# Why are Eurasian and American beavers so different genetically? **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [October 26, 2026, 2:03pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169 "2026-10-26T14:03:08Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Babale](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/babale/32/15666_2.png) [@Babale](https://boards.straightdope.com/u/Babale)\ **Post date:** [October 26, 2026, 2:03pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/1 "2026-10-26T14:03:08Z")
I recently learned that the Eurasian and American beavers are actually super distinct genetically, with one having 48 (24 pairs) chromosomes and the other having only 40 (20 pairs). That seems pretty wild to me - when I think of other somewhat distant cousins, like humans and chimps or horses and donkeys, they are just one pair of chromosomes apart. But while all of those cousins clearly appear to be different species, the two beaver species are so similar that scientists weren’t even in agreement on them being separate species until the mid 20th century. How does a situation like that come about? Or is it actually nowhere near as unusual as I’m making it out to be? ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [October 26, 2026, 4:27pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/2 "2026-10-26T16:27:37Z")
The two species are believed to have diverged about 8 million years ago, which, I am assuming, gives plenty of time for evolutionary and genomic changes. (FWIW, it’s believed that the human ancestor species diverged from chimpanzees and bonobos at about that same time. Humans have 46 chromosomes, while chimpanzees and bonobos have 48.) ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [October 26, 2026, 5:05pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/3 "2026-10-26T17:05:04Z")
Finally a topic I am an expert on. Oh, wait… ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [October 26, 2026, 5:11pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/4 "2026-10-26T17:11:50Z")
> [@Babale](#): > > 48 (24 pairs) chromosomes and the other having only 40 (20 pairs). > [@kenobi\_65](#): > > (FWIW, it’s believed that the human ancestor species diverged from chimpanzees and bonobos at about that same time. Humans have 46 chromosomes, while chimpanzees and bonobos have 48.) How do species even end up with differing amounts chromosomes anyway? I can understand the occasional mutation, but how does that mutation take over the whole species? > [@mixdenny](#): > > Finally a topic I am an expert on. Oh, wait… I laughed. ---
**Author:** ![SCAdian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scadian/32/3292_2.png) [@SCAdian](https://boards.straightdope.com/u/SCAdian)\ **Post date:** [October 26, 2026, 6:11pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/5 "2026-10-26T18:11:04Z")
> [@kenobi\_65](#): > > (FWIW, it’s believed that the human ancestor species diverged from chimpanzees and bonobos at about that same time. Humans have 46 chromosomes, while chimpanzees and bonobos have 48.) Two of the human chromosomes (of which there were two each) fused together, which accounts for the difference. > [@Lucas\_Jackson](#): > > I can understand the occasional mutation, but how does that mutation take over the whole species? For some reason the mutation is beneficial, and leads to more offspring with that mutation. Don’t ask me why having two chromosomes fused would be a particularly good thing, but. . . . ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [October 26, 2026, 6:43pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/6 "2026-10-26T18:43:36Z")
I should probably start a new thread but… > [@SCAdian](#): > > For some reason the mutation is beneficial, and leads to more offspring with that mutation. Don’t ask me why having two chromosomes fused would be a particularly good thing, but. . . . I get that, usually, but from what I’ve read both species do normal beaver stuff. It’s not like the Eurasian beavers are using tools are anything so in this case I wonder what good those extra chromosomes are doing them. ---
**Author:** ![Babale](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/babale/32/15666_2.png) [@Babale](https://boards.straightdope.com/u/Babale)\ **Post date:** [October 26, 2026, 6:48pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/7 "2026-10-26T18:48:16Z")
> [@Lucas\_Jackson](#): > > I get that, usually, but from what I’ve read both species do normal beaver stuff. It’s not like the Eurasian beavers are using tools are anything so in this case I wonder what good those extra chromosomes are doing them. It’s the other way around. Beavers as a clade originate in North America 33 million years ago. But modern beavers both belong to the same genus which originated in Eurasia and migrated back to North America much more recently. So it’s more accurate that North American beavers have fewer chromosomes than they used to rather than Eurasian beavers gaining extra ones. ---
