicellular organisms.

https://en.wikipedia.org/wiki/Haplodiploidy

Now, a Messor ibericus queen can produce eggs with her own genetic material removed, and fertilize these with the single set of chromosomes from a Messor structor male. (It will still have the mitochondria and mitochondrial DNA from the queen.) And because the male only has a single set of chromosomes, the sperm and the resulting offspring has an identical single copy of the father's genetic material (except the mitochondria that came from the mother). So the son is a clone of the father (except for mitochondria).

The queen can also mate with males of her own species, contributing half of her own chromosomes to combine with the full single set of the male chromosomes, to produce to-be-queen female offspring. Here we have the normal genetic recombination (though only on the mother's side) to keep the evolutionary benefits of the variation from sexual reproduction.

What are the odds this behavior is not completely hostile from the side of the builder ant? There seems to be some implication of symbiotic relationship, but maybe I'm reading too much into it.

The interesting part is whether M. ibericus queens do actively remove their own genetic from eggs fertilized with builder sperm. Why would they do this?

M. ibericus queens produce ibericus males and ibericus females, so that these can mate and produce more ibericus queens. This keeps the normal sexual reproduction of the species going on.

M. ibericus queens produce ibericus×structor hybrids as infertile female worker ants.

M. ibericus queens produce structor males, so future queens can keep producing the hybrid worker ants.

My guess is, maybe there is some benefit having the workers to be hybrids and not pure ibericus ("hybrid vigor" [1]). So it's worth the effort of keeping the structor males along, to be able to produce the hybrid workers. But I think the pure ibericus genes in the line of queens are in control.

[1] https://en.wikipedia.org/wiki/Heterosis

I think it's an interesting assumption that that the queen removes her own genetics from the eggs. Normally in biology I would expect the opposite, that species remove competitors in favor of their own dna. So I think the possibility should be explored that it is builder ant's sperm that is somehow removing the dna of the queen.

Still if the foreign DNA is beneficial in keeping the hive going, it will help spread the queen's DNA. Should some allele work against the adoption of the foreign DNA, fitness drops, and that allele would become less frequent.

Who is doing the removing at fertilization is interesting mostly in a mechanical sense. The mechanisms that worked against it are being suppressed or selected out entirely.

It sure is an interesting case that one ant species is having another species promote their males if one looks at it from a gene perspective. A very weird case of symbiosis.

Here's my theory of how it may have happened. Stage 0: 5 million years ago they were the same species. Stage 1: Subspecies. Ibericus and Structor became distinct populations of the same species still capable of mating. Stage 2: Parasitism. Structur became capable of replacing Ibericus dna with their own in eggs. Stage 3: Loss of function. Because of rampant structor parasitism nearly all workers were structor. So when Ibericus lost the ability to make their own workers it was a neutral mutation.

If I understand it correctly their theory is as follows. Stage 0: 5 million years ago they were the same species. Stage 1: Subspecies. Ibericus and Structor became distinct populations of the same species still capable of mating. Stage 2. Loss of function. Ibericus lost the ability to create their own workers, but as Ibericus and Structor existed in the same places hybrid workers allowed Ibericus to survive despite this. Stage 3: Ibericus learns to clone structor males to live in places where there are no Structors naturally.

Kinda interesting that even though the end result is the same who is considered the parasite is different.

One of the author of the study here. Your scenario is very good. The reason why we believe that the second one is way more likely is because there are several other species of the Messor genus that need to hybridize to produce workers, so it's very likely that it's the ancestral state and that male cloning comes after.

Note that this second scenario doesn't necessarily imply that M. structor males are not some sort of parasites. Actually, because it's detrimental to M. structor males to father only workers (they don't transmit their genes to the next generation), it's easy to imagine selection to remove DNA of the mother so they can be cloned in a fertile caste (males). That being said, maternal DNA elimination prompted by the male genome would be nearly immediately also beneficial to M. ibericus (because having M. structor males at home is good for them), so this maternal DNA removing is something that benefits to both partners, which is why it's not that difficult to imagine that it evolved and has been succesfully selected. When something benefits to both partners of any relationship, there are good chances that it will be selected at some point.

That makes a lot of sense! Really appreciate you coming here and explaining

Thank you, that's fascinating.

