Godless Dave

I already answered that question, Eric.

Happy Wonderer

Chordates

These are good questions. I think that a discussion of the evolutionary development of the notochord, which is the first backbone, would be in order. As I understand it, this would be very similar to what you are asking. How could something swimming around quite happily without a bone structure gain one, and would it be an advantage or a disadvantage? (I'd like to see the discussion go that way because I'm interested in fishy things...)

There is a creature in the Pacific Northwest that if you could see it almost screams "evolution!" I believe that it is a class of Sea Slug, and they are very weird looking. It has a clear body, from four to six pseudo-fins, a digestive tract, a primitive backbone, and not a whole heck of a lot else. It 'swims' (to be charitable) by twisting its body and sort of vaguely flapping the blobs that look like proto-fins. It lives quite successfully in the shallows here.

It is quite easy to see how such a creature would benefit from more efficient 'fins', a better 'mouth', etc. It is really suprising that such a lo-tech creature can survive the modern world. (It is nearly transparent, which probably helps.) I had a point here somewhere, what was it? Oh yeah, it indicates how well creatures can survive without all of the fancy 'engineering' that we see in other creatures. You don't need a whole bunch of structure to make your living in the ocean.

Note that this is not an ancestor creature, it is a modern that has probably evolved in very important ways from its distant ancestors. It just hasn't bothered with elaborate bony structures, teeth, ears, fur, and all that other expensive nonsense.

hw

The very very basic point here is that bony evolution happened in the ocean, where physical laws are quite different than on land. A box jellyfish would have no chance as a land creature. A little bit of extra bone here or there might not be any disadvantage at all.

pz

If it is a detriment, natural selection would work against it. However...

1) Even if it is disadvantageous, a structure can become fixed in a population. Look into drift sometime.

2) We already know that 'bonelike substances' can be advantageous to small organisms. Echinoderm plutei, for instance, develop an internal skeleton of small spiny spicules that help the organism maintain its shape. The animals survive quite well.

3) Vertebrates did not develop bony skeletons until they achieved a substantial size. They initially had a springy skeleton of cartilage, which has about the same density as the surrounding tissues. You are working from faulty premises.

4) Your assumptions about why a structure like bone should have evolved may be completely wrong. For instance, one hypothesis about the origin of bone is that it represents a reservoir of calcium, necessary for metabolism and reproduction.

Eric H

Hello pz, thank you for your patience and tolerance in the way that you reply.

I actually like the theory of evolution as an explanation of how life adapts and changes gradually. My questions are in a way to search for the extremes of the theory.

Are there considered to be a maximum amount of changes that one generation could pass on to the next, or does the theory go that if conditions are right most things are possible.

Or does evolution tend to favour small changes between each generation?

Getting back to bones as you describe



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Quote pz one hypothesis about the origin of bone is that it represents a reservoir of calcium, necessary for metabolism and reproduction.
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If cartilage is a benefit to life for a biological purpose then possibly the cartilage could evolve to a best shape and size to aid that purpose.

If the skeleton is advantageous to maintain shape then it may develop a different design.

As you say design of bone substances can drift and maybe then start to serve more than one purpose. But would the overriding priority always be to serve the biological needs first, because the biological needs keeps life going from one second to the next..

You have given a good explanation as to why cartilage and bone may be a biological advantage to have in an organism. But is there any way to also introduce muscles, ligaments and tendons into a body for a biological gain.


Peace

Eric

Eric H

quote Godless Dave. Let's start with a population of 200 animals. Only one of them has to have a trait that the other ones don't have. It mates with (let's say) one of the other ones, and they produce 10 offspring. Maybe 5 of those offspring will inherit that trait. Now, if this trait confers a survival advantage, but does not inhibit the animals' ability to mate, some of those 5 will survive long enough to mate and pass that trait onto their offspring. If the animals that have the new trait are reproducing more than the animals that don't, ultimately they will outnumber the others, and eventually the whole population will have the trait.

It only has to happen one trait at a time, and it only has to start with one organism.
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Take a population of fish that has fins, which can also crawl over land between ponds.

Does one fin on one of the fish adapt leg like qualities to aid its travel on land? Or could a mutation occur that a fish appears for the first time with all four fins that are more like legs?

