What I mean to say is although dolphins and sharks look alike they are not very closely related, one is a fish one is a mammal. I am just curious as to how both wound up with prominent dorsal fins. I mean when I look at drawings of the dolphins ancestors, they had arms, legs and a tal so you can clearly where fins came from, i.e. they evolved from the arms but there is nothing on the animals back that could be considered homologous to a fin. I an just curious.

The reason that they both have prominent dorsal fins is because of convergent evolution. The dorsal fin is absolutely necessary for (large) sea creatures for stability, and those organisms with it were much more likely to have offspring than those that didn't.

Two organisms need not be closely related to evolve many of the same structures, since Natural Selection will select for those traits for the sole fact that they are advantageous in a particular environment. Dolphins and sharks are not the only ones with convergent evolution either; the evolution of the wing on both birds and bats are another example, and The Ancestor's Tale by Richard Dawkins also gives numerous examples of convergent evolution, such as the marsupial mole and it's Eurasian counterpart, or the shrews that evolved independently on Madagascar.

Slowly.

Ergo Proxy wrote:The reason that they both have prominent dorsal fins is because of convergent evolution. The dorsal fin is absolutely necessary for (large) sea creatures for stability

Ever looked at a beluga? Bowhead whale? Northern or Southern right whale dolphin? The Finless porpoise? Narwhal? North Pacific right whale?
Not to mention a few large mesozoic aquatic amniotes without dorsal fins. I think the "absolutely neccessary" should be relativized...

Net Traveller wrote:What I mean to say is although dolphins and sharks look alike they are not very closely related, one is a fish one is a mammal. I am just curious as to how both wound up with prominent dorsal fins. I mean when I look at drawings of the dolphins ancestors, they had arms, legs and a tal so you can clearly where fins came from, i.e. they evolved from the arms but there is nothing on the animals back that could be considered homologous to a fin. I an just curious.

As someone has said its down to convergent evolution. The most effective shape of fin is likely to be reached through natural selection. If you broaden the study of fins to more species then evolution has led to a wide variety of fins that serve various functions depending on the species particular environment and you'll find other examples of convergent evolution in these environments.

As for the fin on the back I don't know but I imagine that there is some kind of mutation that leads to a small raised section along the spine that confers some kind of selective advantage that has led to the dorsal fin but its important to note that not all dolphins have a dorsal fin and that there are some which have a very small one that amounts to little than a lump which indicates that its not vital for the their survival in all environments.

Additional discussion cross-posted at TR:

http://www.talkrational.org/showthread.php?t=27165

As I understand it, dorsal fins are more prominent in faster-moving species - they provide roll stability during quick acceleration and sudden turning. The prominence of these fins can vary greatly, depending on the lifestyle of a given genus / species. Many large cetaceans have greatly reduced dorsal fins, or no dorsal fins at all, likely because their slow-moving swimming style and girth obviates such a fin configuration. In smaller cetaceans, like belugas, the lack of a dorsal fin probably has more to do with their lifestyle. Swimming underneath sheets of ice looking for the occasional air-hole might be easier without a dorsal fin - orcas, with their massive dorsal fins, are often unable to follow belugas under the ice for this very reason. Sharks and fish also display a huge variety of dorsal fin configurations, based on their particular swimming styles / lifestyles.

The convergent evolution of fins between mammals and fish is hardly surprising - if you want a more extreme (and frequently cited) example of convergent evolution, look at human vs. octopus eye structure. That's some pretty incredible stuff.

Net Traveller wrote:What I mean to say is although dolphins and sharks look alike they are not very closely related, one is a fish one is a mammal. I am just curious as to how both wound up with prominent dorsal fins. I mean when I look at drawings of the dolphins ancestors, they had arms, legs and a tal so you can clearly where fins came from, i.e. they evolved from the arms but there is nothing on the animals back that could be considered homologous to a fin. I an just curious.

I am curious to know whether you felt that your question was answered to your satisfaction. Was it?

Quite a few Actinopterygian fishes have a reduced dorsal fin - these are usually fishes whose lifestyle is that of a surface dweller. Fishes such as the Hatchet Fishes (Family Gasteropelecidae), which dwell just below the surface, and which are in addition freshwater flying fishes, have a small dorsal fin combined with a deeply keeled body, a large anal fin and wing-like pectorals, as this image of Carnegiella strigata demonstrates:



Other surface dwelling fishes also have reduced dorsals, such as various Cyprinodonts - this photo of Epiplatys dageti is an example:



Another member of the same Genus is Epiplatys infrafasciatus, shown here:



The same morphology is visible in various Rivulus species, such as Rivulus strigatus below:



Rivulus species will not only dwell very close to the surface, feeding upon fallen insects, or aquatic insects that live at the surface, but will also climb out of the water onto lily pads for short periods of time. In the aquarium, they also have a reputation for being rocket-propelled jumpers, that will leap a good six feet out of the water if the aquarium is left uncovered for any length of time.

A similar small dorsal fin is visible in certain Labyrinth Fishes, which live near the surface and breathe atmospheric air to supplement their oxygen requirements in waters with low dissolved oxygen content. Trichogaster trichopterus is a prime example:



If you're looking for a fish that not only lacks a dorsal fin, but a caudal fin too, how about Gymnotus carapo, the Banded Knife Fish?



