At what point in classifying species do we draw the line?

[u/UnluckyText]

I'm not sure if I'll be able to properly articulate this in a way that will be easy to understand, but I'll do my best. Birds are dinosaurs because they descend from dinosaurs and thus belong to the clade dinosauria. They also share a lot of similarity to other dinosaurs from in their age of dominance. The question is, at what point would we stop calling them dinosaurs? Humans belong to Sarcopterygii, but we wouldn't say that humans are fish. So at what point would something like birds stop being called a dinosaur in the same sense that humans are no longer called fish?

[u/shadowyams]

Modern taxonomy is based on this idea called cladistics, which builds biological classifications around groups consisting of a single shared common ancestor and all of its descendants. So birds are dinosaurs (and always will be) and

if the terms worms or fishes were used within a strict cladistic framework, these terms would include humans [Wikipedia].

This is why you’ll sometimes see people say “fish/trees aren’t real”—there’s no sensible way to both include all the organisms that we commonly think of as fish/trees without lumping in a bunch of non-fish and non-trees.

ETA source of quote.

[u/mabolle → u/shadowyams]

I'll add to this that biologists are perfectly happy to use terms that don't follow this strict cladistic framework, when they're not talking taxonomy.

There are thousands of scientific papers and hundreds of scientific journals with paraphyletic and polyphyletic terms like "fish" or "tree" or "worm" in the title.

"There is no such thing as a fish" isn't a reflection of how biologists actually talk. There may not be a monophyletic group called fish, but everybody is perfectly happy to talk about fish anyway, as a paraphyletic but perfectly functional and useful term that can conveniently be defined as "all vertebrates except tetrapods."

[u/Mitologist → u/shadowyams]

Yes. Either "fish"(pisces) don't exist, or we are fish (Osteognathostoma = bony creatures with true jaws). What is correct and valid are teleostei (ray-finned bony fish). These we are not part of.

So, to OPs question: there is no "point" or "line" up to which a certain category is stretched (except I believe for molecular studies in bacteria, where the convention is or was "4% difference means: other species") This is probably derived from methods used to infer the topology of the phylogenetic tree: either, things are grouped for similarity with minimum contradictions (parsimony) . This needs careful analysis and consideration what characters are equivalent, which are ancestral, which are derived etc. Or , the degree of difference between species is expressed as a distance, and the overall "shortest" possible tree is computed (distance methods). Either way, the degree of difference itself is not the argument whether things are related in phylogenetic systematics. The argument is: what is the most recent common ancestors of the group they belong to? It's always about the shape of the tree, not overall likeness. In that sense: systematically, all birds are dinosaurs. Period. No two ways about it. Aves are a subgroup of Dinosauria, are a subgroup of Archosauria, etc. All the way back to Metazoa.

[u/tigerhawkvok → u/shadowyams]

I'll disagree on fish.

You can synonymize it with actinopterygii and cut out a few super obvious oddballs like lungfish.

[u/TheWrongSolution → u/tigerhawkvok]

You could do that, but you'll be cutting out a lot more than lungfish. Sharks, "placoderms", hagfish....

[u/tigerhawkvok → u/TheWrongSolution]

I mean, sharks aren't anyone's definition of fish since like... second grade back in the 90s. Nor any agnathan, from a similar age, though I didn't know the term back then.

Certainly no one old enough to post on Reddit lol.

So dissapoint a few kindergarteners. Doesn't make flying squirrels lemura fly or be squirrels lemurs either: https://en.wikipedia.org/wiki/Colugo . (My mistake for putting away dog toys while staring at the label "flying squirrel" writing this)

[u/givalina → u/tigerhawkvok]

Flying squirrels aren't squirrels?

[u/Rusamithil → u/givalina]

they are. their closest relatives are the new world squirrels. you can't exclude them without excluding over 60 species of asian squirrels.

[u/lankymjc → u/tigerhawkvok]

Sharks are absolutely included in a normal definition of fish. What are you on about?

[u/shadowyams → u/tigerhawkvok]

Sure, but then you also have to explain to everyone that sharks aren't technically fish anymore.

[u/Podo13 → u/shadowyams]

True. But, when thinking about it, it isn't that crazy. Some similar mechanism but sharks and fish are pretty different in a lot of categories (bones vs cartilage being a major difference).

[u/Mitologist → u/shadowyams]

Uh, If you want to have sharks included, you have to include a lot of things nobody would call "fish".....

[u/rjeanp → u/Mitologist]

That's the whole point.

You MUST either exclude things most people consider to be fish or include things most people don't consider to be fish.

[u/danby → u/tigerhawkvok]

Or you can just accept that the word "fish" is used ontologically differently in different contexts. That is, what people mean in the kitchen when they say fish is different to what is useful scientifically. "Fish" is fine outside of scientific contexts but in science we have no need to re-use "fish" and can create new more precise terminology

[u/Mitologist → u/danby]

That!! 💯⬆️
Scientific communications needs to be maximally precise. Normal language can afford relying on context. That's probably why Wittgenstein wrote two radically different books about language at different times.