Note that many, many animals have non-genetic sex determination. Most fish, amphibians, and reptiles have the same genes for both males and females. Sexual differentiation typically depends on things like the egg temperature or salinity and so on. Some species can even change sex during their adult lifetimes, with external conditions triggering a complex hormonal shift that convert an adult, fertile male into an adult, fertile female.

Having genetic differences between males and females is mostly a bird and mammal thing, at least among vertebrates.

> Having genetic differences between males and females is mostly a bird and mammal thing, at least among vertebrates.

Also: the configuration and function of sex chromosomes is not consistent even within mammals. There are a number of species - primarily rodents - with unusual sex-determining systems, like species with XX/X0 (i.e. where males have an unpaired X chromosome) or even X0/X0.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8617835/

Important plot element in Jurassic Park!

Man, the bible missed all of this when they were talking about the two animals of every species on the Ark. What else did they leave out?

This always struck me as a bit odd, because it was a somewhat common belief around then, and for long after, that many animals reproduced by abiogenesis anyway. Why bother taking two mice on the ark; everyone knows that mice spontaneously emerge from river mud!

(It’s possible that this was just a Greek quirk and never made it to Palestine, I suppose.)

Listen, we still don't know how eels reproduce. Our knowledge has never been all inclusive and properly disseminated. The fearful cave-dwelling scribes who wrote the old testament were clearly not up to date on their biology.

I didn't know either, but hn came to the rescue. In case you're one of today's lucky 10000:

https://www.newyorker.com/magazine/2020/05/25/where-do-eels-...

I think eels are safely outside the domain of knowledge where anyone could safely say "everybody knows that!"

https://xkcd.com/2501/

Well, no, I’m actually surprised that whoever wrote the Old Testament _was_ up enough on their biology (or at least aligned with biology, however accidentally) to realise that most animals reproduce sexually. This certainly wasn’t the conventional view in the Greek world, say, nor was it in the West until the 18th century or so.

What was this view exactly? They would have know their pets and farm animals reproduced sexually. I guess it isn't a leap to think all mammals? So what animals did they think did not?

So, we tend to think that it’s just common sense that most animals reproduce sexually (actually I think most people would assume that _all_ animals do; in fact, as with most things, there are edge cases), but, well, to an extent that’s because we already know that. The historical view was a bit different: https://en.wikipedia.org/wiki/Spontaneous_generation

This isn’t even the only weird idea that people used to have about reproduction; there’s also stuff like https://en.wikipedia.org/wiki/Barnacle_goose_myth

Mice, flies, vermin of various kinds that seemed able to show up anywhere from no obvious parents.

We do now!! It’s fascinating. Look it up!

The idea persisted into the Middle Ages. Can't say for certain that it was continuous, however; the medieval supporters quoted Aristotle et al.

To be fair, you almost always still need two individuals to get reproduction going - you just don't need to be as picky about which two individuals as you might think. There are a rare few animals that can sometimes self-reproduce, but it's not a common strategy in the animal kingdom, even among hermaphroditic animals.

They're less rare than you might think. Parthenogenesis ("virgin birth") occasionally occurs in some domestic birds, including chickens and turkeys. Due to the way sex determination works in birds, the offspring created this way are always male.

https://www.sciencedirect.com/science/article/pii/S003257911...

There are also a number of species of lizards, and one snake, which reproduce exclusively via parthenogenesis: https://en.wikipedia.org/wiki/Parthenogenesis_in_squamates

In some though not all such species, there are no known male examples _at all_ (though in reptiles some forms of parthenogenesis can produce males).

Thank you so much for introducing me to this concept. I knew the word thanks to Shriekback. I used to have ducks. At one point when I only had 3 females, I found a broken egg with a fetus in it. I knew they were all female, but couldn't convince anybody of what I saw.

Parthenogenesis is not uncommon in animals: https://en.wikipedia.org/wiki/List_of_taxa_that_use_partheno... (I am mostly quibbling with "rare few animals" but I can't really say much about the relative prevalence of parthenogenesis compared to sexual reproduction.

Leviticus rightfully instructs you not to eat bats, but it seems to mistake them for special birds rather than mammals.

There can't be much meat on a bat anyway.

there's not much on a snail either, but they're still delicious.

They're fine. Mostly the butter stuff tastes good. I wouldn't put snail up there with badger or owl.

Wait, is there a _specific_ prohibition? Like, they fail Old Testament dietary rules miserably _anyway_.

