Why haven't whales evolved gills?

[Jupiter60]

Why haven't whales evolved gills?

 

[SteamKing]

Why should they?

 

[DiracPool]

Why haven't whales evolved gills?

Because they are mammals and they breath in a different way.

http://nmlc.org/2009/04/how-do-whales-breathe/

But with the terseness of you're inquiry I don't really expect this thread to last long. Hope you got something out of it...

 

[Bystander]

Why haven't whales evolved gills?

Whales evolved from gilled predecessors; should there be any advantage to devolving?

 

[SteamKing]

Whales evolved from gilled predecessors; should there be any advantage to devolving?

Do you have a reference for this?

Cetaceans are thought to have evolved from terrestrial mammals, and early whale ancestors are presently thought to be distantly related to modern-day hippos.

https://en.wikipedia.org/wiki/Evolution_of_cetaceans

As terrestrial mammals, there would have been no creatures with gills in their family tree.

 

[Bystander]

no creatures with gills in their family tree.

None? https://en.wikipedia.org/wiki/Vertebrate_Paleontology_and_Evolution

 

[ShayanJ]

I think the answer is crystal clear if we consider what evolution means.
As I understand it, evolution says that different traits may be introduced into a species by mutation which is an accidental process. Now this means that in the early stages of the introduction of a particular trait into the species, the species can be categorized w.r.t. having that trait or not or various degrees of that trait. Now if that trait is toward making the species less fit to its environment, its simply more probable that evolution of that species goes into a direction that that particular trait becomes less and less in the species. Note that its only a statistical process, species with that trait die sooner,reproduce less, that trait becomes less and less. That simple!
Getting back to the question. To our best knowledge, there is no whale that has gills. By the above reasoning, it simply means that having gills is incompatible with some other trait that whales have. I mean, it makes whales less fit to their environment if they have gills. That's the reason they don't have gills.
So the simple answer is what DiracPool said.

 

[Ygggdrasil]

Do you have a reference for this?

Cetaceans are thought to have evolved from terrestrial mammals, and early whale ancestors are presently thought to be distantly related to modern-day hippos.

https://en.wikipedia.org/wiki/Evolution_of_cetaceans

As terrestrial mammals, there would have been no creatures with gills in their family tree.

Mammals, like all other tetrapod species, evolved from fish. For example, during embryonic development, all vetebrate species show pharyngeal arches, gill like structures which develop into gills in fishes, but end up developing into the jaw and ears of mammals.

 

[SteamKing]

None? https://en.wikipedia.org/wiki/Vertebrate_Paleontology_and_Evolution

I'm talking about in relatively recent times geologically speaking.

I'm sure if you go back far enough, everything evolved from single-celled organisms, but that doesn't mean that humans necessarily will form spores and hibernate like bacteria do.

Dimetrodon is also thought to be distantly related to the ancestors of mammals, but I don't think any mammals will be evolving fancy sail structures along their spines.

https://en.wikipedia.org/wiki/Dimetrodon

Do you have any funny uncles who resemble this critter?

 

[Bystander]

https://en.wikipedia.org/wiki/Bison_latifrons#/media/File:Bison_latifrons_fossil_buffalo.jpg

 

[SteamKing]

https://en.wikipedia.org/wiki/Bison_latifrons#/media/File:Bison_latifrons_fossil_buffalo.jpg

There are similar structures on the vertebrae of many mammals, including humans:

http://anatomy-medicine.com/uploads/posts/2015-08/1440063932_anatomy-human-anatomy-human-body-spine-back-bones.jpg
​

This doesn't mean that human ancestors had sails popping out of their backs.

 

[Ygggdrasil]

Apparently, breathing air with lungs enables much higher rates of oxygen exchange than breathing water with gills, which allows whales and other marine mammals to have higher metabolic rates than fishes: https://www.newscientist.com/blogs/shortsharpscience/2009/08/why-whales-dont-have-gills.html

 

[Bystander]

should there be any advantage to devolving?

but that doesn't mean that humans necessarily will form spores and hibernate like bacteria do.

We're agreed.

Dimetrodon is also thought to be distantly related to the ancestors of mammals,

(Picture of one of my "funny cousins.")

