Bat Origins
Jan. 15th, 2004 09:12 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
herewiss13Man, the mind is racing today.
Was at the bookstore, leafed through a Big Book of Life, looking at all the funky animals, and one small illustration caught my attention.
It was of the earliest known bat.
You wouldn't have been able to tell, though. To the untrained eye, at least, it looked fully modern. I can see where some people find bats tricky to explain through evolutionary theory. Where are the transitional forms? The primitive pre-adaptations?
In actuality, I think, the bat really serves as a good example of evolution. To wit: punctuated equilibrium.
Punc Eek is often misunderstood, but basically can be summed up in these two prinicples:
1) Most of the time, life is static. You're at an optimal bodyplan in an unchanging environment so there is no gradual transformation. Natural selection works to keep you as you are. And when you have the change, you change _fast_. Relatively speaking.
2) Evolution happens fatest in small populations (traits spread through the entire gene pool faster). Often, new species will evolve in small, isolated, fossil poor regions and then spread out from there, replacing the ancestral stock. In the fossil record it looks like a new species popped up out of nowhere, when in reality it just evolved somewhere else on the QT.
This is almost certainly what happened with bats.
Bat anatomy is highly specialized, it resides on a very steep peak on the Fitness landscape. When you start out with fingers and end up with wings, you've got a lot of ground to cover. Intermediary steps _are_ better than the ones before them, but the next step will be exponentially better, so there is a lot of pressure to change and not a lot of pressure to retain the status quo.
While the evolutionary landscape concept speaks of "peaks" of fitness, it really ought to be "pits". As early bats became better bats, the process of becoming the best bats was self-accelerating to the point where fine honing starts to bring fewer returns.
I'm not sure if I'm explaining this well. For one thing, I'm using a lot of shorthand which seems to attribute foreknowledge, intent and/or predestination to completely natural and random processes. Insert the standard Biologist's Caveat about doing so here.
Let's look at this idea of acceleration another way. Compared to the bat, the evolution of the horse is pretty gradual (it's also complicated and bushy, but forget that for the moment). You start with a small, multi-toed horse and, after a number of gradual intermediaries, end up with a large, single-toed one.
Each of those intermediaries represents a species with a long, static history, a species that remained constant until forced to change because of environemental shifts (they also transitioned from woodland shruby browsers (like small antelope today) to grassland grazers. Looking at the big picture, in its time and place, Mesohippus is a perfectly adapted animal just as Eohippus was before it.
Now look at bats. Small, skin-winged insectivores. The wings (and all the associated musculature and flight instincts) couldn't have emerged full-fledged. At some point there was something we'd call a "half-winged bat". At the time, the half-wing still worked quite well for its purpose, which was probably something other than flying. Perhaps it helped them make short glides, or scoop up insects. In any case, while it wasn't a case of this half-winged bat "waiting around" for the rest of its wing to show up.
But
The food and (to an extent) habitat of small grounded insectivores and small winged insectivores is the same. It's a case of placing Eohippus in the middle of a prarie. You need grazing teeth and a good running hoof now. You can't dilly-dally around in the scrub-brush with 3 toes for a while first. A half-winged bat is an effective creature, but any individual with six tenths of a wing is doing a great deal better. There are no good intermediary niches between ground and air insectivores, even though there are intermediary forms.
The selection pressure is constant and never ending, compared with the fits-and-starts of horse evolution. So, geologically speaking, the bat perfects itself really quickly, because a sub-optimal bat is in a much worse position than a sub-optimal horse (think about it: half a wing vs. two toes).
Ergo: In the patchy fossil record, the bat appears to have sprung into existence fully fledged, perfectly suited to its extraordinarily specialized niche because it "needed" to evolve at a much faster speed than most species, relatively speaking.
Interestingly enough, there is another group of species that fits this same pattern of emerging fully-fledged in the fossil record.