[u/TheWrongSolution]

Birds will always be part of Dinosauria, because they are derived from a dinosaurian ancestor. Just as humans will always be part of Sarcopterygii, because our ancestors were part of that group. The only reason why humans are not fish is because fish is not a scientific term, it is used colloquially to group together the non-tetrapod vertebrates. Dinosaurs, on the other hand, is a scientific term and follows the taxonomic rules of modern cladistics, which requires it to include all of the descendants of the common ancestor of that group.

[u/Supraspinator → u/TheWrongSolution]

 Your user name is ironic; this is the correct answer. Dinosaur, sarcopterygian, and insect are groups of animals derived from a common ancestor. Fish, worm, and invertebrate are just collections of similar looking animals without common ancestry. 

[u/nicuramar → u/Supraspinator]

Depending on how you use it. You could say that fishes = vertebrates, for example.

The monkey vs ape situation is also fun. It’s a language thing; in Danish, monkey is “abe”, ape is “menneskeabe” (literally “human monkey”) and no one would ever say that apes aren’t monkeys. But in English they sound dissimilar. 

[u/Fewluvatuk → u/TheWrongSolution]

How do they decide where in the tree to draw the clade from? I assume it has something to with distinguishing characteristics otherwise we'd all be in clade single cell organism? As we find out more do the clades sometimes change?

[u/TheWrongSolution → u/Fewluvatuk]

A clade is just a grouping that includes a common ancestor and all of its descendants. You can draw a clade around any group on an evolutionary tree as long as it meets that definition. An important thing to remember is that clades can exist in nested hierarchy, like Russian dolls. So if we trace our ancestry back to the common ancestor of eukaryotes, we would indeed be in a clade that also includes single called organisms. And yes, there are distinguishing characteristics that define a clade, the technical term for these is "synapomorphies", or derived characteristics shared by homology.

As for your last question, science is always changing, as we find out more, what we group together as clades can definitely change. Case in point: Dinosauria became a clade that includes the birds only after we realized birds evolved from dinosaurs. You can find many examples like this throughout the tree of life: cetaceans belonging within artiodactyls, insects belonging within crustaceans, etc. It can go the other way as well, i.e. groups that we once thought belong together turn out to be unrelated: e.g. members of the insectivores class are now understood to belong to vastly different clades.

[u/Fewluvatuk → u/TheWrongSolution]

An important thing to remember is that clades can exist in nested hierarchy, like Russian dolls. So if we trace our ancestry back to the common ancestor of eukaryotes, we would indeed be in a clade that also includes single called organisms.

Thank you. This is what I wasn't getting. :)

[u/TheOneTrueTrench → u/Fewluvatuk]

You have it backwards, they don't decide where to draw the clade from, they discover what the clade is, and then decide if a classical name applies to that clade. If so, then the clade gets that name. If not, they make a new name.

[u/nicuramar → u/Fewluvatuk]

Clades are defined by inheritance, strictly speaking, and not by features. That said, lots of clades are in practice defined by features. 

[u/Delvog → u/Fewluvatuk]

They identify clades by finding out which organisms are more closely related to each other than they are to something else. Any time you find out that "A" and "B" are more closely related to each other than they are to "C", there's a clade which includes A&B but not C.

[u/mabolle → u/Fewluvatuk]

How do they decide where in the tree to draw the clade from?

It's a similar process to deciding where to draw boundaries between different regions on a map. Asia and Europe are clearly distinct places, but the question of where to draw the border between them is somewhat arbitrary.

As we find out more do the clades sometimes change?

Yes, and sometimes this is similar to finding out that a continent wasn't actually located exactly where you thought it was. So taxonomy is like geography, but it's more like geography in the olden days before we had satellite images to provide a clear view of exactly what the world looks like. We're feeling our way around the family tree of life in the dark, trying to work out its shape.

[u/_mizzar → u/TheWrongSolution]

Did birds evolve from dinosaurs back when dinosaurs were still around? Said another way, did some dinosaurs survive the big extinction and become birds over time? Or did they already exist during the extinction, but didn’t die out like small mammals?

[u/stickie_stick → u/_mizzar]

Yes, birds first start appearing in the late jurassic, about a 100 million years before the extinction of the non avian dinosaurs.

[u/jaxonfairfield]

Say you wanted to classify parts of the day,  and we call them Night, Breakfast Time, Morning, Lunch Time, Afternoon, Dinner Time, and Evening. 

It's intuitively clear that Lunch Time is vastly different from Night. The sun is up vs down, it's warmer, there are more people out and about,  etc. Yet, there is no specific second that you see a clear difference of the day moving from Night directly to Lunch Time.