This doesn't mean that human ancestors had sails popping out of their backs.

http://people.eku.edu/ritchisong/342notes2.htm
Actually, so long as vertebrae include a neural arch (spine) that is subject to whatever evolutionary pressures may favor elongation (or shortening), not only did my greatⁿ grandpappy (n = O(10⁸)) carry a sail, the great^m grandkids might also. Structures that have been highly modified or lost, gills, are not, according to my understanding of current evolutionary thought, likely to be recovered. It is possible, again to my current understanding, that other structures may be modified to handle special/exotic functions; "eyes/photoreceptors" were a popular example in my schooldays, and "No, I do not understand the mechanism for 'loss and recovery of vision' of cavefish in one or two generations." (I've heard discussions of "suppressed gene expression," and given up.)

Metabolic demands are obvious as pointed out by Ygggdrasil ;

Apparently, breathing air with lungs enables much higher rates of oxygen exchange than breathing water with gills, which allows whales and other marine mammals to have higher metabolic rates than fishes:

 

[cosmik debris]

A whale is warm blooded and a reasonable large creature which means its energy consumption is large. This in turn means that a large amount of oxygen is required to burn the fuel that it consumes. I'm guessing the amount of oxygen obtained using gills is just not enough to sustain the sea mammals, so evolving gills would be a backward step.

 

[DiracPool]

so evolving gills would be a backward step.

Yep, we're talking about reversing hundreds of millions of years of evolution. Seems to me much more efficient to float to the surface every now and then and expose your blow-hole to the oxygenated atmosphere with a relaxed attitude than to have to feverishly continue swimming through the water in order to get your oxygen fix.

You know relationships are a lot like this--they're a lot like sharks (which are fish), they have to keep moving or else they die.

 

[Jupiter60]

I would think that because whales are aquatic animals having gills would be an advantage. I guess that's not the case.

 

[DiracPool]

I would think that because whales are aquatic animals having gills would be an advantage.

Yeah, that's kind of the definition of a "naive assumption," aquatic animals=gills. But now you have 15 posts that educate you otherwise

 

[Alcathous]

A lot of what people are saying here is wrong or unsubstantiated. This is all quite difficult to figure out.
Just the mere fact that gills don't evolve doesn't mean having them, and paying their metabolic cost, wouldn't be an advantage. Also, having gills doesn't mean you can't have lungs.

Quite clearly marine mammals develop ways to deal with the problem of having to hold their breath. They just don't do so by evolving gills and they survive fine without them. Same for reptiles like the crocodile, which has evolved very little, meaning it has reached the optimum physiology, evolution can provide, for the niche it occupies.

Are gills not a convergent state of evolution? Can gills evolve into lungs but not lungs into gills? Has there not been enough time to evolve gills de novo? All these are fair questions without answers.

That there is 'no need' to have gills is only true to the extent that obviously whales don't go extinct without them. There is no 'need' in evolution, but that doesn't mean it wouldn't be an adventagous adaptation.
And evolution is happy to go either 'backwards' or 'forwards' depending on what labels we like to use; it is indifferent to this.

Also important to note is that some things just can't evolve no matter what the circumstance. This is why the parts biology uses are often so different from the parts we use to build our machines.

My main guess would be that because lungs already exist, it is hard to evolve something de novo that does the same thing, but differently. Absorbing tiny amounts of oxygen into the blood using skin spots that absorb O2 and act like the most basic of gills are likely to have almost no significance on the total O2 intake of the creature, as it already has lungs. And therefore no significant evolutionary advantage or selection for those genes. And even if it did, not sure if whales have been in the ocean for long enough to match the time it took the first time to evolve gills.

Therefore, adaptations are made to what already exists.

 

[DiracPool]

A lot of what people are saying here is wrong or unsubstantiated. This is all quite difficult to figure out.

It's not really difficult to figure out at all. Aquatic mammals are formerly land-based animals that returned to the sea and retained their lung breathing apparatus because it was sufficient for the job. If it wasn't, it would have evolved a different mechanism. Certainly, if selection demanded it, the aquatic mammals of today could "re-evolve" gills of some sort. I don't quite know what scenario might precipitate that, but that's a reasonable scenario. I don't think it would be a revival of our embrological gill arches, though, it would be a late phlogenetic trait "stacked" on top of everything else.

Can gills evolve into lungs but not lungs into gills?

Lungs would not likely evolve into gills. As you say, they would probably evolve "de novo" if that were to happen. The mammalian lung/respiratory system proper would probably atrophy and become vestigial.

 

[DiracPool]

Also, having gills doesn't mean you can't have lungs.