More interestingly, it's the pterosaurs, which are in _much_ the same boat, in terms of anatomy and niche, as bats. So it's highly likely that exactly the same scenario can be applied to them too.
Bird evolution is somewhat more gradual (or appears so) because feathers are more versatile than membranes spread between digits. You can evolve the feathers first, for warmth or display, and keep your forelimbs doing other things even as they begin to develop.
The interesting question, which will probably never be answered, is which came first for bats: flight or echolocation. There are, I believe, some shrews with a primitive version of click echoing, so I'm inclined to say that the echolocation began perfecting itself first as night-going insectivores tried to see in the dark.
Perhaps (and it's a really big 'perhaps') a group of shrew-like animals ran around in the cenozoic darkness, seeing with sonar and leaping at low-flying insects which they can sense better than any other ground-based nocturnal predator (who usually relies on scent trails or whiskers). Larger forelimbs would help to bat the prey down, webbing between the fingers makes for an even more effective scoop, and the process escalates as the proto-bats leap higher and higher, with larger and larger scoops until the scoops start working as primitive wings, soon to be honed by fierce selection pressure (because each small increment of change is a vast improvement in flight capability, unlike horses which can run nearly as fast with two toes as one).
Phew! If you actually managed to wade through that mess of comma splices, I salute you. And if you did click the link and read this whole idea, could I please ask you to tell me if it was intelligible? I'd say "make sense", but it may be wrong. I'd just like to know if I can be understood by the english-speaking lay-person, or if large pieces of the argument are still contained either in my head or in cryptic code within the post.
Thank you.
Was at the bookstore, leafed through a Big Book of Life, looking at all the funky animals, and one small illustration caught my attention.
It was of the earliest known bat.
You wouldn't have been able to tell, though. To the untrained eye, at least, it looked fully modern. I can see where some people find bats tricky to explain through evolutionary theory. Where are the transitional forms? The primitive pre-adaptations?
In actuality, I think, the bat really serves as a good example of evolution. To wit: punctuated equilibrium.
Punc Eek is often misunderstood, but basically can be summed up in these two prinicples:
1) Most of the time, life is static. You're at an optimal bodyplan in an unchanging environment so there is no gradual transformation. Natural selection works to keep you as you are. And when you have the change, you change _fast_. Relatively speaking.
2) Evolution happens fatest in small populations (traits spread through the entire gene pool faster). Often, new species will evolve in small, isolated, fossil poor regions and then spread out from there, replacing the ancestral stock. In the fossil record it looks like a new species popped up out of nowhere, when in reality it just evolved somewhere else on the QT.
This is almost certainly what happened with bats.
Bat anatomy is highly specialized, it resides on a very steep peak on the Fitness landscape. When you start out with fingers and end up with wings, you've got a lot of ground to cover. Intermediary steps _are_ better than the ones before them, but the next step will be exponentially better, so there is a lot of pressure to change and not a lot of pressure to retain the status quo.
While the evolutionary landscape concept speaks of "peaks" of fitness, it really ought to be "pits". As early bats became better bats, the process of becoming the best bats was self-accelerating to the point where fine honing starts to bring fewer returns.
I'm not sure if I'm explaining this well. For one thing, I'm using a lot of shorthand which seems to attribute foreknowledge, intent and/or predestination to completely natural and random processes. Insert the standard Biologist's Caveat about doing so here.
Let's look at this idea of acceleration another way. Compared to the bat, the evolution of the horse is pretty gradual (it's also complicated and bushy, but forget that for the moment). You start with a small, multi-toed horse and, after a number of gradual intermediaries, end up with a large, single-toed one.
Each of those intermediaries represents a species with a long, static history, a species that remained constant until forced to change because of environemental shifts (they also transitioned from woodland shruby browsers (like small antelope today) to grassland grazers. Looking at the big picture, in its time and place, Mesohippus is a perfectly adapted animal just as Eohippus was before it.
