The way evolution works

I wanted to extemporize a bit more on a tangent from the interesting thread, and thought maybe I should make it a new topic. At the other bbs I was poor at guessing what might merit it. But this place is new, it’s all on us to make them, so I figured why not.

This made me wonder, how often does evolution work this way? The basic possibility is definitely there, with groups rising and falling in prominence all the time. The lycophytes once formed vast forests towering over other plants, and now the survivors are only little plants in the shadow of their descendants.

On the other hand, in the example I gave it sounded like a reasonably one-way slide. The shallower seas are home to diverse animals undergoing constant evolution and turn over, with the deep ocean holding refugees from the process. That might just be how things have worked out, but it might be some property of the habitat that encourages migrations in but not out, so that lineages in some sense get stuck.

Like I said, I haven’t found much about that, and as Wanderfound’s original link pointed out it’s not a good place for fossils. But I did see a paper (pdf) about fish leaving the ocean. Except for coelacanths, most of the “ancient” fish lineages – like lungfish, sturgeons, and gars – have ended up solely in freshwater, so apparently it too can be a sort of refuge. They found fish go both ways, but this one more than the other. It’s probably a lot easier to find a stream without much competition than a sea.

So then I was trying to think of where evolution actually did swing back and forth – where the same group fell and then recovered. I don’t think there are many, though that may be because we can’t see long enough. The best I can think of is our own lineage, the synapsids.

These include what used to be called mammal-like reptiles, which were dominant during the Permian. The early mammals were much more diverse than once thought: there were at some point hunters like Repenomamus, burrowers like Docofossor, swimmers like Castorocauda, even gliders like Volaticotherium. But there is no question they were overshadowed by true reptiles like dinosaurs and pterosaurs until the K-T extinction.

After…well, their lineage hasn’t done that badly. There are some 10000 species of birds and 10000 lizards and snakes, compared to 5400 mammals, a bit less than frogs. The wild animals I see most often (except bugs) are dinosaurs. But if you talk about rulers, the larger animals are all mammals, including the biggest ever known.

This switch might just be an extremely lucky break; nothing ruins the usual odds like a giant meteor, which is why it is a popular candidate these days. But in the aftermath of K-T there were also new giant birds like Gastornis, giant snakes like Titanoboa, giant turtles and crocodiles, all also since replaced. So maybe the giant niche is more prone to swinging back and forth, or maybe mammals weren’t really that sidelined, or maybe sidelining isn’t really such a thing in at least in the very long run.

It’s tough to generalize from so few examples, but it’s interesting to consider. For me, anyway. I hope there’s something in all that for you too, and if not, thanks for letting me ramble. :slight_smile:


The hardest thing about answering this question is the sampling. People build lineage through time (plots showing the number of expected lineages in a group over some period of time) plots all the time. But being able to build a good lineage through time plot is pretty hard. If we assume that the underlying evolutionary process has resulted in some evolutionary tree, we usually have pretty good data for the tips. Maybe we don’t have DNA sequences or morphology for all of them, but we usually know how many there are, ballpark. Very few groups, though, have any sort of good fossil record. We end up backfilling that knowledge by assuming a model of how speciation, extinction and fossil preservation occur through time.

Models for doing that sort of prediction of numbers of extinct lineages are much better than they used to be, but on some level, we’re still predicting from really scarce data. If you ask a paleontologist, everything on earth is dead, and the extinction rate is so high that if it’s not dead, it will be soon. If you ask an entomologist, we’re discovering new things every day and paleontologists should just study more successful groups.

I don’t think we’re going to see more giant mammals for as long as humans are around, though.


Thank you for writing this up! I was definitely thinking about the synapsids vs sauropsids thing, I actually thought they went back and forth a few times, but my biological knowledge is pretty thin so I probably over-generalized to: this is the kind of thing that often happens


Evolution is not so much about what works as what doesn’t fail.


markets seem to work the same way, unfortunately.


Why not look at it from Gene’s perspective.