These are the birds you are to regard as unclean and not eat because they are unclean: the eagle, the vulture, [a bunch of other birds] and the bat"

https://www.biblegateway.com/passage/?search=Leviticus%2011:...

Aaah, right, I suppose if you’re assuming it’s a bird it _would_ need a specific call-out, yeah. I was assuming it’d be covered by the hooves-and-stomachs stuff, but if you don’t think it’s a mammal in the first place that wouldn’t work.

(From the above: “Bible Gateway is currently unavailable to consumers in the United Kingdom and European Union due to technical issues.” I am now very curious just which EU regulation the bible website was worried about.)

> What else did they leave out?

Plants. Fungi.

The latter they brought with an especially rank pot of Grandma Noah's sauerkraut.

It was an early deployment of RAID1. Two copies of everything

They were actually intelligently designed this way any animal you can’t sex easily as a hermaphrodite

Here's some punctuation: ....,,,;;;:::!!!???

I don't have the space to help on your other issues.

That's ok, the space key seems to work just fine :troll:

> It was very difficult, because in lab conditions, it’s nearly impossible to have males,” says co-author Jonathan Romiguier, an ecologist at the University of Montpellier in France, to New Scientist’s Tim Vernimmen. “We had something like 50 colonies and monitored them for two years without a single male being born. Then we got lucky.

This confuses me too.

Did the queen once mate with one of these males and save the sperm for two years? Or are the queens somehow born with a copy of the genetic material.

Or does the old queen produce one, which mates with the new queen, and then dies off. And the new queen is able to hold onto that sperm for years (forever?). And they only produce a handful of males for this purpose?

Also why is it so difficult to have males in lab conditions?

Yup; in most ants the queen has a bit of an orgy, once, and then saves the sperm for the rest of her life. Male ants (in most species) are really only useful for starting new nests.

Ants seem to be nature's favorite way to experiment

The Bacteria Kingdom coughs and smiles knowingly at the Archaean supercluster.

Care to expand?

I suggest you look into the origin of eukaryotes.

It may not be as weird as all that (of course, it’s still spectacularly weird) because fertile male ants are haploid!

I read about this a some other source. I was so confused and lost.

Imagine this on a alien planetary civilization scale.. and the real Zerg and Tyranids and Xenomorphs that must be out there...

I'm no expert, but why does the female need to remove her own genetic material from her eggs to produce clones? Isn't it possible that during the DNA recombination phase the male DNA somehow dominates?

This ability of the female to give birth to "multiple species" seems to me best understood as the two "species" not having yet actually become distinct, since the only meaningful definition of speciation is when two sub-populations of a species have genetically drifted so far apart that they can no longer successfully interbreed and produce fertile young.

During the process of speciation (one species splitting into two) there are going to be various messy half-way stages such as lions and tigers still able to interbreed and so not fully speciated (even if well along, and not going to typically interbreed), horses and donkeys still able to interbreed but producing infertile young (mules), and these ants in this strange state where interbreeding apparently only results in males. It would be cool to be able to speed up the evolutionary timescale to see the process happen, but what we have here is like a still frame from a movie.

Ants, bees, and wasps operate on a genetic system called haplodiploidy. It works like this.

Queens don't actually mate to produce male offspring.

Females are Diploid: They are created from a fertilized egg. They have two sets of chromosomes - one set from the mother (the queen) and one from the father's sperm, which the queen releases from the spermatheca when she wants a female.

Males are Haploid: They are created from an unfertilized egg. They have only one set of chromosomes from the queen located in the nucleas of the egg. The queen does not release the male's sperm when she wants a male offspring. They have no father. They hatch from an egg that contains only the mother's genetic material, meaning they are essentially a haploid (single chromosome set) version of the queen.

The M. ibiricus queen produces 2 kinds of offspring with the M. structor:

Sterile Female Hybrid Workers: These are produced in the standard way. The queen lays an egg (containing her genes) and fertilizes it with the sperm from the M. structor male. The resulting worker has DNA from both parents. It's a true hybrid. There is no "dominance"; it's a merger of two different species' DNA.

Fertile Male M.structor Clones: This is where things get really bizzare.

Remember that in the normal case:

- The queen does not use a male's sperm to produce male offspring.

- Joining both DNA always results in a female (males do not have two sets of chromosomes)

There can only be one conclusion. The queen creates this special clone from the male's DNA only, probably by somehow purging her DNA from the nucleus of her egg.

