evolution: hard to fathom instances

[btilly]

Note that the top of the tail is consistently larger than the bottom. That is to provide the extra downwards push which is needed to keep them from sinking.

Erm, citation needed on that point.

I linked to a picture that shows you diagrams of what various shark tails look like. Click on that link, and look at the picture and you have your citation. Finding additional verification is as simple as http://images.google.com/images?client= ... a=N&tab=wi.

It's my understanding that a DOWNWARD push will push you DOWN, making you sink. And it is also my understanding that sharks are neutrally buoyant. I could be wrong.

Repeat after me, For every action there is an equal and opposite reaction. If you push down, you are pushed up.

Aha!

Locomotion and buoyancy

Sharks swim by moving their caudal fin from side to side in a sweeping motion, which propels them forward through the water. The large upper lobe of the caudal fin of most sharks provides most of the forward thrust. Sharks, like makos, which sometimes need to swim at high speed, also have a well-developed lower caudal fin lobe for greater thrust. As a shark moves through the water, it angles the pectoral fins to change direction.

Sharks are slightly heavier than water, so they naturally tend to sink. Buoyancy or lift is provided in two ways. First, sharks store large quantities of oil in their liver. Because oil is less dense than water, storing this oil decreases the overall density of the shark, and increases its buoyancy. Second, as a shark swims, its pectoral fins provide lift, in much the same way the wings of an airplane does. If a shark stops swimming it will sink, but its stored oil and relatively light skeleton help it to float and decreases the amount of energy that must be expended on swimming.

It seems sharks are actually slightly negative, that their fins provide upward thrust, and they increase their buoyancy by storing oil in their livers. Neat. Sharks are cool. I see nothing about being less efficient swimmers for this, just that they can't stop swimming.

The fins and tails provide upwards thrust on the shark by providing downwards thrust on the water. As the diagram by headprogrammingczar indicates, any need to push in a direction other than forwards represents wasted energy, and wasted energy means you need to put out more energy to swim. Sharks don't generally waste that much energy, but it still is an inefficiency that some other marine mammals don't have to deal with.

[Heisenberg]

It seems sharks are actually slightly negative, that their fins provide upward thrust, and they increase their buoyancy by storing oil in their livers. Neat. Sharks are cool. I see nothing about being less efficient swimmers for this, just that they can't stop swimming.

As btilly said, this is wasted energy. An analogue would be a plane compared with a train. The plane, like the shark, must expend a certain amount of energy staying up. A train, or a neutrally buoyant animal, does not have this requirement. The train has the potential to be more efficient because it needs only to expend the necessary energy for movement, and doesn't have to fight gravity.

[Izawwlgood]

Repeat after me, For every action there is an equal and opposite reaction. If you push down, you are pushed up.

I obviously misunderstood what you were saying, as we were discussing forward thrust.
Still, show me where it is mentioned that the upward flare of a sharks fin is used to induce a downward push, keeping the shark aloft. The wikipedia entry on shark tails makes no mention of a sharks tail being used to provide downward push, but does mention that the fins are best suited towards the lifestyle of the shark (cruising efficiency, sprinting efficiency).

An analogue would be a plane compared with a train. The plane, like the shark, must expend a certain amount of energy staying up. A train, or a neutrally buoyant animal, does not have this requirement. The train has the potential to be more efficient because it needs only to expend the necessary energy for movement, and doesn't have to fight gravity.

The requirement to continue forward momentum does not strike me as an impediment towards the efficiency of forward momentum. Explain to me how this is incorrect?

[Mr_Rose]

An analogue would be a plane compared with a train. The plane, like the shark, must expend a certain amount of energy staying up. A train, or a neutrally buoyant animal, does not have this requirement. The train has the potential to be more efficient because it needs only to expend the necessary energy for movement, and doesn't have to fight gravity.

The requirement to continue forward momentum does not strike me as an impediment towards the efficiency of forward momentum. Explain to me how this is incorrect?

Two vehicles (fish) have the and same total power available from identical engines. In fact they are identical in all respects except one:
Vehicle A has a neutral buoyancy.
Vehicle B as negative buoyancy.

Vehicle A can therefore dedicate all available power to moving forward.
Vehicle B must however direct some of the available power downwards to maintain altitude. That quantity "some" being directed downwards is an identical quantity that is not being directed backwards to produce forward thrust.

Therefore if you measure efficiency as percentage available power directed into forward thrust, Vehicle B is going to be less efficient.