Evolution isn't 100% efficient by any means, but it is by necessity, parsimonious, and the odds are vanishingly small that it would retain two mutually distinct respiratory systems, especially in larger aquatic mammals.

 

[johnnymorales]

Why haven't whales evolved gills?

As long as whales are warm blooded, gills will be incompatible with their make up.

That they are warm blooded is why there has been no evolutionary changes that would make a whale able to breathe water despite the wholesale evolutionary makeover the ancestors of whales underwent to become modern whales.

Every single animal that has gills is cold blooded or mostly coldblooded (some sharks and Tuna and sea turtles) whose warmer body temps. lie in their interior, are far from the gills where they are less affected by the extreme cooling effects of breathing with gills, and restricted to certain organs and tissues (muscle, brain). Even then their ability to warm those specific regions is limited, and only as needed.

When you consider Whales have a body temperature similar to ours, and that many of them live in the high latitudes with water temps. around freezing, having gills with the amount of water necessary to keep them oxygenated would kill them via hypothermia without fundamental changes to its morphology. Even in much warmer waters the dying from hypothermia is a serious risk, because the water is much better at sucking the heat of something than much less dense air is. Humans for example can die of hypothermia in water as warm as 85 degrees or so if we are in long enough.

So if you think it makes sense for a whale to have gills, you need to figure out how a whale could keep a high body temperature. If you can't then you have the answer to your question.

Of course it's possible to imagine an extreme evolutionary tangent where somehow beginning with first being able to breath through their skin that something similar to gills could develop, but as long as whales are as big as they are, no such development would require so many changes to their current morphology that the creature that resulted would no longer be recognizable as a whale or even a mammal.

Also gills and lungs have nothing to do with each other anatomically. Lungs did not develop from gills.

Lungs began in terms of evolution as air sacs in fish that had become vascularized enabling them to extract oxygen from air. This ability became better and better as fish/amphibians moved into water with poor oxygen levels. Over 100s of millions of years those air sacs became the first very inefficient lungs.

It took millions of years more before a creature developed lungs so efficient they could dispense with gills. In the interim many had both lungs and gills, and some amphibians still have that set up.

So with fish evolution had fully functioning gills to breath with while a completely different part of their anatomy began the long process of evolving into lungs.

So if you still think whales should be able to develop gills you need to find some sort of starting point on their anatomy as it exists today, and think logically and rationally how that current anatomical feature might develop the ability to extract oxygen from water and then from there eventually turn into a gill like structure that could do what fishes do with their gills and do so without simply assuming that lungs would evolve into gills.

If you do this you'll realize it's practically impossible to imagine all the steps necessary to develop gill-like structures on a modern whale that wouldn't severely compromise its ability to survive as a whale.

Regardless, assuming someone could lay out all the hypothetical steps in a logical and rational manner for a whale to breathe water, without violating what we know regarding how evolution works, the wholesale changes to a whale's physiology would likely be so profound, that the resulting creature could not longer be called a whale or probably not even a mammal.

 

[DiracPool]

It took millions of years more before a creature developed lungs so efficient they could dispense with gills. In the interim many had both lungs and gills, and some amphibians still have that set up.

I like your post, and I kind of anticipated your quote above, especially in the transitory amphibians, which is why I revised my statement in my earlier post to ,"especially in larger aquatic mammals." However, I defend my position that evolution is parsimonious. But in the transitionary phases, if a trait or a redundancy does not negatively affect selection it does stand a chance of persisting, as in the amphibian cases you mentioned. Although, for the record, I have not researched this so I'm taking johnny's word here for the meantime.

 

[johnnymorales]

I like your post, and I kind of anticipated your quote above, especially in the transitory amphibians, which is why I revised my statement in my earlier post to ,"especially in larger aquatic mammals." However, I defend my position that evolution is parsimonious. But in the transitionary phases, if a trait or a redundancy does not negatively affect selection it does stand a chance of persisting, as in the amphibian cases you mentioned. Although, for the record, I have not researched this so I'm taking johnny's word here for the meantime.

Thanks. About the only thing I would take issue with is your use of the word parsimonious. I'm sure I get your point, but only by extrapolation.

 

[DiracPool]

About the only thing I would take issue with is your use of the word parsimonious

That's about the only thing you shouldn't take issue with. As sure as natural selection, evolution is built on parsimony. Do you disagree with this?

 

[johnnymorales]

That's about the only thing you shouldn't take issue with. As sure as natural selection, evolution is built on parsimony. Do you disagree with this?