Now look at bats. Small, skin-winged insectivores. The wings (and all the associated musculature and flight instincts) couldn't have emerged full-fledged. At some point there was something we'd call a "half-winged bat". At the time, the half-wing still worked quite well for its purpose, which was probably something other than flying. Perhaps it helped them make short glides, or scoop up insects. In any case, while it wasn't a case of this half-winged bat "waiting around" for the rest of its wing to show up.
But
The food and (to an extent) habitat of small grounded insectivores and small winged insectivores is the same. It's a case of placing Eohippus in the middle of a prarie. You need grazing teeth and a good running hoof now. You can't dilly-dally around in the scrub-brush with 3 toes for a while first. A half-winged bat is an effective creature, but any individual with six tenths of a wing is doing a great deal better. There are no good intermediary niches between ground and air insectivores, even though there are intermediary forms.
The selection pressure is constant and never ending, compared with the fits-and-starts of horse evolution. So, geologically speaking, the bat perfects itself really quickly, because a sub-optimal bat is in a much worse position than a sub-optimal horse (think about it: half a wing vs. two toes).
Ergo: In the patchy fossil record, the bat appears to have sprung into existence fully fledged, perfectly suited to its extraordinarily specialized niche because it "needed" to evolve at a much faster speed than most species, relatively speaking.
Interestingly enough, there is another group of species that fits this same pattern of emerging fully-fledged in the fossil record.
More interestingly, it's the pterosaurs, which are in _much_ the same boat, in terms of anatomy and niche, as bats. So it's highly likely that exactly the same scenario can be applied to them too.
Bird evolution is somewhat more gradual (or appears so) because feathers are more versatile than membranes spread between digits. You can evolve the feathers first, for warmth or display, and keep your forelimbs doing other things even as they begin to develop.
The interesting question, which will probably never be answered, is which came first for bats: flight or echolocation. There are, I believe, some shrews with a primitive version of click echoing, so I'm inclined to say that the echolocation began perfecting itself first as night-going insectivores tried to see in the dark.
Perhaps (and it's a really big 'perhaps') a group of shrew-like animals ran around in the cenozoic darkness, seeing with sonar and leaping at low-flying insects which they can sense better than any other ground-based nocturnal predator (who usually relies on scent trails or whiskers). Larger forelimbs would help to bat the prey down, webbing between the fingers makes for an even more effective scoop, and the process escalates as the proto-bats leap higher and higher, with larger and larger scoops until the scoops start working as primitive wings, soon to be honed by fierce selection pressure (because each small increment of change is a vast improvement in flight capability, unlike horses which can run nearly as fast with two toes as one).
Phew! If you actually managed to wade through that mess of comma splices, I salute you. And if you did click the link and read this whole idea, could I please ask you to tell me if it was intelligible? I'd say "make sense", but it may be wrong. I'd just like to know if I can be understood by the english-speaking lay-person, or if large pieces of the argument are still contained either in my head or in cryptic code within the post.
Thank you.
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no subject
Date: 2004-01-17 06:27 pm (UTC)Interesting article, thanks! And I chucked at this expression:
the bat appears to have sprung into existence fully fledged as it literally means "fully feathered". ];-)
Bats are an area of interest of mine. A couple of points, which might refine your hypothesis:
-- Rather than going from fingers to wings, exactly, bat wings went from foreleg-hindleg membranes to focus more on the forelegs. They were, it seems, leaping gliders.
-- What we call "bats", one group, are really two not-closely-related groups, whose origins are separated by millions of years. The larger bats, Megachiroptera, have no echolocation, completely different brain and eye wiring, different skeletons, and are primitive primates. In other words, bats evolved twice from early-mammal scratch.
This last point brings them even more in line with your pterasaur example, as it is yet another group that seems to have gotten through intermediate stages quickly.
