Why haven't whales evolved gills?
Why should they?
Because they are mammals and they breath in a different way.
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...
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.
As terrestrial mammals, there would have been no creatures with gills in their family tree.
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.
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.
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.
Do you have any funny uncles who resemble this critter?
There are similar structures on the vertebrae of many mammals, including humans:
This doesn't mean that human ancestors had sails popping out of their backs.
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
(Picture of one of my "funny cousins.")
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 greatm 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 ;
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.
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.
I would think that because whales are aquatic animals having gills would be an advantage. I guess that's not the case.
Yeah, that's kind of the definition of a "naive assumption," aquatic animals=gills. But now you have 15 posts that educate you otherwise
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.
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.
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.
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.
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