**Author:** ![kenobi\_65](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kenobi_65/32/13178_2.png) [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)\ **Post date:** [October 26, 2026, 6:49pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/8 "2026-10-26T18:49:11Z")
> [@Lucas\_Jackson](#): > > It’s not like the Eurasian beavers are using tools are anything so in this case I wonder what good those extra chromosomes are doing them. IANA biologist, but I might guess that, just as it’s believed that humans had chromosomes which merged (which is why we have 46 and our great ape relatives have 48), the American beavers underwent something similar. Fewer chromosomes, but maybe the same (or similar) number of total genes. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [October 26, 2026, 7:58pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/9 "2026-10-26T19:58:32Z")
> [@Babale](#): > > So it’s more accurate that North American beavers have fewer chromosomes than they used to rather than Eurasian beavers gaining extra ones. Got it, thanks. I guess I just assumed that the total would go up. In my mind that was the obvious thing. But I appreciate the clarification. And, yes, I know about assumptions… 😉 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 26, 2026, 9:38pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/10 "2026-10-26T21:38:22Z")
It’s not necessarily that the new gene was more advantageous. Sometimes it’s just dumb luck, especially when a relatively small population colonizes a new environment. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [October 26, 2026, 9:42pm UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/11 "2026-10-26T21:42:49Z")
Does a difference in the number of chromosomes necessarily imply a genetic difference? The genes are stored differently, but it could be the same genes. ---
**Author:** ![SCAdian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scadian/32/3292_2.png) [@SCAdian](https://boards.straightdope.com/u/SCAdian)\ **Post date:** [October 27, 2026, 3:49am UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/12 "2026-10-27T03:49:32Z")
Too bad we no longer have _Castoroides_ or _Trogontherium_. ---
**Author:** ![puzzlegal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/puzzlegal/32/3827_2.png) [@puzzlegal](https://boards.straightdope.com/u/puzzlegal)\ **Post date:** [October 27, 2026, 3:55am UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/13 "2026-10-27T03:55:01Z")
We’ll, I’m sit there are some generic differences, but your are right that those differences might not be huge, if they have most of the same genes, just stored in a different number of chromosomes. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 27, 2026, 7:28am UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/14 "2026-10-27T07:28:48Z")
> [@Babale](#): > > Or is it actually nowhere near as unusual as I’m making it out to be? One of the cool things I learned in school is that genes don’t really closely relate to body plans — or rather, that different genes can still result in morphologically similar, lookalike species that occupy similar niches, whether in the same place or scattered across the world. In the case of beavers, “aquatic tree-felling rodent” worked well enough in both the Old World & New that they kept the same basic body plan despite genetic drift over time. Think sharks, cockroaches, etc. too — many different genes and species, same basic body plans (for the most part, with specialist niches within them though). A lot of birds are like that too. When you think “owl”, that’s not really a species of bird, but a whole order of 200+ different species that look similar and often have the same “big-eyed, night-hootin’, rodent-killin’” body plan, but are still distinct species. And can you tell apart eagles and falcons readily? I cannot. But eagles are actually closer to owls, while falcons are closer to parrots. These are actually distinct species. The beavers, for example, cannot interbreed, and when mixed, the American beavers can outcompete the Eurasian ones, screwing them over instead of, well, screwing them. This [happened in Finland](https://niche-canada.org/2025/03/07/canadian-beavers-in-the-nordic-countries/) and resulted in a proposal to wipe out American beavers there… The different owl species mostly don’t interbreed either. Sometimes they _can_ produce fertile hybrids, but don’t typically do so in the wild, apparently because they like different kinds of music — their songs are different enough that they don’t recognize each other as potential mates, like in humans. Then there’s the case of convergent evolution, when _completely_ different lineages nonetheless end up looking similar over time, and occupying similar niches — like how wings appear in birds, bats, and pterodactyls, but good luck getting KFC to sell those. Or how [crab-like things have independently evolved many times](https://www.sciencealert.com/evolution-keeps-making-crabs-and-nobody-knows-why), and been lost even more times. In the plant world, you have [cacti vs euphorbia](https://www.succulentsandmore.com/2024/11/succulents-101-cacti-vs-cactus-like.html) — superficially similar “green, spiky succulent things” that are completely different. “[Snake-like lizards](https://reptilesmagazine.com/six-legless-lizards/)” keep showing up across distant groups (or more [sciency](https://evolution.berkeley.edu/evo-news/the-legless-lizards-of-lax/) version). Two [completely different types of frogs](https://news.mongabay.com/2005/08/convergent-evolution-of-poison-frogs-and-ants/) both evolved poison by eating completely different types of poisonous ants on different continents. There’s many more examples like this across all of biology. Basically, I think the overall principle (as far as I know) is that environments cause niches, niches require adaptations, and genes that result in successful adaptations in those niches survive — but there can be more than one set of genes that result in the same adaptations. Our mistake (as scientists and everyday people, I mean, before DNA sequencing was the norm) was assuming the opposite: that morphology indicates relatedness. It _can_ but doesn’t have, and there were many false relatives that eventually had to be reclassified once DNA data proved earlier assumptions of relatedness wrong. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 27, 2026, 7:51am UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/15 "2026-10-27T07:51:52Z")
And on the specific subquestion of “related species changing chromosomes over time while keeping the same body plan”, beavers aren’t the only ones to do that. AI found a few other examples: [Muntjac barking deer](https://india.mongabay.com/2019/05/the-barking-deer-enigma/): > The muntjac group has the most diversity in their chromosome number among mammals. While the Chinese or Reeves muntjac has 46 chromosomes, the black muntjac and the Gongshan muntjac have eight chromosomes in females and nine in males and Fea’s muntjac has 13 chromosomes in females and 14 in males. In 1970s, it was discovered that the Indian muntjacs from Garo Hills, from the northern part of southeast Asia and southern China had just six chromosomes in females, seven in males, the lowest for any mammal. Mice on madeira with much fewer chromosomes, a more scientific look at the reasons: [Probing “Selfish” Centromeres Unveils an Evolutionary Arms Race | The Scientist](https://www.the-scientist.com/probing-selfish-centromeres-unveils-an-evolutionary-arms-race-71017) > The Portuguese island of Madeira is home to six different chromosomal races of mice, each with dramatically reduced diploid chromosome numbers compared to mice elsewhere. This striking diversity, first [identified](https://www.nature.com/articles/35003116) at the turn of the 21st century, can be explained by the repeated fusions of separate chromosomes. Each race has a different set of fusions, and a hybrid between two races would likely have reduced fertility or be sterile because of problems with chromosome pairing. Such reproductive isolation among populations is a key step on the road to speciation—and in the mice’s case, these chromosomal changes have all occurred within the 1,000 years since their ancestors arrived on the island, possibly on Viking ships. Rock wallabies: [North Queensland Rock-wallabies cause a rethink of biological theory - The Australian Museum Blog](https://australian.museum/blog-archive/amri-news/amri-north-queensland-rock-wallabies-cause-a-rethink-of-biological-theory/) > Understanding the detailed mechanisms by which new species form (i.e., the process of speciation) remains one of the great mysteries of biology. The iconic Australian rock-wallabies (_Petrogale_) show extreme chromosome diversity compared to other marsupials, particularly the six species in northeast Queensland found from Rockhampton to Cape York Peninsula. These closely related species have been thought a clear example of chromosomal speciation as they are otherwise indistinguishable but differ markedly in the shape and number of their chromosomes. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [October 27, 2026, 9:14am UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/16 "2026-10-27T09:14:40Z")
> [@Reply](#): > > Sometimes they _can_ produce fertile hybrids, but don’t typically do so in the wild, apparently because they like different kinds of music — their songs are different enough that they don’t recognize each other as potential mates, like in humans. You’ve never hung out with music hipsters, have you 🙂 ? ---
**Author:** ![Filbert](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/filbert/32/3433_2.png) [@Filbert](https://boards.straightdope.com/u/Filbert)\ **Post date:** [October 27, 2026, 9:56am UTC](https://boards.straightdope.com/t/why-are-eurasian-and-american-beavers-so-different-genetically/1034169/17 "2026-10-27T09:56:41Z")
> [@Pleonast](#): > > Does a difference in the number of chromosomes necessarily imply a genetic difference? The genes are stored differently, but it could be the same genes. Yup. I’ve only worked with plant genetics, but they do this stuff all the time. Chromosomes split, merge, duplicate… it doesn’t change the function, just the arrangement. With mammals, more than plants, having one chromosome split into two or two merge into one can be enough to make the two individuals barely or not interfertile- basically although they have the same genes, they don’t line up right in fertilisation. It’s common for different species to have a different chromosome number, as part of the traditional definition of a species is that they don’t typically interbreed, but it doesn’t necessarily confer a benefit or a negative. In other words, if they didn’t have a different chromosome number they may well be interfertile and so we wouldn’t consider them two species.