The problem is I'm not sure YOU know EXACTLY what parsimony or parsimonious means, though it's clear you have a general idea. I DO know what it means, and how the word is supposed to be used. For that reason it is hard to just assume what you mean.

Thus I'm not absolutely certain what you mean when you use it to describe the process of evolution.

You're using the word in an incongruent context where what it measures doesn't exist.

You are using the word "parsimony Etc." as if it is a general use word with a meaning that can be applied in different contexts, and that's not right.

Parsimony is NOT a general use word.

The meaning of parsimony is tied to money and how you use it or your attitude about money.

In this context it's meaning is ambiguous. I can extrapolate from how it's used in that context to get an idea of what you mean, but have to assume a lot leaving much room for error.

Before I could answer your question as to what you think about evolution is right or wrong you should find a better word or phrase to describe what you think, because parsimony is definitely not it.

 

[Ygggdrasil]

That's about the only thing you shouldn't take issue with. As sure as natural selection, evolution is built on parsimony. Do you disagree with this?

If evolution is parsimonious, why do we have vestigial structures like a tailbone or an appendix? Why is about half of the human genome composed of repetitive sequences that are derived from viruses or mobile genetic elements? I get your general point and agree that an organism with both lungs and gills would likely vestigialize one of the organs if it used only one (it is much easier for random mutation to degrade unecessary function), but I'd disagree with the general statement that evolution is parsimonious.

Parsimony is NOT a general use word.

I also disagree that usage of the word parsimony is tied to money. The http://dictionary.reference.com/browse/law+of+parsimony, commonly referred to as https://en.wikipedia.org/wiki/Occam's_razor]Occam's[/PLAIN] razor or lex parsimoniae, is very often cited in many fields of science, including evolutionary biology.

 

[DiracPool]

If evolution is parsimonious, why do we have vestigial structures like a tailbone or an appendix? Why is about half of the human genome composed of repetitive sequences that are derived from viruses or mobile genetic elements?

Well, I'm using the term as a substitute perhaps for "extreme conservatism." It's pretty much the equivalent to the "least action" principle in physics. As I mentioned in an earlier post and as I'm sure you're aware, in evolution, if a vestigial structure doesn't have a deleterious effect on selection it's not necessary going to be phased out. If it takes more energy to remove the structure than it does just to ignore it, evolution is just going to ignore it. This goes the same for the genome. What you seem to be talking about is some form of an active vs. passive parsimony whereby the body sculpts itself to maximum efficiency. This is not how evolution works, so to say that evolution is parsimonious but to use vestigial structures as an argument to counter that assertion is think is wrong.

 

[Chronos]

My impression is whales were among a group of mammals who returned to the sea after evolving for life on land. Environmental factors probably favored their return to aquatic dwelling as a survival response.

 

[DiracPool]

Environmental factors probably favored their return to aquatic dwelling as a survival response.

What's your evidence for this? I see none. It doesn't have to be an environmental driver that pushed them back into the sea. Maybe they just tired of competing with the other land mammals and said I'm going to jump into the water and see what's down there.

 

[Alcathous]

Evolution cannot be generalised to be either parsimonious or 'extremely conservative'. Without selection pressure, change is inevitable.
And it is often postulated how evolution may be able to occur in bursts.

The best way to describe evolution is to describe the entire process, not to stick on it a few words.

It may actually be the case that one of the best pathways to evolve whales is to do it through land animals. Some adaptations whales have could not happen with only the selection pressures occur in the sea. Being warm-blooded and breathing air may be one among many.

 

[Pythagorean]

If it takes more energy to remove the structure than it does just to ignore it, evolution is just going to ignore it. This goes the same for the genome

I don't think this is necessarily true, though. Evolution gets caught in local minimums, it may never find the global minimums. It doesn't really matter whether one takes more than the other unless they do so in a way that influences selection in the present moment. For instance, imagine that removing our tongues increased fitness and reduce energy expenditure in ways we can't calculate as a social creature, but if there's no gradual path of increased fitness there, evolution will not likely find that solution through random mutation and selection. Evolution can be wasteful as long as its waste isn't affecting fitness relative to its competitors; and even then, it may only lead to a population reduction, not an elimination of the entire population containing such a mutation.

What's your evidence for this? I see none. It doesn't have to be an environmental driver that pushed them back into the sea. Maybe they just tired of competing with the other land mammals and said I'm going to jump into the water and see what's down there.