> M. ibericus and M. structor are not closely related, evolutionarily speaking. The two species diverged more than five million years ago, according to the paper.

Time means nothing - species can stay stable for very long periods of time (e.g. coleocanth), and more to the point it makes no sense to call two animals different species if they can still successfully interbreed, since there then still remains the possibility that they could recombine. NOT being able to interbreed successfully (donkey & horse) marks the point of no return where they are now bound to genetically drift further apart over time.

Regarding coleocanths... we have no data on how much drift has happened in their DNA. Our only real data is that they are morphologically very similar to their ancestors over a long range.

Your last sentence correctly points out the frailty of our definition of "species". However, this is not the only time our data has confounded our artificial, if often useful, definition of species boundaries.

True - I guess examples like these ants, or lions and tigers, where we have DNA available for both, give a better idea of the speed of genetic drift, or at least some datapoints. We can compare the DNA, and estimate how long those numbers of changes took to accumulate, without yet having got to the point of no return.

I wonder what are the most visually, or structurally, or genetically, different animals that can still interbreed. Things like lions & tigers, polar bears & grizzlies, and zebras & horses, come to mind ... what else ?!

The American paddlefish and the Russian sturgeon is a pretty wild one. They're in different families (your examples at least share genuses). As far as looking really different (but actually being pretty recently related) beluga whales and narwhals can hybridize.

The definition of speciation is more complicated than your highschool bio class lead you to believe. There's a dozen definitions, and if you choose one you end up with at least a couple of exceptional cases.

For example, American bison and domesticated cattle can interbreed to produce fertile female beefalos, but the males are sterile. Are domesticated cattle the same species as buffalo?

Then there's ring species: populations of animals where population A can interbreed with populations B and D, but not with C, but C can interbreed with B and D. (often the rings are larger than that). For example, the genus Ensatina salamanders here in California can interbreed with neighboring populations as you go around the mountains, but if you drove one from one side of the central valley to the other it couldn't interbreed. We've mostly decided in that case to call them a bunch of different species, but it's a weird case.

Shit gets even weirder when you leave the animal kingdom. All varieties of pepper will cross pollinate. Bacteria just sort of spread their genetic material to anything that's nearby. Don't even get me started on the absurdity of declaring all the asexually reproducing organisms as being single species individuals.

Basically, a species is a group of animals that has enough of the following characteristics that biologists can agree they're sufficiently different things:

  1) They appear distinct from other things
  2) They exclusively select mates from their group
  3) They exclusively produce fertile offspring with their group
  4) They occupy a distinct niche in their ecosystem
  5) They are more genetically similar to other members of their group than to other things we consider distinct species
  6) Their common ancestor with another group we identify as a species is extinct and considered a different species
  7) They really seem like they should be a species

Interesting - thanks.

I hadn't really considered the definition of asexually reproducing species - it seems that things are much more clear cut for ones that sexually reproduce since then we can use the more clear cut "point of no return" definition.

I suppose in cases like beefalos and mules, or these ring species, this "point of no return" comes down to is there any path for to the DNA of these divergent animals to recombine, so a fertile female beefalo (or the occasional fertile female mule) still provides that chance.

It seems that in general it's rare for widely divergent animals like zebras and horses to interbreed in the wild, but apparently western wolf-coyote hybrids are not that uncommon, so it's more than just a theoretical possibility. Who knows, maybe global warming will force polar bears to adapt to warmer climates and increasingly interbreed with grizzlies.

Even in the cases of species that roughly meet the naive high school definition, it is at least sometimes the case that they can still interbreed, they just don't. Usually, this is because of geographic isolation. Take African and Indian elephants, for instance. They diverged long enough ago that they're morphologically distinct, not just genetically distinct, but they can still interbreed. They don't because they live on different continents, and they probably wouldn't if they were put together outside of captivity because they're intelligent, social animals with culture and learned histories who rely on not entirely biological cues regarding who to breed with, just as much as humans do.

In reality, we first categorized life into species because they either looked different or we found them exclusively in different places, and only centuries later did we attempt to figure out exactly why and how this was the case and reverse engineer some sensible definition onto the pre-existing categories, but it turns out there is no single definition that works universally and has zero exceptions. It's frustrating if you're a language pedant who likes clarity, but a lot of categories and definitions are like this.

I guess you missed the point that M. ibericus and M. structor are at this point of no return: like donkey and horse, they can't interbreed successfully, as hybrids are all workers and can't mate.

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