Natural Selection

[Thistles]

ONE AND ALL -

See? Another convert. I'm thinking of starting my own original religion. Things with 8 legs. I'll be the Pope.

:roflmao:

It won't be a proper religion unless you incorporate some way to profit from it. I say you require an income-based tithe and an annual donation of 5 tarantulas.

 

[Stan Schultz]

It won't be a proper religion unless you incorporate some way to profit from it. I say you require an income-based tithe and an annual donation of 5 tarantulas.

EVERYBODY TAKE NOTE -----

I DIDN'T SAY THAT!


:biggrin:

 

[jecraque]

Late to the party but wanted to add a thing or two...

On the Origin--if you read the free version, the first edition is best to start. A number of changes were made in later editions that can serve to make it even more confusing. To those with unpleasant memories of muddling through it in school or on their own, a really thorough annotated version* (The Annotated Origin) came out a few years ago and it is well worth picking up if you've got the spare change.

If you're into learning about evolution and not sure where to start, an excellent resource is Berkeley's Evolution 101 site. You sound like you're on the right track so far, and the previous comments are helping!


Lastly, and more along the lines of the original questions, spider silk is probably something that happened waaaay back in the lineage (think 400mya or earlier), before there was anything recognizable as an araneomorph spider. I'm no expert, and I'll try to keep the technical stuff to a minimum, but my understanding of the current literature is that silk was initially associated with appendages (like Stan pointed out), and that spinnerets are evolved from appendages (think forked legs like in horseshoe crabs). Not just that, but the segmented plates on the undersides of early arachnids are not evolutionarily the same as the abdominal plates you see on roaches and other insects, but fused appendages as well. Weird, right?

So here's where it gets interesting--in the earliest spiders, silk isn't associated with spinnerets (that we know of) but comes straight out of spigots on those belly plates. Google Attercopus or check out the links posted by RakuenVI for more. If you know anything about hox genes, it's easy to make the leap in thinking about the position of silk production moving from one part of the body to another, or in this case, from covering a large area to a smaller, more focused/flexible location. Flexibility and the increased accuracy that comes with it would definitely have been a major selection pressure in more modern spiders with spinnerets on the end of the abdomen.

Unfortunately, Attercopus already had silk--in one fossil fragment there's even a strand coming out of a spigot--so the origin of silk, like others have said, is still under debate. I've heard of a couple of ideas in the literature, but basically, all agree it would have happened on dry land, so we have a general idea of the timing (which is where the 400mya estimate comes in). One idea is that pre-silk was a covering for the gills (before book lungs) that allowed for movement on land with no drying out of your essential oxygen-getting equipment; another is that pre-silk was a protective covering for eggs. I've also heard the burrow-lining thing, but I'm pretty sure this is thought to be a secondary use.

As for learning and behavior... we may never know, beyond what we can glean from living spider species. Interesting questions though!

*Full disclosure: the annotator is a lovely fellow who I'd consider a mentor (but the book is really good, you guys).

 

[Kaimetsu]

On the origin of species is one of the most important books in the history of science and a must have for any collection, but I don't recommend using it to learn about evolution. For one thing the victorian era wordy writing is difficult to get through, and we know infinitely more about evolution today then what Darwin knew, he didn't even know how heredity worked or anything.
If your really interested in how amazing evolution, natural selection, genetics, and the history of life on earth is their are tons of amazing modern books on the topic. One introduction to evolution that I enjoyed was Evolution: the triumph of an idea by Carl Zimmer. For a much more involved book that details how natural selection truly works by altering how an embryo develops and how similar all living animals are to each other I recommend Endless Forms Most Beautiful by Sean Carroll.
Also if your interested in really heavy informative reads you can't go wrong with Richard Dawkins, you don't have to agree with his views on religion to enjoy his books on evolution, although hes definitely right about religion! My favorite book of his is the Ancestors Tail, which tells the story of the history of life starting with humans and tracking the family tree back in time to the last common ancestor of all life. The Selfish Gene is great for explaining how the unit that natural selection acts on is really individual gene's in DNA rather than just a species, population, or individual, and the Blind Watchmaker does a good job of explaining how natural selection can create complex ordered structures like living things through accumulated small changes over time.

 

[Stan Schultz]

... but the segmented plates on the undersides of early arachnids are not evolutionarily the same as the abdominal plates you see on roaches and other insects, but fused appendages as well. Weird, right? ...

References please?

... all agree it would have happened on dry land, ...

I wonder from where "they" made this leap of faith? Given that so many arthropods (e.g., caddisflies) make silk underwater (even several spiders, see Diving Bell Spiders and possibly some species of the family Pisauridae), I find it equally likely that silk production preceded land/air living. I can spin (Sorry. Bad pun) as likely a story that silk was evolved by aquatic arthropods as holdfasts and safety lines in water currents, and later became adopted to a terrestrial/air existence as the arthropods eventually adapted to that new environment.

Just stirring the pot a little...


"The magnitude of our ignorance [about tarantulas] is staggering."
- S. A. Schultz, TKG3

 

[jecraque]

References please?

Dunlop, J.A. 1998. The origins of tetrapulmonate book lungs and their significance for chelicerate phylogeny. In Proceedings of the 17th European Colloquium of Arachnology, P.A. Selden, ed. (Edinburgh, Scotland), pp. 9–16. PDF here
Selden, P.A., W.A. Shear & M.D. Sutton 2008. Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order. Proceedings of the National Academy of Sciences 105 (52): 20781–20785. PDF here

You know, having studied caddisflies pretty extensively I also wondered the same thing--why not an underwater ancestor for the first silk-producer? I'm afraid I don't know enough about the physiology of silk production to speculate, but it does seem strange that it's often taken for granted that silk was a post-land adaptation. Good point and not one I can readily account for.