That would be an envrionmental driver, no? Indeed, the history of evolutionary development (which started with their teeth while they were still amphibious) does indicate that they were capitalizing on the easy pickings in the ocean, but this is a matter of ecology (resource availability) which is a subset of environment, no?

http://www.nytimes.com/1994/05/03/s...egs-and-returned-to-the-sea.html?pagewanted=2

 

[DiracPool]

That would be an envrionmental driver, no?

Well, I think my point in that post was drawing a distinction of "constraint" versus "opportunity." As the first sentence in the reference you posted alluded to. There's a big difference between being constrained to jump back into the sea for whatever reason, and jumping back into the sea as an opportunity. The former is a negative reinforcement condition versus the latter which is a positive reinforcement condition. Are they both driven by environmental conditions, aka "environmental drivers?" I guess you could look at it that way. I was looking at an environmental driver as more being driven by negative reinforcement.

 

[julian kaye]

Why haven't whales evolved gills?

good question, ignore the ignorani that question your motive.
The fact that they have to surface to breath could definitely be a disadvantage, such as ice sheets or predators etc.
Darwin will sort this out in due time.

 

[Fernando L.]

Evolving gills would be a backwards step and requires too many structural changes in the body and its design.

 

[D H]

Evolving gills would be a backwards step and requires too many structural changes in the body and its design.

Evolving gills would not be a backward step. For a whale to evolve gills would be an absolutely huge step forward. Evolution doesn't take giant steps. It can't. Most evolutionary steps are little tiny baby steps. Every once in a while, evolution takes a bigger stride. But unlike Superman, it never, ever leaps across the Grand Canyon in a single bound.

Whales cannot evolve gills.

 

[Torbjorn_L]

There is a lot of unreferenced assertions in this thread, most erroneous, and unfortunately due to time constraints I am going to add my own unreferenced (and therefore likely erroneous) assertions. :-/

devolving?

a backwards step

a backward step.

we're talking about reversing hundreds of millions of years of evolution.

As already commented on, evolution always move forward in time. As a process how can it not?

If something like gills evolved again, they could be convergent, as gill arches are now used to form the throat, larynx et cetera. Mind that some land living vertebrates do retain some of their gill openings in development by accident, it is a known phenomena in humans, so such a pathway could use some or all of the earlier trait basis. [The "Why Evolution Is True" blog related such a case a few months ago.]

It is a teleological idea that evolution has a goal, often relying on pre-evolutionary ideas of a "ladder of descent". Sticking pity labels on process characteristics is always problematic, and sticking erroneous labels on it is confusing.

Similarly already commented on, selective pressures on a population may preserve traits or evolve new ones, but the population need not be at a local (or global) optimum but simply surviving. The global optimum (most biomass) for evolution acting on individual populations is the prokaryote unicellular 'body' form. (Ecologically we can expect a spread, and it is beneficial since the world with plants is the most globally productive - seen as net primary productivity - yet.)

I would think that because whales are aquatic animals having gills would be an advantage. I guess that's not the case.

And again as already commented on, evolving several complementary ways of air uptake may be advantageous and is how tetrapods switched to lungs. Animals may use skin (amphibians), air sacks (various tetrapods) or intestinal tissue (swim bladder) as supplement. The mechanism may work best under water (gills), with access to water (skin uptake) or be independent of water life (intestinal uptake).

As long as whales are warm blooded, gills will be incompatible with their make up.

Every single animal that has gills is cold blooded or mostly coldblooded (some sharks and Tuna and sea turtles) whose warmer body temps. lie in their interior, are far from the gills where they are less affected by the extreme cooling effects of breathing with gills, and restricted to certain organs and tissues (muscle, brain).

Arguably, no longer. A few months ago the find that there is at least one warmblooded species of fish, the opha, was released, an accidental finding. "Heated blood makes opah a high performance predator that swims faster, sees better" [ https://swfsc.noaa.gov/news.aspx?ParentMenuId=39&id=20466 ]

You can argue that it would be harder, perhaps impossible for whales to evolve similar counterstreaming heat exchangers as the opah use, since the opah body temperature is a lot lower than in mammals. (But it is a recurring development, IIRC the gnu antilope has evolved that to shield its brain from its unusually high body temperature of some 44 deg C or so. And we have some similar heat exchange mechanisms for the brain at a lesser efficiency level.) But it could be possible.

