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 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)