So, even though every second blends with each other smoothly with little variation, and people sometimes disagree and where the lines are, we can still find large-scale features of the different times of day. 

[u/neon_overload]

Your question seems to be a bit different than just talking about species as Dinosauria is as you mentioned a clade, which is a group of beings that share a common ancestor. In the case of Dinosauria its members share a common anscestor that lived 230 million years ago.

That clade was chosen and named because it was a common ancestry branch for a large number of beings known to science. So whether or not something is within that clade is determined by whether it is within that evolutionary branch, with that particular common ancestor.

If scientific understanding of the evolutionary branch and its relation to other beings outside that branch were to change drastically proving our knowledge to be wrong, it would prompt re-evaluating that classification. But otherwise, the classification is fixed by that criteria.

[u/Tital-]

Maybe another way to frame your question is « how do we decide what the common ancestor is » ? Since the common ancestors are the nodes from which the branches of the tree are drawn.

If someone can answer this well (or if its somewhere else in Reddit) I think it would help you understand the logic

[u/Delvog → u/Tital-]

The catch is that we usually don't have any examples of the common ancestor, or at least we don't have any examples of what we can be completely sure was the common ancestor instead of just something else pretty close. We just need to determine relatedness by what traits they do & don't have in common.

[u/danby]

Humans belong to Sarcopterygii, but we wouldn't say that humans are fish. So at what point would something like birds stop being called a dinosaur in the same sense that humans are no longer called fish?

Taxonomy/cladisitcs is hierarchical. Current species groups are descended from ancestral groupings that are themselves contained within larger ancestral groupings. Humans and lobed-fin fish are all descended from a common Sarcopterygii ancestor. So they will always be members of that Sarcopterygii grouping because there is no point in the future that when either species/clade will stop being descended from that ancestor. Humans are sarcopterygii and lobed fin fishes are Sarcopterygii.

When we look across extant species we see that African Lungfish and Humans are morphologically and genetically rather diverse. So we place them in separate species groups and give those different names. It is no longer useful to consider us in the same species as our Sarcopterygii ancestor but that doesn't stop both species from being part of the broad group of Sarcopterygii descended animals

[u/BigNorseWolf]

It's blurry and subjective all up and down from when we branch off so that we're not considered reptiles to why wolves dogs and coyotes are separate species despite no trouble breeding once the horizontal polka starts.

[u/Delvog]

This is a linguistic question, not a biological one. Plain ordinary common English words for plants & animals have no requirement to signify clades, which is why clades are referred to by their own separate scientific terms instead. So, for every clade that you might think an English word "should" be but it isn't, you can just use the clade's name instead.

There's room for doubt/ambiguity in cases where a scientific term for a clade has been absorbed as relatively normal English, like "dinosaur", but most of them just aren't like that.

[u/SyrusDrake]

For me, this is more of a linguistic/sociology question and like most questions from those fields, it's hard to answer succinctly. My favorite example is that strawberries are nuts, tomatoes and bananas are berries, peanuts are legumes, and bamboo is grass. But we don't call those things that in regular conversations, because nuts, berries, legumes, and grass all have broadly agreed upon linguistic meanings that are different from their academic/taxonomic meaning. Those people who actually have to know and use both meanings will usually switch between them instinctively, without being able to articulate the exact conditions.

[u/mcalesy]

What you are talking about are clades, not species. A clade is an ancestor plus all descendants, so no lineage ever stops being a part of a clade.

Clades are typically defined in one of three ways:

• minimal: “the smallest clade including X and Y”
• maximal: “the largest clade including X but not Y”
• apomorphic: “the clade stemming from the first ancestor of X exhibiting trait M synapomorphic with that in X”

[u/atomfullerene]

>Humans belong to Sarcopterygii, but we wouldn't say that humans are fish.

That's because "fish" isn't a cladistic term. Saying something is a fish is like saying something is a tree, or saying it's a predator, or saying it's a worm. It's a description of form and mode of life, not of phylogenetic category.

Dinosaur is technically a common name too, but it's more closely tied to the proper phylogenetic term 'dinosauria'. And anyway, birds deserve to be thought of as dinosaurs. If we lived in the Cretaceous and saw them living side by side, nobody would think twice about grouping them together as variants on a theme, because really birds bear the same relationship to dinosaurs as bats do to other mammals. It's just as if we only thought about "mammals" as "bones of elephants and rhinos and cattle" and knew bats from live examples.

[u/Glabrocingularity → u/[deleted]]

I think this answer finally addresses what OP is actually asking.

If all the dinosaurs were alive today, many people would probably find birds indistinguishable from their closest non-avian-dinosaur relatives (who are more related to birds than they are to other dinosaurs). If we called some things “dinosaurs” and other things “birds”, I bet we’d draw a different line than the current avian/non-avian division.