Entirely agreed. Reviews of prehistoric life by necessity sweep through everything so quickly, it’s easy to forget that for any given group and time, fossils will be known from relatively few places on earth. I found PDBD navigator a neat tool that really brought this home; that there are ghost lineages is not more surprising then that we can piece some together into a near-continuum.

I’m not sure that’s quite what it sounds like, though. No question everything dies on that scale. Well, if you count clones as one then who knows about things like amoebae; but otherwise the very oldest individuals are trees from a few millennia ago. And plainly that’s not what we mean by something dying out. We generally mean whether they have no surviving descendants, right?

To paleontologists, though, extinction rate measures species that have no survivors of the same form, so that the species itself is gone. It’s a meaningful concept, and one much more useful to them, because the descent of one species to another is mostly impossible to verify. It could probably be done in some very recent cases, but for instance even in our own well-known Homo, you can argue whether we came from H. erectus or some other unrepresented relatives. So it makes sense not to presume.

But I also wouldn’t presume the opposite, that the ancestors of any living species can’t have fossilized and they must all come from ghost lineages instead. And so saying all species go extinct is potentially very different from the above notion of dying out. Just as we don’t expect individuals to live forever, I don’t expect anything to go millions of years without substantial change.

I think there might be a lot hidden in that, too. We know evolution doesn’t optimize, but in theory it could in the long term; we know enough cases where one group replaced another to suppose less efficient forms couldn’t be displaced in the long term.

In ecology they talk about the competitive exclusion principle, where ultimately only one species should hold a niche, and so presumably the reason others endure is by escaping to others. I think this fits with the idea of refuges, where “worse” designs like slit shells have lasted because they’ve found a habitat where they either have some other advantage or it just doesn’t matter.

But even more, the principle turns out to be really limited in practice. The largest habitat on earth is the open ocean, and here there’s what is called the paradox of the plankton, where even though conditions are relatively uniform there is tremendous diversity. It turns out living things can generate a lot of different ways not to fail, as it were, in how they interact with each other – it’s still an active area of research.


The reason others survive is that the species occupying the niche eventually modifies it so that it is no longer the fittest - something I think Darwin didn’t anticipate.
The human race seems to be involved in a big experiment to prove exactly this point. Of course we may out-adapt everything else - if we don’t become extinct first. Hence all the jokes about cockroaches inheriting the Earth.

I however prefer the theory that the most successful life forms are e coli - which have evolved us to act as convenient containers that also supply them with food - and the tobacco plant, which has successfully demonstrated a form of parasitism in which the human race expends a lot of resources to grow tobacco despite it’s growth being nothing but downside. Even the opium poppy gives something back for the effort expended in growing it.


It sounds to me like many of them work in clades where the extinction rate is higher than the speciation rate (at least over the time span they study; many paleontologists specialize in one specific time period), and so many paleos work in lineages where they observe a winnowing-down of lineages present, if not a total ablation of the group.

It depends on who you ask. I think the fashion has much more swung towards measuring extinction as the rate at which lineages on a tree go extinct; not as much whether or not a specific morphospecies is never observed at a later time. A lineage can be a single species, but it could be a larger or smaller unit. This definition moves the concept more towards a phylogenetic definition of lineage, as opposed to a strictly morphological one. I prefer that.

But we’re getting much better at it, even for very ancient clades, as a function of improved models.

There was probably interbreeding between what we call ourselves and Homo erectus, and other unobserved lineages. I think something that contributes to the rise of lineage-based definitions is a greater knowledge of pop gen among paleontologists (i.e., more paleos are coming from biology these days). Even if the particular morphospecies is extinct, that doesn’t mean its descendants and the other morphospecies with which it interbred are extinct. I don’t know nearly as much about data availability for ancient hominids, but my general feeling would be that if you were building a lineage through time plot, or estimating origination and extinction rates for the group, you would want to co-estimate that with the tree, and experiment with collapsing tips into more or fewer lineages (i.e., collapse together Neanderthals and humans, or Denisovans and humans) to test the sensitivity of the model to the sampling.