Similarly, we have Eigenmannia virescens, the Glass Knife Fish:



An interesting variation is Malapterurus electricus, the electric catfish. This has a large adipose fin near the caudal peduncle, but has lost its rayed dorsal fin altogether:



This fish is a bottom dweller, and is usually slow moving, but then it has an advantage with respect to its predatory habits - it can generate a 350 volt electric shock to knock out its prey.

http://www.talkrational.org/showthread. ... post984302

Perhaps that was interesting in response to your question Net Traveller?

I don't know that the answer 'convergent evolution' is satisfactory, it answers it but leaves me disgruntled. That they are completely different structures situated in similar situations is interesting.

"Convergent evolution" is not satisfactory. It sounds like a shitty answer because it is a shitty answer, as bad as creationist jargon. Anyone who IS satisfied with that as an explanation should be ashamed.

susu.exp wrote:

Ergo Proxy wrote:The reason that they both have prominent dorsal fins is because of convergent evolution. The dorsal fin is absolutely necessary for (large) sea creatures for stability

Ever looked at a beluga? Bowhead whale? Northern or Southern right whale dolphin? The Finless porpoise? Narwhal? North Pacific right whale?

All those have ancestors with dorsal fins.

It's certainly not needed for stability though. It just helps prevent some animals from rolling. Lots of cetaceans, belugas in particular, love to spin through the water and travel upsidedown anyways. It's super helpful too because when mammal eating orca are present they (and narhwals) can tuck themselves right up under the ice. Orca can't pursue them due to their dorsal fins forcing them to travel far deeper under the water.

The dorsal fin helps with sudden turns too. Most cetaceans just aren't great at turning quickly. Sharks, in general, are much better at it. Watching hammerheads turn is magnificent.

I watched a tagging operation of a big beautiful male beluga that absolutely broke my heart. Popping a couple holes through a dorsal fin to insert a tag is easy. Try doing that to someone with a dorsal ridge. It has far further to travel from one side to the other and is much deeper into the animal's back.

I spent the day with one of my favourite cetacean families so this is all very appropriate.

Perhaps this paper provides some answers ...

Sonic Hedgehog Function In Chondrichthyan Fins And The Evolution Of Appendage Patterning by Randall D. Dahn, Marcus C. Davis, William N. Pappano and Neil H. Shubin, Nature, 445: 311-314 (18th January 2007)

Dahn et al, 2007 wrote:The genetic mechanisms regulating tetrapod limb development are well characterized, but how they were assembled during evolution and their function in basal vertebrates is poorly understood. Initial studies report that chondrichthyans, the most primitive extant vertebrates with paired appendages, differ from ray-finned fish and tetrapods in having Sonic hedgehog (Shh)-independent patterning of the appendage skeleton1. Here we demonstrate that chondrichthyans share patterns of appendage Shh expression, Shh appendage-specific regulatory DNA, and Shh function with rayfinned fish and tetrapods2–10. These studies demonstrate that some aspects of Shh function are deeply conserved in vertebrate phylogeny, but also highlight how the evolution of Shh regulation may underlie major morphological changes during appendage evolution.

Another relevant paper is this one:

Fins, Limbs, And Tails: Outgrowths And Axial Patterning In Vertebrate Evolution by Michael I. Coates and Martin J. Cohn, BioEssays, 20(5): 371-381 (1998)

Coates & Cohn, 1998 wrote:Summary

Current phylogenies show that paired fins and limbs are unique to jawed vertebrates and their immediate ancestry. Such fins evolved first as a single pair extending from an anterior location, and later stabilized as two pairs at pectoral and pelvic levels. Fin number, identity, and position are therefore key issues in vertebrate developmental evolution. Localization of the AP levels at which developmental signals initiate outgrowth from the body wall may be determined by Hox gene expression patterns along the lateral plate mesoderm. This regionalization appears to be regulated independently of that in the paraxial mesoderm and axial skeleton. When combined with current hypotheses of Hox gene phylogenetic and functional diversity, these data suggest a new model of fin/limb developmental evolution. This coordinates body wall regions of outgrowth with primitive boundaries
established in the gut, as well as the fundamental nonequivalence of pectoral and pelvic structures.

Since that paper establishes that Shh and Wnt genes, in combination with Hox genes, are responsible for regulating limb patterning, this correlates with the findings of Shh expression in the earlier paper, and in the earlier paper, Shh expression has an effect upon the development of unpaired fins as well.

Other possible clues may be obtained from this paper and this paper.

EDIT: From that latter paper:

Abstract

Detailed examples of how hierarchical assemblages of modules change over time are few. We found broadly conserved phylogenetic patterns in the directions of development within the median fins of fishes. From these, we identify four modules involved in their positioning and patterning. The evolutionary sequence of their hierarchical assembly and secondary dissociation is described. The changes in these modules during the evolution of fishes appear to be produced through dissociation, duplication and divergence, and co-option. Although the relationship between identified median fin modules and underlying mechanisms is unclear, Hox addresses may be correlated. Comparing homologous gene expression and function in various fishes may test these predictions.

