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This question already has an answer here:

It seems at first glance that it would be an evolutionary disadvantage for a sea creature to have to come up to the surface on a regular basis in order to breathe, so why are there animals (e.g. whales and dolphins) which still need to do so?

If there is some advantage to breathing from the air instead of using gills, why haven't animals such as sharks (or indeed fish in general) evolved to do this instead?

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marked as duplicate by Remi.b evolution Apr 20 '18 at 20:17

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    $\begingroup$ Just to be sure; are you aware of the evolutionary history of whales (i.e. their phylogenetic relationships)? $\endgroup$ – fileunderwater Feb 23 '15 at 15:27
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    $\begingroup$ @Cliff Whales and dolphins are not just land animals; they are mammals (in fact they are phylogenetically related to hippopotamus). Gills are not present in higher animals. $\endgroup$ – WYSIWYG Feb 23 '15 at 16:50
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    $\begingroup$ @Cliff It is because whales would require too many mutations to restructure their respiratory and cardiovascular system to develop back gills whereas they could survive in an aquatic environment with lungs and few mutations. The likelihood of selection of that many mutations without an obvious advantage is too less. $\endgroup$ – WYSIWYG Feb 23 '15 at 20:13
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    $\begingroup$ Another part of the answer is that it's probably a lot more difficult to re-evolve gills than to evolve the simple streamlining &c that is necessary to become air-breathing aquatic mammals. There'd also seem to be no real pressure/advantage, since air-breathing whales, dolphins and so on are pretty successful top-of-their-food-chain creatures. $\endgroup$ – jamesqf Feb 23 '15 at 20:22
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    $\begingroup$ Looking at deepsea fish with gills, they tend to be sluggish and at a bare minimum of activity/expenditure due to the utter lack of oxygen at depths whales are known to frequent. So you have these enormous marine mammals, with enormous concentrations of myoglobin packing literal tons of oxygen, it elicits a much greater capacity for hunting etc. at greater depths. There are advantages to lungs in whales. $\endgroup$ – CKM Feb 23 '15 at 21:53
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I think you're looking at this the wrong way.

Animals evolve to fill niches. Many, many land animals returned to the water as a place to live, including:

whales, dolphins, porpoises, seals, sea lions, walruses, manatees, dugongs, otters, hippos, pygmy hippos, crocodiles, sea snakes, sea turtles, river turtles, river snakes, etc. etc. etc.

Their success in this environment points to no need whatsoever to "evolve" gills to be better suited. Without the interference of man, there would be yet many more of them.

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    $\begingroup$ Not to mention penguins (and should we count diving birds like ducks?), and in prehistory the ichthyosaurs, plesiosaurs, pliosaurs, mosasaurs, and probably more that I can't remember. Becoming an ocean-going air breather seems to be a pretty successful ecological niche. $\endgroup$ – jamesqf Feb 24 '15 at 17:58
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    $\begingroup$ Gills are a really inefficient means of obtaining oxygen, while you have some cold-blooded creatures on your list, warm-blooded animals have an oxygen requirement that probably could not be supported by gills. Also, we have pharyngeal arches during development. Our developmental pattern produces ears and parts of the jaw, and fish get gills. I think it is less about no need than it would be unsustainable for a warm-blooded organism to try and supply its oxygen requirement with gills. $\endgroup$ – AMR Oct 7 '15 at 18:15
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    $\begingroup$ @AMR - you may have a point there! My point is that evolution is influenced by selective pressure, not "(un)common sense." Land animals returning to the sea started with lungs, and kept them. Fish started with gills, and kept them. You state, "it would be unsustainable for a warm-blooded organism to try and supply its oxygen requirement with gills." (The great white predator seems to do fine. It is still about niches in my mind. However...) If you could add an answer that covers your point, the OP might get all the information they need. $\endgroup$ – anongoodnurse Oct 7 '15 at 18:20
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Natural selection can only choose from the diversity available. Sometimes there are physiological or other constraints that prevent a solution we think should be optimal to arise. For instance, insects are the most diversified group of animals, but there are no marine insects. Why? It seems their respiratory tract (a tracheal system) can't deal with salt water. Or we can think of flying as something very advantageous, but flight has only evolved four times (insects, pteurosaurs, birds and bats; or five, if we include our flying machines). So "you can't always have what you want".

The precursor of gills can yet be seen in the early stages of mammal embryological development (they lead to the channel that allows you to unclog your ears by swallowing). But to turn them back into fully functioning gills would take so many mutations and the re-adaptation of so many working systems, that it's not a much viable option for large animals such as cetaceans. Remember that the gills first formed in the very first, very small vertebrates, and slowly acquired it's present shape we see in bigger fishes.

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Welcome to evolution.

Just because the idea is great, the path obvious (in our minds), does not mean it will happen.

Evolution works by random mutations followed by selection to weed out nearly all phenotypes that are less successful than the original.

So you need both the mutations and right selection environment.

So while gills on a sea going mammals is a great idea. The path towards that is not an easy outcome without some form of intelligent design. You see those gills arches fishes have to support their gills... well the tetrapod lineage has since re-purpose those bones to make several structures such as the thyroid gland, part of the jaw, the larynx, and the bones in mammalian ear. https://en.wikipedia.org/wiki/Branchial_arch

So a whale that simply reconverts to gills, would be without a larynx (mute), or inner ear (bad hearing) and parts of the jaw (Can't eat). So you can see alot of other work will need to be done to make such an animal work. And if this process is left to nature, each intermediate must also work successfully in some environment.

Such are the constrains of evolution.

However if intelligent design were available. No problem. It can be done. We can ignore temporary lowered fitness, and work with a desired outcome in mind.

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