The only problem with this that I see, is in its natural habitat the water, were it presumably feeds, any number of legs would be a step backwards, and it would not swim so well,

Presumably if this first fish appeared with legs to survive in a better way on land it would also have to change its gills to lungs. And adapt other parts of its body to a new food chain. To the rest of the fish species it is going to look like a real out cast, how is it going to attract a mate in these conditions?

There is one way, if the changes occurred first in a female, because we all know that males aren’t so fussy and they will mate with anything given the chance.

But is it the male or the female that passes on most of its characteristics to the next generation?

Peace

Eric

lpetrich

Eric H:
Does one fin on one of the fish adapt leg like qualities to aid its travel on land? Or could a mutation occur that a fish appears for the first time with all four fins that are more like legs?

Your first possibility is a beanbag view of developmental genetics; your second possibility is much more reasonable.

The only problem with this that I see, is in its natural habitat the water, were it presumably feeds, any number of legs would be a step backwards, and it would not swim so well,

Except if this fish is living in a swamp, where there would be lots of fallen logs and branches to climb over.

Presumably if this first fish appeared with legs to survive in a better way on land it would also have to change its gills to lungs. And adapt other parts of its body to a new food chain.

Actually, there would be selection pressure for acquiring lungs while still in the water. Swamp water tends to get low on oxygen as a result of all the decaying that occurs in swamps, and being able to breathe air can be very helpful.

To the rest of the fish species it is going to look like a real out cast, how is it going to attract a mate in these conditions?

I think that it's very important not to project human stereotypes onto other creatures.

But is it the male or the female that passes on most of its characteristics to the next generation?

It is very close to 50:50 in nearly all cases.

Eric H

Hello lpetrich

Quote Eric H: Does one fin on one of the fish adapt leg like qualities to aid its travel on land? Or could a mutation occur that a fish appears for the first time with all four fins that are more like legs?
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quote Ipetrich, Your first possibility is a beanbag view of developmental genetics; your second possibility is much more reasonable.
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I agree with you, if evolution happened, I believe it had to happen with vast changes at once.


For the first three or four billion years, give or take a week or two, life on Earth progressed at a very slow pace. It seems after all this amount of time it had only managed to make simple algae and bacteria.

Then during the short period of the next three hundred million years in the Palaeozoic era, evolution takes of exponentially to create complex life. Only then to have most of it wiped out.

Then exponentially it goes of again over a shorter period of 180 million years, to end up with even more complex life, most of which becomes extinct.

These extinct dinosaurs had adapted very well and were successful, so when they became extinct it seems strange that evolution would start of with virtually a whole new set of species.

These also evolved exponentially over an even shorter time of 65 million years.

It is at this point that I have a problem; the traditional view of evolution that has been quoted on this forum is that life evolved tiny little bits at a time. And over a very long time scale.


peace

Eric

Wounded King

Some good points Eric, but also a couple of misconceptions.

This might be the case were evolution to be 'directed' as in some intelligent design scenarios. However evolution as it is generally concieved today does not refer back to past successes or failures. The ancestor from which the dinosaurs evolved was itself long gone, although species resembling it may still have been extant subsequent to extinction. The small mammals were just in the right niche at the right time and jumped in to fill the gap. Had there not been any small mammals around who knows what might have evolved. The idea of being able to rewind natural history and see if things work out the same again is a common, though impossible, one. Perhaps these sort of mass extinctions are the closest we will ever get.

'Only' is a bit harsh, unicellular organism always have been, and probably always will be, the most successful form of life. It is an anthropocentric conceit to think that the larger and more complex an organism is the better evolved it is. This time also has to allow for abiogenesis to occur, a pretty staggeringly succesful bit of work from nature, although arguably not evolution.

Personally I think that 300 Myears is a pretty large length of time, I realise that it is fairly moderate in terms of geological spans, but in terms of generation times of organisms it is pretty big.

As far as gradualism goes, and it sounds as if you are pretty much smack in the SJ Gould Punctuated equilibrium camp, this is a term open to some interpretation. It is certainly true that there are a number of major changes in body form and organisation which can be caused by only 1 or 2 mutations. The HOX genes previously mentioned are an especially good example of this. This doesnt mean there isn't a whole lot of evolution going on which doesn't have such a drastic effect. Genetics in general, and developmental genetics in particular, has shown us the wide spectrum of change possible, from minor changes in relative organ size or limb development to radical restructuring of the body plan. These mutations can be as simple as a point mutation in a vital regulatory region or as large scale as the duplication of a whole cluster of genes, the HOX cluster being a primary example of this.

