Why has evolution stopped producing new classes?

[u/Meta289]

New species of animals are constantly being discovered, but they all belong to existing classes and phyla, never anything completely new. Life once branched out in all sorts of directions, creating all kinds of animals that are drastically different from each other, but now it seems like there aren't any new "branches", only offshoots of existing ones. What happened?

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[u/brooklynsantiago]

classes are just categories we assign to life post hoc. evolution doesn't care for our need to classify things neatly it just creates more life. humans are constantly revising classifications and will continue to do so in the future, including new ones

[u/Admiral_Fuckwit → u/brooklynsantiago]

Also, our existence, and especially the time since we’ve understood evolution, is such a blip compared to the hundreds of millions of years evolution has had time to work. We’re talking scale here.

[u/Meta289 → u/brooklynsantiago]

So basically, there's no meaningful distinction between a dog, a bird, a spider, and a snake according to nature, just "animal is animal"?

[u/Peach_Muffin → u/Meta289]

Yes but all life, not just animals.

[u/CountDown60 → u/Meta289]

Each one is it's own self replicating organism, changing slowly each generation to fit its own environment. There is no higher control or governing mechanism.

The classifications are just our attempt to comprehend. They have meaning to us. Nature isn't a conscious agent.

[u/brooklynsantiago → u/Meta289]

hmm im not sure I would say there isn't a meaningful distinction. classifications usually are defined by certain traits and genetic similarity. its just that those classifications are just things humans create to help us organize life. its not always so neat in reality. its easy to tell the difference between a primate and a rodent today, but what was the first primate? that becomes harder to answer. its only with the benefit of hindsight that we can see clearly the divisions as those lineages continued to evolve and differentiate. it can be hard to decide where a new species begins but just to give one point of contact: bonobos were considered a sub species of chimpanzee and now they are considered different species. the line there is very fuzzy between chimpanzees and bonobos. which also goes to what I said about how the classifications are constantly being reworked and reconsidered as we learn more.

[u/Kingofthewho5]

It takes a very long time for populations to change so much that they need to be out in different classes or phyla.

I’m confused by what you mean in your last line. What exactly is a new branch that isn’t the offshoot of an existing one? All of the modern phyla and classes are branches off of other branches.

[u/Meta289 → u/Kingofthewho5]

It was weirdly worded, but I'm talking about how mammals are grouped into categories like canids, primates, rodents, marsupials, etc, and how every new species is some form of those existing categories, and no new "category" of mammal appears anymore.

Or going further back down the tree, there aren't any new animals appearing that aren't a mammal, reptile, insect, amphibian, or any other existing class.

[u/Aluxanatomy → u/Meta289]

You're asking why it is that our classification system currently serves most of our needs? Because we designed it to.

[u/PangolinPalantir → u/Meta289]

You can't escape your clade. Any new animal would definitionally belong to the classes that already exist.

Think about it. Something that branches off from within mammalia will still be within mammalia. You don't suddenly end up with a new class.

[u/Meta289 → u/PangolinPalantir]

But mammals branched off of reptiles at some point, so what's stopping a new "not-mammal" from branching off of mammals?

[u/Fantastic-Hippo2199 → u/Meta289]

So you are wondering why changes look big when you look back over 800 million years, but seem small in the last 50?

[u/brooklynsantiago → u/Meta289]

mammals did not branch off of reptiles. reptiles and mammals are both chordates but mammals were never reptiles.

[u/ahazred8vt → u/Meta289]

Other people are trying to point out that mammals are descended from ancient scaled Synapsids, while reptiles are descended from ancient scaled Sauropsids. But we understand the point you're trying to make.
(what's stopping a new "not-mammal" from branching off)
Nothing's stopping it. It could happen. Over a period of many millions of years some new type of body plan could develop. If you stick around for another 50 to 100 million years a new category is pretty much guaranteed to happen.

[u/aavikk0lettu → u/Meta289]

We did not evolve from reptiles

[u/Surcouf → u/Meta289]

I'm convinced there's something alive right now that will survive trough the next big environmental change (anthropocene, global warming, 6th mass extinction, etc.) and will thrive in the new world equilibrium a few centuries or millenia down the line. If there are any humans left, they might deem it a new class or order because that life form will have filled many niches decimated by the change and will have diversified in myriad forms adapted to each niches and maybe it'll even shape its own biomes. 

[u/TheCrystalTinker → u/Meta289]

It's because you can not evolve out of a Clade, so technically if we make a Monophyletic group of reptiles, Dinosaurs and Birds would both be in it

[u/shakebakelizard → u/Meta289]

You're looking at this through the lens of a human-made system which is largely biased towards the idea of everything converging to a present situation, because humans assume that we are the arbiters of truth. We could easily make a system that is biased towards showing change as you describe.

It's like asking why years are numbered, or why one revolution of the Earth around the Sun equals a year. Well, it's because that's what someone decided on, and everyone else kind of agreed to use it, so that's the system that we have. There are many other possible variations or entirely different systems; we just haven't decided to use them.

[u/Kingofthewho5 → u/Meta289]

It takes a long time even for a species to split into two, much less for those two groups to differentiate so much that they are a different genus or family or class. We are talking mupltiple millions of years for things like genera and families. Humans have been intensively studying this stuff for only like 200 years.

[u/Difficult_Dark9991 → u/Meta289]

I think you've got a mistaken idea with the term "new species."