[Izawwlgood]

But no evidence of sharks exhibiting downward thrust has been given so far.

I think a better analogy would be two darts, one with flights and one without. If you fling the flighted dart forward, the flights help keep it 'aloft', i.e., moving in a straight line. If you fling the dart without flights forward, it will fall to the ground more rapidly (as well as spiral or such). The other fins on the underside of the shark (pectoral and pelvicl) are partially responsible for providing upward force during forward thrust.

An airplane does not angle it's jets upward to stay aloft. Edit: Coworker points out that drag is induced from the wings to impart an upward force, point made and taken.

The upward force which allows sharks to stay in a plane (and not sink) I would imagine is dictated by their pectoral fins, not their caudal fin.


Also, just to point out, a few percentage points of lost efficiency is a moot point to be making about a shark, as they are some of the oldest organisms still around, pointing to their extreme efficiency at what they do. But yes, buoyancy control would likely remove this impediment.

[crowey]

from the natural history museum

Sharks swim by passing a series of waves down the body. First the head oscillates from side to side, and then the amplitude of the movement becomes progressively greater towards the tail. This pushes a series of inclined surfaces outwards and backwards against the water which, when pushed aside, causes the shark to move forwards. The tail shape varies with lifestyle. Fast-swimming sharks, such as the makos and the great white, have tail fins with an almost equal upper and lower lobe, a feature they share with other fast swimmers, such as swordfish and sailfish. Others, such as the tiger shark, have a tail with a long upper lobe, its function a subject of debate amongst shark scientists. Some think that it creates a thrust that is directed ventrally through the shark's centre of gravity, its body rather than its side fins providing lift. Others believe that it produces a downward thrust that is countered by the shark's pectoral fins.

so in conclusion: maybe?

[btilly]

But no evidence of sharks exhibiting downward thrust has been given so far.

The fact that they have negative buoyancy and do not sink is pretty solid evidence that they exhibit downward thrust.

[Izawwlgood]

@btilly: Or that they generate forward thrust and use their pectoral fins like 'wings' and angle upward? Read the previous post to yours.

[btilly]

@btilly: Or that they generate forward thrust and use their pectoral fins like 'wings' and angle upward? Read the previous post to yours.

I read the previous post, and see that you're still missing the point.

Whether the downwards thrust is generated by the shape of the tail or by the pectoral fins doesn't matter. Whichever way they do it they are actively displacing water downwards and that is going to negatively affect their efficiency. Not by much, granted. But by enough that I'm not surprised that a dolphin is more efficient.

[0.0]

@btilly: Or that they generate forward thrust and use their pectoral fins like 'wings' and angle upward? Read the previous post to yours.

I read the previous post, and see that you're still missing the point.

Whether the downward thrust lift is generated by the shape of the tail or by the pectoral fins doesn't matter. Whichever way they do it they are actively displacing water downwards and that is going to negatively affect their efficiency. Not by much, granted. But by enough that I'm not surprised that a dolphin is more efficient.

FTFY!
If I had more time to kill, I would go back and fix every mention of "downward thrust" in this thread, which is a lot.

[Izawwlgood]

Coworker points out that drag is induced from the wings to impart an upward force, point made and taken.

The upward force which allows sharks to stay in a plane (and not sink) I would imagine is dictated by their pectoral fins, not their caudal fin.

So, you didn't see that?

Moving on, because we're no longer disagreeing on anything:

Stem cells differentiate according to the stiffness of the substrates they find themselves in. External mechanical cues translate to transcription changes...

[btilly]

@btilly: Or that they generate forward thrust and use their pectoral fins like 'wings' and angle upward? Read the previous post to yours.

I read the previous post, and see that you're still missing the point.

Whether the downward thrust lift is generated by the shape of the tail or by the pectoral fins doesn't matter. Whichever way they do it they are actively displacing water downwards and that is going to negatively affect their efficiency. Not by much, granted. But by enough that I'm not surprised that a dolphin is more efficient.

FTFY!
If I had more time to kill, I would go back and fix every mention of "downward thrust" in this thread, which is a lot.

Clue for you, they are the same thanks to Newton's Third Law. A downwards thrust on the water is the same as lifting the shark. So no fixes are necessary.

[Heisenberg]

For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much - the wheel, New York, wars and so on - whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man - for precisely the same reasons.