This is still life evolving 'tiny bits at a time' but once something has evolved it provides a plastic resource which can be built on, as in the metameric organisation of arthropods.

The topic of central pattern generation and the feedback from large numbers of linbs is fascinating. I always loved the, rather cruel, experiment where by removing limbs from a lobster, or somesuch, you could get it to reproduce the heterochronic feeding pattern of the cirrepedia of brine shrimp.

Peez

If I may stick my nose in here... That depends on how one defines "vast changes" and "all at once."That depends on how one assesses the rate of evolution. A great deal of evolution likely occurred during this time.Probably what happened is that multicellularity evolved, and this opened up many new niches. As a result there was "rapid" evolution of many new forms.Why do you think that life was more complex after this extinction event?How could it be otherwise? If these dinosaurs are all dead, then they are not going to leave any more descendants. Evolution will simply continue with populations that are still alive. These are not the same populations that started out after the previous period of evolution, and the conditions in which they evolved was different, so we should not be surprised that the results were different.I do not understand what you mean by "evolved exponentially," but you are implying that the evolution of these organisms proceeded at a constant rate over the time periods between extinction events. Why not have organisms evolving over, say, 20 million years, then remain more or less unchanged for the remainder of the period?That depends on what you mean by "tiny bits" and "a very long time scale." As an example, during horse evolution there was a substantial increase in size. The width of one particular tooth appears to have increased from about 8 mm to about 34 mm over about 15 million years (it became over four times bigger, along with similar increases in the rest of the animal) . If we assume generations of about three years (and, for simplicity, a linear increase), this works out to about 0.000005 mm per generation (this is about the size of some small molecules). Thus, we see substantial change (an increase in size of more than four times) through the accumulation of tiny bits (so small that they could not possibly be detected) over a very long time scale (about 3,000 times longer than it has been since the earliest pyramids were built).

Peez

Eric H

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Quote Eric, For the first three or four billion years, give or take a week or two, life on Earth progressed at a very slow pace. It seems after all this amount of time it had only managed to make simple algae and bacteria.
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quote Wounded King, 'Only' is a bit harsh, unicellular organism always have been, and probably always will be, the most successful form of life.
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Sorry I was being a bit flippant with that remark, I thought by adding a couple of weeks onto billions of years might have shown this.

Anyway,




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Quote Wounded King, the wide spectrum of change possible, from minor changes in relative organ size or limb development to radical restructuring of the body plan.
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You talk of the possibilities of a radical restructuring of body parts, and that is the point that interests me most.

It has been suggested how adding cartilage to an organism could be considered a biological gain.

One of my last points was to ask, can there be a biological gain by introducing muscle, tendons, and ligaments around the same time to an organism.

These additions would have to serve the over riding laws of natural selection and also not be a burden in anyway to the host.

Would it make sense that if the cartilage or bone, plus muscles, tendons, ligaments were introduced primarily as a biological aid then their design would evolve to suit this purpose?

If the original design of these components were to drift of course, would they still have to satisfy biological needs first?


For movement to take place in a body then Evolution would have to comply with some very simple and basic laws of movement.

Take the leg bones, there are one set of muscles that straighten these bones up, which gives it one step forward. That’s the end of the story, unless there are another set of muscles working in opposition to bend the bones back again. Also all the tendons and ligaments must work as well.

I can see no symmetry of design in the leg bones, muscle groups, tendons and ligaments.

Then there are another set of basic design laws that evolution would have to comply with. If the components aren’t tough enough for the job, they brake.

The fish does not require strong bones or muscles in the water, and so the original ones need only be tough enough for this purpose.

Take away the buoyancy of water then the moving parts have to increase in strength.

I can understand how strength can improve gradually with a change in use. It is in the same way as an athlete trains to improve strength.


Then there are another set of laws for evolution to comply with. If nothing tells all the muscles in a group to contract, and at the same time tells the opposing muscle group to relax, then there is no movement.
This set of instructions needs to be given to all four legs, otherwise they become useless and the creature would have to drag any leg that don’t work.
Then there is natural selection again.

Evolution would have to satisfy all these opposing needs.

Unfortunately my time is limited over the next three days, and then I am away from a computer for a week.


Peace

Eric