The earliest actinopterygians likely had dorsal and anal fins that were symmetrically positioned via a positioning module. The common patterning (differentiation) of skeletal elements within the dorsal and anal fins may have been set into motion by linkage to this positioning module. Frequent evolutionary changes in dorsal and anal fin position indicate a high level of dissociability of the positioning module from the patterning module. In contrast, the patterning of the dorsal and anal fins remains linked: In nearly all fishes, the endo- and exoskeletal elements of the two fins co-differentiate. In all fishes, the exoskeletal fin rays differentiate in the same directions as the endoskeletal supports, indicating complete developmental integration. In acanthopterygians, a new first dorsal fin module evolved via duplication and divergence. The median fins provide an example of how basic modularity is maintained over 400 million years of evolution.

Jehannum wrote:"Convergent evolution" is not satisfactory. It sounds like a shitty answer because it is a shitty answer, as bad as creationist jargon. Anyone who IS satisfied with that as an explanation should be ashamed.

Ashamed? That's rather a strong emotion to have to feel for a process that's well documented in other cases.

Convergent evolution is invoked to explain how they look like each other, not how they got to be like each other - for that, you just need the 'evolution' bit.

Well don't forget that dophins and also humans are fish (I guess I should say sarcopterygii as fish is paraphyletic)

Jehannum wrote:"Convergent evolution" is not satisfactory. It sounds like a shitty answer because it is a shitty answer, as bad as creationist jargon. Anyone who IS satisfied with that as an explanation should be ashamed.

Sure it's not satisfactory in the sense that it doesn't explain why they have dorsal fins. It's simply a word we use to describe the fact that they both evolved dorsal fins. What that probably means is that sharks and dolphins were under a similar kind of selective pressure to develop that kind of aquadynamic stabilization.

The underlying genetic mechanics are probably going to be vastly different between how the two. I know almost nothing about shark evolution so I'm not going to speculate on them. Dolphins, being a mammal that evolved from land-mammals, obviosly didn't have any kind of noticable structure on their backs to begin with(before they were dolphins, of couse), for selective evolution to modify.

It seems to me this leads to a pretty obvious explanation for the ultimate origin of the dolphin dorsal fin : In a population of pre-dolphin whales, a member was born with a mutation that resulted in some kind of, although probably tiny and smooth, potruding piece of tissue somewhere along the spine. Maybe a lump of cartilage or something similar under the skin, providing the basis for the structure for selection to start working upon.



These are sperm-whales. Being a toothed whale, I would expect it to be evolutionarily closer related to dolphins than baleen-whales. It has exactly the kind of lump on it's back that could have served as a precursor to the dorsal fin, for selection to modify and refine over many generations.


Here's a Gray Whale, showing a significantly less developed pre-dorsal lump, several actually.

I don't think it's really a stretch of the imagination to suggest such lumps could have served as the ancestors of Dolphin dorsal fins.

The theory of evolution has an enormous amount of confirmatory evidence and I do not question its scientific usefulness. But the explanations offered thus far still leave puzzling questions. The concept of "conversion" (that significantly divergent genetic lines can develop similar structures due to similar environmental conditions) is, of course, a description, not an explanation.

The discussion of Hox genes is getting closer to explanation. Hox genes turn on or off the development of various genes in different segments of the developing organism. How and why this operates is not fully understood. Different segments have specialized genes for different functions. The first segment has genes for developing head structures. for example, sight, taking in air or food. It is unable to develop legs. The next segment can grow legs, but not sight. A middle section can grow a gut but not legs, etc. There can be great variability within a section over time governed by Hox genes turning on or off genes already in that section. Of course mutations can also occur by accidental gene mutations or by environmental radiation, etc. But even these modifications are limited in scope. They cannot change a stomach gene into an eye gene. What usually results is a mutant that is not environmentally fit.

So how did the dorsal fin of of the cetaceans evolve? Some genes during evolution do become inactive as a particular feature may no longer be useful. Five toes of early horses became useless as the horse developed hooves but the five toes gene still exists in a dormant state and could be reenacted at a future time. But the dorsal fin gene is not to be found at any time in the evolution of mammals. It was suggested that the gene might have been carried dormant from mammal's fish ancestry. Possibly but not plausible. Fish fins are boney structures and found on various parts of the body. Dorsal fins are fleshy and found on only one area. Furthermore, the other cetacean appendages developed from leg and tail bone structures already highly developed in prior mammals. The dorsal fin is totally unique. It is not boney and is located on the gut segment of the body that never had any appendage genes during the whole of mammal evolution.

Although the Orca's fin might help in stabilizing motion, It is difficult to understand how a small "accidental lump" could in anyway be beneficial to survival. A small lump provides no stabilization benefit, thus no survival benefit and, therefore, no natural selection could promote its further development.

I have not come across any answer to this question. I'm still looking. But please don't just say that "evolution explains it".

Um.... Since when did anyone say the dorsal fin of marine mammals evolved solely, or even primarily, for stability?

And why assume they all have them?