[Interactive Civilian]

Wow. I didn't realize there would be so much controversy in stating that dolphins are the most efficient swimmers* in the sea.

_{*based on length specific swim speed (body lengths / tail cycle), as cited in my post with the links to journal articles}

[tantalum]

And therefore, evolution is a theory in crisis >.>


Back on topic, I have another question: why is it that we've evolved to like the taste of cooked meat? Maybe i'm just being naive because I don't routinely eat raw meat, but there's a whole field of science around determining the unusual compounds formed during the cooking process (look up Maillard reaction). I can't imagine cooking or standing around wildfires/chasing animals that were on fire was one of our pastimes.

[danpilon54]

Cooked meat is easier to digest, not to mention a lot more sanitary. I dont know if taste is really determined by evolution, however. People can learn to like a lot of different things, and I think its more that all the meat you eat as a kid is cooked. Raw fish is eaten in things like sushi and beef is served raw in things like steak tartare. Both of which are very tasty. But it makes sense evolutionarily. People figured out how to cook meat. It probably started as burning hair off an animal, and led to cooking the meat but that is just a guess. Some liked it so they ate it more than raw; some didnt so they ate more raw meat. The ones who ate less raw meat digested the meat easier and got less food-born diseases.

[jmorgan3]

I would guess the process was

-warm (recently killed) meat > cold meat (freshness means less time for bacteria growth, etc.)
-fire used to warm non-fresh meat to make it more palatable
-some meat is left in fire too long, becomes cooked
-this kills bacteria
-the people who like the new cooked meat are subject to less bacteria and can eat old food more easily
-the trait "likes cooked meat" is selected for
-?
-profit!

[danpilon54]

-open successful restaurant chain
-profit!

fix'd

[jmorgan3]

You just broke a meme. I'm telling.

[iop]

-the people who like the new cooked meat are subject to less bacteria and can eat old food more easily
-the trait "likes cooked meat" is selected for

I doubt that a "likes cooked meat" trait has been selected for. There is certainly a tradition of cooking meat that has been successfully passed down, but that is more a behavioral trait (a meme, if you want).

And therefore, evolution is a theory in crisis >.>

What about this line from an article in Cell?
"Our results suggest a substantial rethinking of several widely credited hypotheses in molecular evolution."

[Izawwlgood]

I recall hearing that spices have antimicrobial properties, thus explaining a selection for them tasting 'good'. One's gotta reason that our perception of good/bad scents is an evolutionary trait too; putrefied corpses smell bad to us, it is important we stay away from them because they harbor diseases. Flies however, must love them.

Cooking meat and consuming spices likely started as a means for sanitation benefited those diners.
Animals (bears and primates come to mind for this) will consume low-calorie herbs that have anti-parasitic properties to curb stomach problems.

nom nom nom.

[jmorgan3]

I also remember hearing about fungi and bacteria evolving scents that repel animals, so that the decomposers don't get eaten and killed.

[Interactive Civilian]

I would guess the process was

-warm (recently killed) meat > cold meat (freshness means less time for bacteria growth, etc.)
-fire used to warm non-fresh meat to make it more palatable
-some meat is left in fire too long, becomes cooked
-this kills bacteria
-the people who like the new cooked meat are subject to less bacteria and can eat old food more easily
-the trait "likes cooked meat" is selected for
-?
-profit!

Maybe. I'm kind of a fan of the forest fire hypothesis*, i.e. that cooked meat was discovered and enjoyed by chance. Of course, it could be a combination of both.
_{*probably not the real name}

As far as the evolution of taste goes, I'd say to an extent that evolution and taste go very well together, though it isn't always exact, as you can have toxic compounds that taste good. I think of it along the lines of the idea of why salt and starchy foods like potatoes, rice, etc. go well together and taste good together.

**Caveat: this is just an example from my own imagination based on some facts. Please take it as such***

Starch is digested by the enzyme amylase, which is present in your saliva (as well as produced by the pancreas and added in the duodenum). Chlorine ions are a co-factor of amylase, which means it works better when there is chlorine present. Salt is NaCl, which dissociates in solution. Therefore, when eating starchy food, it is good to have salt around to activate the amylase.

How does that work with evolution and taste? Well, imagine two populations, one with a mutation that makes salt and starch taste better than just starch and the other without such a mutation so, while it may not taste bad, there is no difference in taste with or without salt. The former is more likely to eat starch with salt while the later may or may not. If the latter does not, then it will not break down the starch as efficiently and will get less energy from it. The former will break down starch very efficiently and get more energy from it.

Granted, this is not a strong selective pressure, but perhaps when dealing with other selective pressures, that little energy boost is enough to allow that trait to be selected for.

I'm sure this is very over simplified, and possibly not even real at all, but it makes for a decent example. In reality, I have no idea which organisms started using amylase to break down starch or when it started happening, so the concept of them "tasting" may be moot. Like I said, it's just a possible example. Please take it as such.

[btilly]

Let me guess. You wrote that while eating potato chips?

[Interactive Civilian]

Let me guess. You wrote that while eating potato chips?

Heh. Actually it struck me a few weeks ago when I was starting to teach cellular respiration to my 11th graders. Before we could get into the details of things, we had to do a review of energy transfer, redox, enzymes, and co-factors. One of the examples for an inorganic ion as a co-factor given in the text book was Chlorine ions and amylase.

I read that and though, "Hey, wait a minute!"

[danpilon54]

You just broke a meme. I'm telling.

I know, maybe I'm trying to start my own anti-meme meme

[Izawwlgood]

Mitch Hedburg wrote: I'm against picketing but don't know how to show it

[0.0]

@btilly: Or that they generate forward thrust and use their pectoral fins like 'wings' and angle upward? Read the previous post to yours.

I read the previous post, and see that you're still missing the point.

Whether the downward thrust lift is generated by the shape of the tail or by the pectoral fins doesn't matter. Whichever way they do it they are actively displacing water downwards and that is going to negatively affect their efficiency. Not by much, granted. But by enough that I'm not surprised that a dolphin is more efficient.

FTFY!
If I had more time to kill, I would go back and fix every mention of "downward thrust" in this thread, which is a lot.

Clue for you, they are the same thanks to Newton's Third Law. A downwards thrust on the water is the same as lifting the shark. So no fixes are necessary.

I wasn't trying to fix the vectors of your forces, merely improving upon the poor choice of terminology that was running rampant and creating confusion.

[qetzal]

What about this line from an article in Cell?
"Our results suggest a substantial rethinking of several widely credited hypotheses in molecular evolution."

That looks like a fascinating article. Can't wait to pull it up when I'm back at work on Mon.

Thanks, iop!!

[iop]

i've always been curious about the evolution of fangs that inject venom. i'm not sure how the mechanism for delivering poison through the teeth could have been beneficial before it was fully operational or how the modified teeth and glands developed in a coordinated fashion.

There is an article about the evolution of fangs (not specifically venom, though) in this week's Nature. Here's the abstract

Nature 454, 630-633 (31 July 2008)

Many advanced snakes use fangs—specialized teeth associated with a venom gland1, 2—to introduce venom into prey or attacker. Various front- and rear-fanged groups are recognized, according to whether their fangs are positioned anterior (for example cobras and vipers) or posterior (for example grass snakes) in the upper jaw3, 4, 5. A fundamental controversy in snake evolution is whether or not front and rear fangs share the same evolutionary and developmental origin3, 4, 5, 6, 7, 8, 9. Resolving this controversy could identify a major evolutionary transition underlying the massive radiation of advanced snakes, and the associated developmental events. Here we examine this issue by visualizing the tooth-forming epithelium in the upper jaw of 96 snake embryos, covering eight species. We use the sonic hedgehog gene as a marker10, 11, 12, 13, and three-dimensionally reconstruct the development in 41 of the embryos. We show that front fangs develop from the posterior end of the upper jaw, and are strikingly similar in morphogenesis to rear fangs. This is consistent with their being homologous. In front-fanged snakes, the anterior part of the upper jaw lacks sonic hedgehog expression, and ontogenetic allometry displaces the fang from its posterior developmental origin to its adult front position—consistent with an ancestral posterior position of the front fang. In rear-fanged snakes, the fangs develop from an independent posterior dental lamina and retain their posterior position. In light of our findings, we put forward a new model for the evolution of snake fangs: a posterior subregion of the tooth-forming epithelium became developmentally uncoupled from the remaining dentition, which allowed the posterior teeth to evolve independently and in close association with the venom gland, becoming highly modified in different lineages. This developmental event could have facilitated the massive radiation of advanced snakes in the Cenozoic era, resulting in the spectacular diversity of snakes seen today6, 14, 15.