 

[Stan Schultz]

Dunlop, J.A. 1998. The origins of tetrapulmonate book lungs and their significance for chelicerate phylogeny. In Proceedings of the 17th European Colloquium of Arachnology, P.A. Selden, ed. (Edinburgh, Scotland), pp. 9–16. PDF here
Selden, P.A., W.A. Shear & M.D. Sutton 2008. Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order. Proceedings of the National Academy of Sciences 105 (52): 20781–20785. PDF here ...

Thanks. Now I have 14 unread pdfs on my desktop!

:laugh:

... You know, having studied caddisflies pretty extensively I also wondered the same thing--why not an underwater ancestor for the first silk-producer? I'm afraid I don't know enough about the physiology of silk production to speculate, but it does seem strange that it's often taken for granted that silk was a post-land adaptation. Good point and not one I can readily account for.

That's so obvious an hypothesis that I'd be shocked if no one had ever thought of it before. The major problem seems to be that no one has ever been able to demonstrate silk or silk spigots in fossils of aquatic arthropods. This is not too surprising given the fragility of silk, and its difficulty of recognition. Similarly, I wouldn't be too terribly surprised to hear of someone who went looking for evidence of it who would actually find it, perhaps as one of those enigmatic, unidentifiable fossils that litter every museum's shelves (or maybe thrown in the trash heap!), or misidentified as worm castings, leaf vein networks, crystalline formations, or something equally obscure.

Drat! Another line of research and another potential PhD thesis! Will it never end?

:roflmao:


“The difference between utility and utility plus beauty is the difference between telephone wires and the spider web.”
-- Edwin Way Teale

 

[Gel]

Once again, thanks everybody for the great posts!

Keep the conversation going!

 

[TomM]

You may want to read this book to get a better grasp of evolution:

http://www.amazon.com/The-Magic-Reality-Whats-Really/dp/1451675046

It is geared towards a younger audience, but is surprisingly helpful and does a great job of explaining things.

 

[Gel]

You may want to read this book to get a better grasp of evolution:

http://www.amazon.com/The-Magic-Reality-Whats-Really/dp/1451675046

It is geared towards a younger audience, but is surprisingly helpful and does a great job of explaining things.

Thank you. I will check it out

 

[The Snark]

Thanks. Now I have 14 unread pdfs on my desktop!

:laugh:



That's so obvious an hypothesis that I'd be shocked if no one had ever thought of it before. The major problem seems to be that no one has ever been able to demonstrate silk or silk spigots in fossils of aquatic arthropods. This is not too surprising given the fragility of silk, and its difficulty of recognition. Similarly, I wouldn't be too terribly surprised to hear of someone who went looking for evidence of it who would actually find it, perhaps as one of those enigmatic, unidentifiable fossils that litter every museum's shelves (or maybe thrown in the trash heap!), or misidentified as worm castings, leaf vein networks, crystalline formations, or something equally obscure.

Drat! Another line of research and another potential PhD thesis! Will it never end?

:roflmao:


“The difference between utility and utility plus beauty is the difference between telephone wires and the spider web.”
-- Edwin Way Teale

Upon reflecting on this... Okay, first and foremost, the evidence of actual spider silk dates back to the early Cretaceous period with an actual fossil in amber identified. It has been theorized that spiders moved from the water to dry land during the early Devonian period. This was prior to the carboniferous period (no amber available to preserve webs) and there was no habitat suitable for a land based arachnid prior to that time. Therefore it can be assumed that aquatic arthropoda were the first to utilize webbing. This is as logical, or more so, than an eight legged critter crawling out of a swamp and going, 'Oh look! A tree! I think I'll squirt something out of my butt over it. So Pikaia's leap of faith is actually a very safe surmise.

Re: Another line of research... Reminds me of a new advisor dragging me to his office to point out I appeared to be working on 5 different majors. I retorted that Albert got his start fiddling with refrigerators.

 

[Micrathena]

Just to clarify, these zigzags that argiopes spin are called stabilimentae (stabilimentum singular) and they are to allow birds to see, and so avoid, the web. For more info, read Thomas Eisner's "For Love of Insects." I highly recommend that book.

 

[jecraque]

Just to clarify, these zigzags that argiopes spin are called stabilimentae (stabilimentum singular) and they are to allow birds to see, and so avoid, the web. For more info, read Thomas Eisner's "For Love of Insects." I highly recommend that book.

AFAIK the jury's still out on the bird thing. It doesn't make much sense to me and there are some competing hypotheses. My research-dad is pretty vehemently in Camp UV Patterning and anti-anti-bird-avoidance. I don't know enough about orb-webs to speculate much.

Funny you should mention the Eisner book. My partner is way into it right now (as in this week). I'll have to give it a read!

 

[Stan Schultz]

AFAIK the jury's still out on the bird thing. It doesn't make much sense to me and there are some competing hypotheses. My research-dad is pretty vehemently in Camp UV Patterning and anti-anti-bird-avoidance. I don't know enough about orb-webs to speculate much. ...

What's wrong with the hypothesis that they just think it's cool to have that thing in the middle of the web? After all, we do silly things like that, why not them?

:biggrin: