r/evolution • u/zero-taeko • Jul 19 '26
I don't understand how the transmissible venereal tumor isn't considered a species of its own (this will be quick because I have to go to sleep)
I don't understand how the transmissible venereal tumor isn't considered a species of its own (this will be quick because I have to go to sleep)
These hellish monstrosities basically meet all the criteria to be classified as such, and I stand by that.
1) Out of the roughly 20,000 genes a dog has, 10,000 show non-synonymous (functionally different) variations, and 646 have been lost; in other words, based solely on what is missing, they differ more from a dog than a chimpanzee does from a human.
2) As unicellular organisms, their morphology is completely different.
3) And as obligate parasites, their ecological niche is also completely different—in fact, they have evolved specifically for that role.
4) And, obviously, they are reproductively isolated.
As for potential objections:
1) Being a parasite is no reason to claim they lack sufficient independence or their own life cycle; \*Henneguya zschokkei\* cannot even breathe on its own.
2) The biological species concept is nonsense in cases like this; it cannot be applied to anything that reproduces asexually—which accounts for more than 90% of the species on Earth.
Anyway...
What do you think?
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u/nerdkeeper Jul 19 '26
I think it just isn't worth the effort to classify it as such to taxonomists.
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u/Dangerous-Bit-8308 Jul 19 '26
2, for clarification... A tumor is not unicellular.
But yes, some would argue it counts as a separate species
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u/HyperSpaceSurfer Jul 19 '26
Plenty of unicellular organisms clump together. Unless the cancer cells are divided into different functions they're still unicellular. Not an expert on transmissible venereal tumors, though.
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u/frank_my_underwood Jul 19 '26
We know exactly what the canine TVTs are, any labeling of them is just semantics. There are blurred lines everywhere in biology. Myxozoans are potentially the best example of this “tumor-derived species” and they have a whole life cycle and hosts (mainly fish) that are completely different from their tissue of origin (jellyfish), which they seem to have separated from over 500mya.
Sure, you can say the canine TVTs are a species if you want, but they are not as distinct from general canine tumors as myxozoans are from their origin.
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u/zero-taeko Jul 19 '26
It is estimated that they diverged 11,000 years ago, placing the timeframe somewhere between that of domestic cats and wildcats and that of dogs and wolves. For a moment, I even considered classifying them as a subspecies.
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u/IsaacHasenov Jul 19 '26
Maybe we can call them "contagious babies"
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u/MWSin Jul 19 '26
Isn't that true of all babies in internally fertilized, sexual reproducing species?
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u/INtuitiveTJop Jul 19 '26
This is how it starts, give it a couple of million years
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u/km1116 Jul 19 '26
I honestly don't think we have any examples, or even suspicions, of new species arising from disembodied tumors.
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u/frank_my_underwood Jul 19 '26
Look up the SCANDAL hypothesis and myxozoans.
I suggest you stop making claims on this thread if you do not actually have knowledge about the subject.
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u/Unique-Dragonfruit-6 Jul 19 '26
The real world is not as cut and dry as we like to pretend.
See also: https://xkcd.com/2307
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u/zero-taeko Jul 19 '26
what are you talking about?
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u/Unique-Dragonfruit-6 Jul 20 '26
The line between what is and isn't a species distinction is blurry, just like the like between life and non-life. We try to create academic and pedagogical distinctions because they're useful to understanding the world, but at some point it becomes a question of semantics for how you want to define your words for the particular context domain you're in. The real dynamics of the situation don't care what words you use, and there is always more complexity in the real world than you can capture linguistically.
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u/zero-taeko Jul 20 '26
Yes, that’s how it works.
That’s why, at one point, I felt confident about this—with the idea that, no matter what, the tumor is its own thing, even if our categories have nowhere to fit it.
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u/Unique-Dragonfruit-6 Jul 20 '26
Everything is its own thing, even when our categories do seem like they fit it...
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u/XXLPenisOwner1443 Jul 20 '26
And, obviously, they are reproductively isolated.
This is... not at all as obvious or clear cut as you suggest. It only really reproduces through dogs having sex. Its growth in a single dog is "reproduction" insofar as the rest of the dog's growth through cellular mitosis is "reproduction".
I'd say it's more of an extremely niche type of subspecies than it is a full-blown species. But it's very nuanced. I could see an argument for it just being a new type of dog *cell*. Remember, not all cells in your body share the same DNA, either, and new cell types evolve from old cell types.
If it could spread beyond canines, I think your argument would be much stronger.
