Why haven't whales evolved 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.

 

[DaveC426913]

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.

Perhaps not mutually distinct, but many animals have multiple respiratory systems. The common Bettafish and the Gourami have what's called a labyrinthine organ, allowing them to breathe air. (In fact, they must breathe air, or they will drown. They still use their gills, it's just that the gills aren't sufficient.)