[Izawwlgood]

I wouldn't be surprised if venomous saliva or such started first, with a meandering evolution towards beveled fangs to deliver more toxin into the bite. Or maybe that order reversed, with beveled fangs resulting in a bloodier bite if the snake hangs onto it's prey, resulting in quicker death?

[cephalopod9]

One hard to fathom instance of evlolution that weirds me out: mammary glands.
"I was just sitting on my nest of hungry babies, and I had the best idea ever. Secreting fluid from my under belly with wich to feed them!"
I've seen it explained that boobies might have evolved from sebaceous glands, (first there was a bald spot to help keep eggs warm via direct skin contact, then there was dent to collect extra nutritious sweat, then there was a sticky-out part to better reach the babies' mouths, and we have milk. As I understood it anyhow) It's still pretty freaking weird.

[crowey]

One hard to fathom instance of evlolution that weirds me out: mammary glands.
"I was just sitting on my nest of hungry babies, and I had the best idea ever. Secreting fluid from my under belly with wich to feed them!"
I've seen it explained that boobies might have evolved from sebaceous glands, (first there was a bald spot to help keep eggs warm via direct skin contact, then there was dent to collect extra nutritious sweat, then there was a sticky-out part to better reach the babies' mouths, and we have milk. As I understood it anyhow) It's still pretty freaking weird.

That's more the evolution of nipples isn't it? The platypus (and echidna I think?) produce milk but have no nipples, it kind of sweats out from where the nipple would be.

[Master Gunner]

When talking about evolution, I don't think the Platypus is the best thing to draw comparisons to...being a platypus and all.

[crowey]

True, but I don't think it lost nipples, more that they never evolved in the prototherian line.
So, either the common ancestor for prototherians, metatherians and eutherians had nipples and produced milk, then the prototherianss lost the nipples. Or it didn't have nipples but produced milk then the eutherianss and metatherianss evolved nipples as an addition to the system while the prototherians stayed on the no nipple front. I'm pretty sure that the second is the accepted course? I have no sources for that though, I'm just recalling my undergrad course from a few years ago .

My point still stands though, that cephalopod's example was of nipples evolving not of milk production evolving.

[GhostWolfe]

My point still stands though, that cephalopod's example was of nipples evolving not of milk production evolving.

I think cephalopod was talking about the whole milk/nipple/mammary gland deal. i.e. how we got from [no internal provision for young once they are birthed/hatched] to [milk supply].

/angell

[cephalopod9]

...
My point still stands though, that cephalopod's example was of nipples evolving not of milk production evolving.

There's not much point of having nipples without milk production, so I sort of considered them a stage in the evolution of lactation. Plus I obviously skipped some steps (I remember skimming over something about concave nipples, I think), and I did say "boobies" there.

Another anatomical feature I've never fully understood, the whole neck arrangement. I guess I can see some advantages in range of vision and such, but putting all the thinking* (seeing, smelling, hearing, tasting) matter in one place, and that place being the far end of the body connected by only the neck seems like kind of a bad idea. That would probably make more sense if I understood biology better tho'.

*data processing, and nerve controls, not necessarily sentience.

[btilly]

Another anatomical feature I've never fully understood, the whole neck arrangement. I guess I can see some advantages in range of vision and such, but putting all the thinking* (seeing, smelling, hearing, tasting) matter in one place, and that place being the far end of the body connected by only the neck seems like kind of a bad idea. That would probably make more sense if I understood biology better tho'.

Think about it from the point of view of a fish. You have a mouth. For ease of reaching prey, your mouth is at the tip in front of you. For ease in sensing food and getting your mouth to it, you want your eyes and ears to be close to that mouth. Both for simplicity of wiring and for reaction speed, it now makes sense for your control center to be close to the eyes and ears, which means close to the mouth.

Congratulations, you now have a head.

We haven't changed that basic arrangement since. We've just elaborated it and adapted it for life out of the water.

[Izawwlgood]

The evolution of the thinking bits came after the evolution of the "I've got a mouth here and I'm not afraid to use it!" bits. There's no reason to internalize the brain, so why would it happen?

Early critters that laid the bauplan for this jazz had a brain that should really be described as a neuronal network chunk.

[Guest]

I obviously believe in evolution, but from my high school biology days I was left with the impression that a lot of people just pull random explanations for things out of the air without really knowing. Things like "X developed long necks so it could look over tall grass better". Is there actually solid evidence for stuff like this or is it just an educated guess? I don't really know much about it so if it's a misconception feel free to clear it up.