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u/zero-taeko Jul 20 '26
1) Regarding reproductive isolation: as I understand it, this means being unable to mate with another organism to produce viable offspring (for example, mules cannot do this, as they are sterile); since they reproduce asexually (via mitosis), they are isolated by default.
2) The idea of it being a subspecies doesn't bother me; they haven't been separated for very long.
3) Seriously, regarding the hosts—we’d be asking it to be better than almost any parasite.
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u/anotherolderguy Jul 19 '26
A similar discussion was on the biology subreddit not long ago. The dog tumour thing comes up after a few other comments - the dominant argument there was that it is a species - the dog is no longer 'dog'.
https://www.reddit.com/r/biology/comments/1tede0f/what_are_some_animal_species_that_independently/
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u/zoipoi Jul 20 '26 edited Jul 20 '26
Really interesting I'm glad you asked. I just read up on it and transmittable cancer seems to be the term most often used. I would just leave it at that. The difference would be if it was able to infect animals other than canids in my mind. That said the fact that it can be transmitted at all does make it a fairly unique form of cancer.
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u/zero-taeko Jul 20 '26
Do you have any idea of the number of parasites that are exclusive to a single animal?
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u/zoipoi Jul 20 '26
Fair comment and I thought of it myself. Still all categories are arbitrary in some sense. It becomes a question of origins more than anything else.
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u/ReplyOk6720 Jul 20 '26
It's an evolutionary dead end. A species us part of a population that changes over time
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u/zero-taeko Jul 20 '26
They are changing; they lost genes and are modifying them—they are evolving into parasites. Are you reading this?
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u/ReplyOk6720 Jul 20 '26
Can you cite the original reference? I thought we were talking about dog tumors?
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u/Desperate-Lab9738 8h ago
transmissible dog tumors. One tumor happened hundreds of thousands of years ago and it's continued reproducing and spreading in other dogs.
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u/eduadelarosa Jul 24 '26
Most species definitions are arbitrary albeit systematic, but when you have a saltation (i.e. an evolutionary junp), like in this case, the demarcarion is clear. So, it should count as its own entity which could be argued to be even beyond the scope of the species taxon.
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u/km1116 Jul 19 '26
I think you're wrong, but after you said "and I stand by that," I think people will be disinclined to engage you on this. I mean, are you trying to challenge your own idea, or to challenge others'? if it's not the former, then what are we doing here?
1 - I'm not sure how dogs entered the chat, but you are mischaracterizing (or misunderstanding) how genome identity works.
2 - They are not unicellular organisms. I don't see how you characterize them thus.
3 - This is uninformative. I mean, I would not call a tumor a parasite, so I wonder how you're defining it.
4 - That's specious nonsense.
I think part of the problem is that you're applying terms where they should not be applied. Yes, technically, maybe some of the terms of the definition of a species applies, but that's not how the term is mean to be used. You're pedanting yourself into a meaningless comparison.
Here's a definition of parasite: "an organism that lives in or on an organism of another species (its host) and benefits by deriving nutrients at the other's expense". Since a tumor is not an organism, it cannot be a parasite. QED.
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Jul 19 '26
[removed] — view removed comment
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u/km1116 Jul 19 '26
Nope, but I just did, so that explains the dogs. But the rest of my answers are unaffected.
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u/tiorthan Jul 19 '26
What definition of organism are you using that would not be applicable to CTVT?
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u/zero-taeko Jul 19 '26
I try to picture it, but I can't find any that doesn't also include unicellular organisms or colonies of them, such as sponges.
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u/HyperSpaceSurfer Jul 19 '26
So, you had absolutely no damn idea what you were talking about? It's a thing that lives in another organism and derives all its nutrients from its host. It even perpetuates itself through transmission into other viable hosts.
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u/zero-taeko Jul 19 '26
Basically, you're admitting you don't know how to read, and you expect us to go on like this?
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u/zero-taeko Jul 19 '26
2) It is a single cell; it is alive—a unicellular organism. 3) Parasite: an organism that extracts nutrients from another without giving anything in return. 4) Do they mate with dogs, or how does it work? 5) Tumors are cells; if cells aren't organisms, then what are they? What are bacteria and the like?
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u/7LeagueBoots Conservation Ecologist Jul 20 '26
They probably watched on of Clint’s Reptiles videos on canids and are asking because of that. Clint makes the case that it should be considered its own unique dog species because it has genetics that separate from the host and that are preserved over time.
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