Curiously, the often-energetic lifestyle of both bat groups has caused them to evolve similar DNA base pair mixes, heavy in TA and much reduced in CG. This is also true of hummingbirds to an extent, and it lead the early DNA researchers to conclude (mistakenly) that big bats and little bats were more closely related than previously thought.
We're now getting some idea as to why high energy use would favor one type of DNA link over another.
I have some bat enthusiasts on my list -- I'll post part of this there and lead them here.
===|==============/ Level Head
no subject
Date: 2004-01-17 07:07 pm (UTC)Thanks.
Um...if I'm known to you (presumably from the evolutiongroup list) are you known to me? This isn't Lenny, is it? ;-)
And I chucked at this expression:
the bat appears to have sprung into existence fully fledged as it literally means "fully feathered".
Yeah, I know. Pun was probably unconciously intended. ;-)
What's the evidence we have for bats originally being forelimb-hindlimb gliders? In retrospect, that sounds much more plausible than my "webby fingered shrews", but is it some morphological observation or do we have fossils?
I'd heard something about the whole "ape bat" idea before, but dismissed it as controversy and plain old "weird science". Huh. That amount of covergent evolution simply boggles the mind. If primates and insectivores can latch onto practically the exact same body plan (on an external level, at least), we shouldn't be at all surprised if non-terrestrial life looks extremely familiar; Gould's objections about historic contingency be damned. ;-)
This last point brings them even more in line with your pterasaur example, as it is yet another group that seems to have gotten through intermediate stages quickly.
Yeah, I suppose it does (still trying to acclamate myself to flying simians). The pterasaur tie-in comes from a half-remembered mention in "Dinosaur Heresies" by Bakker. While the example of "early bat" species I ran across looked like it was probabaly an ordinary chiropteran, I'm guessing then that the most primitive Megachiroptera fossils are equally advanced, then?
We're now getting some idea as to why high energy use would favor one type of DNA link over another.
That does seem like a puzzler. Is it that amino acids and proteins associated with high energy use are high in TA codons or are the TA pairs more thermally stable or what? How far does it correlate? Would, say, sloths be CG heavy then?
I'd love to see a graph of metabolic rate vs. TA/CG ratio for a bunch of organisms, from turtle to shrew.
Anyhow, glad you found it interesting and quite glad to have friended you. My friends list is high on fandom and low on science at the moment. Time for that to start changing. ;-)
no subject
Date: 2004-01-17 07:34 pm (UTC)But we're on the same side when it comes to the evolution debate.
Here's a clade tree showing the two groups of bats. Unfortunately, I'll be tied up for a few hours.
http://tolweb.org/tree?group=Chiroptera&contgroup=Eutheria
===|==============/ Level Head
umm...
Date: 2004-01-17 07:47 pm (UTC)The number of studies supporting monophyly vs. those supporting diphyly seems overwhelming, and appears to contain all the DNA analyses as well.
So, and meaning no offense, is the webpage out of date or are you?
Re: umm...
Date: 2004-01-17 08:14 pm (UTC)Look for articles on:
megachiroptera, "optic nerve", primate
as a good starting point.
Also -- the early studies on DNA produced an artifactual similarity because of the tendency for the AT pairs to be used more frequently in high-energy critters in general, from what I have seen. The dual-origin hypothesis is recent, so of course there have been more studies supporting the monophylitic concept.
But even going back and reading *those* with new eyes shows things emphasized and other things glossed over.
Even the details of wing/forelimb bone structure are consistent within the two groups -- but not across. It is only when you stand far enough back that they look the same.
I read a couple of dozen of these studies (on both sides of the fence), about a year ago. I'd say that dual origin will have a history like the introduction of plate tectonics. ];-)
But, as with any science on the edge, much can be learned, and perceptions can change. I'd give "two evolutions of flight in mammals" about an 85% chance at this point.
===|==============/ Level Head
Re: umm...
Date: 2004-01-17 08:16 pm (UTC)