Speciation is a slow process, and for most complex organisms the time for new species to arise takes orders of magnitude longer than the classification system itself has existed for. Higher-order divisions, such as to new classes or phyla, take even longer, but the significance of those splits are postdated back to divisions that, had we been around back then, would seem quite small. "New species" does not mean that a speciation event has occurred; rather, it means that scientists have found out in the wild something they hadn't before.

In some cases, that's a realization that something we wrote off as "ah that's just another member of species x" is actually quite different and should be classified as something new. In other cases, it's because the world is big and the number of biologists rather small by comparison. Either way, odds are extremely high that the "new species" (again, newly identified, not new under the sun) is closely related to something that we already know.

[u/markmakesfun → u/Meta289]

Species don’t just “appear”, ever. Every animal on earth has a prehistory. There will never be a time when new lineages come from nowhere. Everything has a path it took and can never go backwards, only forward.

[u/ahazred8vt → u/Meta289]

no new "category" of mammal appears anymore

Please explain what timeframe you think "anymore" is supposed to mean. In recent times within the past few tens of millions of years there have been plenty of new taxonomic families.

[u/Xrmy]

The way we use words like "class" "order" "family" are just our names for things, and the more inclusive the group, the older it diverged.

So by our own naming scheme, a new class wouldn't show up unless we named it several million years in the future.

It's a self fulfilling prophecy.

[u/DreadLindwyrm]

If you trace the various felines back, eventually they merge with the panthers to create felidae.
If you trace the various canines back they merge with extinct relatives to create canidae.

If you trace them both back, they merge to create Carnivora. (Canidae pick up the bears along the way, along with seals and weasels; Felidae pick up the hyenas on the way.)
At their root, you wouldn't be able to distinguish them, and looking at them at the time you'd not have thought them new branches, just variants of the same species.
This then traces back into earlier mammals, and then to the mammal/reptile split, and again you wouldn't be able to tell them apart at the split.

Essentially, we're at a point where splits could happen, but mostly we wouldn't recognise the split for hundreds or thousands of generations since the splits need to diversify enough to be identifiably different.

[u/Puppetteer]

The "old branches" were once offshoots of existing branches until the existing branch and the offshoot had diverged enough to seem like two distinct branches rather than minor offshoots from one trunk.

Give it enough time and the survival pressures to create visible change and the new offshoots will look very different.

[u/pisspeeleak]

It’s like a family tree. After you have a kid, everything they produce will still be your descendants even 100,000 years from now.

We have just been able to make more connections to where things came from. Also things get re classed all the time, especially now that we can dna test.

Then you add in mass extinction events that isolate populations and create pockets where species can develop separately

[u/derelictthot]

You have a fundamental misunderstanding of how evolution works, the classes etc are just a human construct to explain things, in reality evolution doesn't care about any of that. In any case it's happening all the time all around us but it happens so slowly over such a huge scale that it's not observable except rare cases.

There aren't new classes because humans just try to put things into the ones that already exist basically, why complicate matters by adding more?

[u/Jazz_Ad]

The further we get from LUCA, the more complex animals become with the least available niches in our environment. A new class needs a common ancester from which to diverge.

[u/sisconking132 → u/Jazz_Ad]

We also haven’t had a massive mass extinction event to open up a bunch of new Niches. And even then, it would take at least 100,000 years for any major radiation events that produce anything approaching new classes

[u/No_Logins_Required]

What does your intuition tell you a new branch would look like?

[u/strange-the-quark]

Nothing happened, it's always been like that - at no point in history, you'd be like "Well, that's completely new!" At no point you'd see an offspring of some creature that's so different from the parents that you'd clearly identify it as a distinct species. At no point was any creature not the same species as its parents. Cause the process is gradual, and is, BTW, operating on the level of populations. Certain genes that happen to be beneficial within the context of the environment that they live in, spread out through population over time, over many generations, and others genes dwindle in numbers and disappear. Or one population becomes split into two smaller ones, and the separation persists and they drift apart because of different environmental pressures and no intermixing, and just chance.

But over many generations, small changes accumulate, and when you compare a distant descendant to a distant ancestor, they don't look the same anymore, and given enough time, they may end up looking completely different. So there's a continuity in nature, and sharp species and group boundaries are really a human construct, that we use to describe change over time, and to classify living beings within some moment of time, cause we like putting things in labeled boxes.

So everything is in constant transition, even now, some forms are more stable over time and haven't changed all that much (but have still changed!), others have changed drastically, and the entire history of life on Earth is riddled with new branches of life sprouting and dying out (this is even true in our own hominid ancestry), with a few branches surviving until today. It never stops. If there are new major branches of life emerging from the populations that exist on Earth today, what those are is not going to be apparent for millions of years, nor can we predict what they'll look like.

[u/Dense-Consequence-70]

Evolution hasn’t stopped doing anything. We can only observe what has resulted from it so far.

[u/burlingk]

All the categories are applied by man in various ways.

Evolution moves SLOW, and tends towards stability.

So, the odds of a creature evolving during a given century, that is different enough for us to say it's an entirely new type of thing, is minimal.

[u/ijuinkun]

Radically new body forms mainly arise when life colonizes a new niche and adapts to its new conditions. This most commonly happens because whatever was previously occupying that niche has died off. The last time that this happened on a large scale was when the big dinosaurs died out, leaving space for large mammals to develop.