Mostly, evolving gill analogs would be under several constraints (contributes relatively little oxygen; must be shielded against heat losses).

 

[BillyT]

Whales were large land animals that only recently (on typical evolutionary time scales) became whales - Thus all the intermediate stages* of fossil skeletons have been found. - Possibly the most complete set of "intermediate stages" that exist. They did slightly evolve their air breathing structures. Their "nose" was near the front of the land animal they evolved from, but migrated towards their developing tail so that it was in the air when most of the animal was under water (and water was supporting its weight).

* Vestiges of the once massive hind legs still remain in most modern whales - just tiny useless internal bone structures that are "floating inside" their red meat flesh - not any more even attached to the back bone. In some of the later stages of the whale evolution fossils, these diminished in size former hind leg bones are still attached to the back bone but are 100% internal, so with little if any utility. Here is that almost to whale stage skeleton with the residual hip bones and greatly reduced hind leg entirely internal, but unlike the whale, these bone structures are attached to the back bone still.

Probably a still born baby that sank immediately into the bottom mud and was well fossilized.

 

[votingmachine]

There are some excellent answers here, but many that imply a direction to evolution. Evolution requires variation and selection. The variation is pointless variation (from genetic mistakes). The selection among variants is what leads to differentiation.

Generally, there is step-wise progression. If an entire chromosome is deleted or duplicated, there can be a multi-step change, but generally, single-steps accumulate variation, and sexual reproduction mixes variants.

Gills may be incompatible with lungs via a single step process. The process of acquiring dry-air tolerance by gills may be simpler than the process of acquiring lung tolerance for water (generally called drowning). Perhaps there has been a variant whale born that COULD have inhaled water, but that whale never forgot to hold its breath. And maybe that whale was particularly infertile.

The warm-blooded explanation makes a lot of sense. Whales require thermo-regulation. A whale that took in sea-water would drop in temperature. Again, evolution is step-wise. So if you need to develop a mechanism for oxygenation of blood, and a thermo-regulatory change simultaneously, the odds are long.

There also needs to be selective pressure. If a whale is born with a first step in a hypothetical path to gills ... say some blood-skin patch that oxygenates poorly ... then that genetic variant has to be passed on, and survive. As random genetic drift, there is a good chance that articular variant is lost. If it is preserved, then a second step has to happen to add to that first-step.

Google tells me that whales have existed for 35 million years, and that there are 84 current living species, along with another 400 that are extinct. If any of the 400 were on a path to gills, that soft tissue change is not available. Perhaps they all were and that particular path is always a dead-end.

A gill is a highly evolved structure, with a suite of genes involved. And there has to be a stepwise path to that structure that is not somehow also a selective problem.

Many mammals live in or around water (beavers, otters, seals, etc). A few live completely in the water (dolphins, whales). It is relatively easy to see how a sea-otter population could evolve to a "seal" population (better food in the water, greater predation on land, etc.). And to see how a seal population could evolve to a pure marine mammal (problem of child-biirth, but evading land predation, perhaps better food). The variation of adding fat layers, and mutations for water maneuvering can be simpler single-step mutations.

So in addition to the problems people pointed out here, where entire gills might be selectively disadvantageous (from cooling, from loss of bouyancy (drowning again!)), I would also point out that gills are variation-ally complex, and intermediate steps may be disadvantageous.

 

[D H]

It is a teleological idea that evolution has a goal, often relying on pre-evolutionary ideas of a "ladder of descent".

Given the rest of the cited post, I'm sure @Torbjorn_L would agree that evolution has no goal. There is no evolutionary god that thinks "Those whales would be better off if they had gills. Let's evolve some for them!"

Embryonic whales (and embryonic humans) have pharyngeal structures that are homologous to gill slits. Every once in a while those pharyngeal structures don't take the right developmental turn and a baby whale (or baby human) is born with things that look like gill slits. They are non-functional; a deformed baby whale with what looks like gill slits won't be able to breath through those things. The features that those pharyngeal structures should have become but didn't most likely would result in a quick death. There is no evolutionary pathway forward for an organism that doesn't produce offspring, or whose offspring don't produce offspring.

 

[D H]

The warm-blooded explanation makes a lot of sense.

Warm bloodedness has nothing to do with it. Sea snakes and sea turtles aren't warm blooded. While sea snakes are a relatively new appearance, sea turtles aren't. They've been around for a lot longer than have whales. Pliosaurs and ichthyosaurs also existed for a much longer span of time than whales.

Over the ages, a number of reptiles, birds, and mammals have returned to the seas. None have evolved gills.

 

[votingmachine]

Warm bloodedness has nothing to do with it. Sea snakes and sea turtles aren't warm blooded. While sea snakes are a relatively new appearance, sea turtles aren't. They've been around for a lot longer than have whales. Pliosaurs and ichthyosaurs also existed for a much longer span of time than whales.

Over the ages, a number of reptiles, birds, and mammals have returned to the seas. None have evolved gills.

It is a barrier. Mammals need to thermoregulate. Extracting oxygen from cold water also has a heat loss. You can say that has nothing to do with it, but I think mechanistically, if you ignore that heat loss, you are ignoring something important.

It is true that you could be cold-blooded and not evolve gills. That doesn't mean that being warm-blooded and not evolving gills is an exact equivalent barrier. But really ... I don't care. It is a difficult evolutionary path either way. I don't need to figure out which is the easiest of two very difficult paths. I think the one with thermoregulation is more difficult, but I don't know that as a fact.

It was pure speculation ... sorry.

EDIT: A sloppy analogy occurred to me.
People might give the following barriers why more people are not climbing Mt Everest:
1 It takes an expensive trip to get there.
2 It takes a lot of time away from work.
3 It takes a lot of fitness to do the activity

But: People that live there don't climb Mt Everest. Unemployed people don't climb Mt Everest. Some fit people don't climb Mt Everest.

Thermoregulations seems like a barrier that might be listed in the path to gills. Certainly not the only one. Possibly not a large one. Be careful in the logic of shooting it down. I'm not sure your logic shoots down warm-blooded-ness as a barrier, but it does show that the process is not one that is simple for any creature.

 

[julian kaye]

There is no forwards and backwards as regards evolution, its all about adapting to survival under the current conditions.

 

[KenJackson]

I've searched this whole thread, and as far as I can see, no one has mentioned "base pair", "nucleobase", "amino acid", or "DNA". Why not? They're key to the answer.

In order for a whale to have functioning gills, many many base pairs of it's DNA must change. How may? I have no idea, so let me guess a million.

To evolve gills, a whale would start with one base pair mutation. It would have to be a beneficial change or the laws of evolution would get rid of it. In due course, it's descendant would have another mutation. It too would have to be beneficial in and of itself or the laws of evolution would get rid of it. Repeat a million times. Now your whale has gills.

What are the chances that a path for a million individual beneficial base-pair changes exists?

Also important to note is that some things just can't evolve no matter what the circumstance.

Exactly!

 

[D H]

I've searched this whole thread, and as far as I can see, no one has mentioned "base pair", "nucleobase", "amino acid", or "DNA". Why not? They're key to the answer.

In order for a whale to have functioning gills, many many base pairs of it's DNA must change.

I said pretty much the same (without getting technical) when I wrote

Evolving gills would not be a backward step. For a whale to evolve gills would be an absolutely huge step forward. Evolution doesn't take giant steps. It can't.

Even though many reptiles, birds, and mammals have taken up an aquatic lifestyle, none of them have evolved gills. Pliosaurs and ichthyosaurs, like whales, were fully aquatic. The same embryonic structures that become gills in fish have been expropriated by evolution to serve other essential functions in tetrapods. Those could not be reappropriated to once again form gills; too many base pair changes need to happen at once.

Primitive gills evolved in tiny, slow moving animals. Primitive animals without gills absorbed oxygen from water and exchanged ions with water all over the surface of their bodies. The cube-square law says that while this might work for small animals (and for larger animals with a large surface area to mass ratio such as jelly fish), it won't work for larger, more compact animals. Evolution of a gill (even a primitive one) provided a huge advantage in the world of small, slow moving, osmoconforming animals.

 

[Astronuc]

Some interesting information on whale evolution
http://www.amnh.org/exhibitions/past-exhibitions/whales-giants-of-the-deep/whale-evolution
http://evolution.berkeley.edu/evolibrary/article/evograms_03
http://www.pbs.org/wgbh/evolution/library/03/4/l_034_05.html

It's interesting to ponder the whale shark, basking shark and megamouth shark and their evolution and environment. The whale shark can get very large, ~12 m and 21.5 te. The basking and megamouth sharks are smaller. All three move very slowly in the water. Whale sharks apparently prefer the equatorial regions, or middle latitudes, while basking sharks and megamouth sharks seem to prefer the high latitudes.
https://en.wikipedia.org/wiki/Whale_shark (There is a comment that the whale shark evolved about 60 million years ago, in contrast to the comment that whales evolved since then as inferred in the American Museum of Natural History article).
https://en.wikipedia.org/wiki/Basking_shark
https://en.wikipedia.org/wiki/Megamouth_shark

It seems that each of the different species has been successful in it's niche.

 

[votingmachine]

Some interesting information on whale evolution
...
(There is a comment that the whale shark evolved about 60 million years ago, in contrast to the comment that whales evolved since then as inferred in the American Museum of Natural History article).

Those comments are probably just juxtaposed strangely ... but the whale shark is not even closely related to whales. Sharks are a vary ancient creation of evolution, and do have gills.
http://www.arkive.org/whale-shark/rhincodon-typus/video-03.html

 

[Astronuc]

Those comments are probably just juxtaposed strangely ... but the whale shark is not even closely related to whales.

Nor did the statement infer that whale sharks are related to whales.

I was reflecting on

Apparently, breathing air with lungs enables much higher rates of oxygen exchange than breathing water with gills, which allows whales and other marine mammals to have higher metabolic rates than fishes:

Higher rates of oxygen would be needed by large animals. But here we see large whales with lungs, and a large shark, or whale shark, with gills, and both species, unrelated, have been successful.

The original question of "why haven't . . . ?" could have been asked, "why didn't . . . ?" Well they developed/evolved in parallel, although whale sharks apparently had a head start.

Lungs enable a greater rate of oxygen absorption, which enables faster metabolic rate, and ostensibly greater speed.
http://www.speedofanimals.com/animals/blue_whale (up to about 30 mph/ 48 kmph) but cruise at 12 mph (~19 kmph).
http://acsonline.org/fact-sheets/blue-whale-2/

Apparently the fin whale can reach about 35 mph. I'd like to verify that and the top speed of the blue whale.

In contrast, whale sharks have a top speed is only about 5 miles (8 kilometers) per hour, but apparently cruise at about 3 mph (5 kmph).

In terms of capability, gills would be disadvantageous, especially for large creatures.

 

[Ygggdrasil]

Higher rates of oxygen would be needed by large animals. But here we see large whales with lungs, and a large shark, or whale shark, with gills, and both species, unrelated, have been successful.

Part of the reason why whales can maintain a higher metabolism is that they are warm blooded whereas sharks and other fish are cold-blooded. As others in the thread have mentioned, this poses additional challenges for gills which would incur significant heat loss as opposed to lungs. So, there are multiple factors favoring lungs over gills for whales. Because sharks are cold-blooded, gills are not as disadvantageous to them as they would be for whales. Warm-blooded whales with lungs versus cold-blooded sharks with gills are two independent solutions to similar evolutionary pressures.

 

[Torbjorn_L]

Warm bloodedness has nothing to do with it. Sea snakes and sea turtles aren't warm blooded. While sea snakes are a relatively new appearance, sea turtles aren't. They've been around for a lot longer than have whales. Pliosaurs and ichthyosaurs also existed for a much longer span of time than whales.

Over the ages, a number of reptiles, birds, and mammals have returned to the seas. None have evolved gills.

That is an excellent point! On the other hand we have larval stages in amphibians that have evolved gills all over again, whether supplements or not. So it could (should) be harder for warmblooded animals. (Mammal fetal stages are constrained by having access to placental oxygen; )

 

[votingmachine]

That is an excellent point! On the other hand we have larval stages in amphibians that have evolved gills all over again, whether supplements or not. So it could (should) be harder for warmblooded animals. (Mammal fetal stages are constrained by having access to placental oxygen; )

I think the disagreement I have with that point is that the barrier is large. I might say there are many reasons why I can't jump over an obstacle, including my legs are out of shape. You might point out a snake cannot jump over it, so my legs have nothing to do with it. There can be different levels of difficulty.

There are many reasons why there are no gilled species with ancestral whales. And many reasons why there are no gilled species with ancestral sea turtles. Some of them may be the same. I hesitate to say that they must have the exact same answer set.

The question "why haven't whales evolved gills?" has many answers. It is a LARGE evolutionary change in morphology and biology. There is no correct answer. It is something that has not happened. We can surmise that evolutionary selection has selected against anything along that path, for many different survival reasons.