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Crocodiles have supposedly remained unchanged for millions of years, and several other species are considered as "living fossils". How do such species remain so constant over time given that they will have had so much time to accumulate new mutations?

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marked as duplicate by Christiaan, March Ho, Jack Aidley, kmm, James Feb 29 at 3:33

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I'd add that although they may appear morphologically the same as their ancestors they may well be substantially genetically different. "Unchanged" on the surface does not necessarily mean the same under the hood. – James Elderfield Feb 26 at 11:53
Molecular evolution != phenotypic evolution! – Vérace Feb 27 at 10:38
up vote 30 down vote accepted

Evolution is a process of change by four mechanisms; mutation, migration, drift, and selection.

You are correct in thinking that, because crocodiles have been around for a long time, they could have accumulated many new mutations in that time, relative to other more recent species. However, mutation is only one of the important mechanisms underlying evolution.

How different are ancestral and modern crocodiles?

It seems the appearance of crocodiles has been fairly unchanged since their occurrence ~85 million years ago (mya).

How can they remain so unchanged?

Genetic variation may have been low in the ancestral population, this would reduce the potential for evolutionary change, as most change would have to occur through new mutations. It seems the populations of ancestral crocodiles were quite small, such that a genetic bottleneck may have occurred (which would reduce genetic variation). Mutation rates seem to be relatively low in crocodiles (also see here) which would reduce the rate at which novel mutation occurs, reducing the potential for evolution.

Note that many mutations will be neutral in their effect (or "synonymous") so won't have an obvious phenotypic effect, so there may be substantial evolution at the genetic level despite the phenotypic similarity.

Low rates of evolutionary change could suggests some other things may have also played a factor. Given that the populations have been through multiple genetic bottlenecks, genetic drift could have slowed down rates of evolution eroding genetic variance, removing rare mutations from the population.

If selection has been fairly constant over time then there is less chance that changes will occur. If selection were to change and favour new adaptations then these are likely to spread, but if selection remains fairly constant over time then it will continue to favour the same mutations. After a long time of consistent selection it is likely that most mutations will be deleterious (have a negative effect) and be removed from the population by selection. Darwin suggested that living fossils could occur because the environment they are in has remained fairly constant (from this link).

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When the species is not affected by disease, The species has masterd the environment and the mutations should entirely cease. The salt water croc has a incredible immunity and a wide variety of food – user5434678 Feb 26 at 13:59
@TheVoid - diseases have basically no effect on mutations. What are you even talking about? – Davor Feb 26 at 14:41
@TheVoid Mutations don't happen on purpose, they just do. They are not trying to make any species better. – Tomáš Zato Feb 26 at 15:35
The quality of a crocodiles immune system won't have any effect on mutation rates. If the immune system is very well adapted then most mutations will be deleterious and selected away, but mutations will still occur – rg255 Feb 26 at 15:40
I think The Void wants to say that a perfectly (as far as biologically/physically possible) adapted species won’t change because all mutations would only yield a disadvantage (so there would be no permanent mutations). Because of that there would even be a strong selective pressure against mutations once a species has reached that point. – Michael Feb 27 at 14:05

Crocodylomorpha were actually once a lot more varied than they are today, so their group isn't immune to change or evolution. picture by my uni lecturer!

The tongue in cheek answer is to say, there were a lot more forms of crocodile in the past so chances are the single form we see today would look like one of them!

A better answer is: Species often evolve to fit a niche, they become specialised in their form both externally and internally. As long as this niche stays the same and neighbouring niches remain filled, the species will only become more suited to its niche. Over time you'd expect new mutations which made an individual fitter to become rarer, or have a smaller impact on survival so be selected for less strongly, thus the longer a niche exists, the more stable the form of animals living in it will become.

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Looking at adapting to niches from the other side, we could ask why dolphins look so much like ichthyosaurs. – jamesqf Feb 26 at 19:00
I particularly like your first paragraph in your "better answer" - it's a good clear and simple summary of the effect of constant selection! (+1) Do you he any references and links to external material to add so people can follow it up with further reading if they want? – rg255 Feb 26 at 23:19
That's a good point. – David Blomstrom Feb 27 at 1:05
@rg255 Unfortunately I didn't have any particular papers in mind when writing that answer, I could probably dig out my old uni notes and find the lecturers references if you were really interested? – Troyseph Feb 28 at 20:22

I think in the case of crocodiles in particular, this is a question of body type. Crocodiles have fixed jaws, meaning that they have lost the mechanism used to move their lower jaw from side to side. This simplification means that they can exert a huge amount of power, making it a great adaptation for their particular strategy of hiding in water and ambushing large land mammals that drink from it.

However, it also means that it's very hard for them to adapt to any other niche. Anything that would require eating smaller prey, or eating on land, would be untenable, because crocodiles can't chew. (Instead they close their mouth and rotate their whole body in the water to rip chunks of flesh off.) They can't easily re-evolve the ability to chew, because the mechanism needed to do that is pretty complicated, so their fixed jaw is more or less "locked in" evolutionarily.

So crocodiles have remained constant because they are very well adapted to a particular niche; because they've lost features that would allow them to adapt to other niches; and because the niche they occupy has been around for a very long time.

Personally I think these are the primary reasons. The low mutation rate mentioned in the other answers seems to me more likely to be effect than cause. If you are very tightly adapted to a particular niche then there is less advantage in genetically exploring other possibilities, so the descendants of individuals with lower mutation rates would have an advantage over those with higher mutation rates. This would provide an evolutionary pressure for a low mutation rate, which might explain the observation. (But this last paragraph is speculation on my part. I work on modelling evolution and I know that this kind of selection on mutation rates can occur, but I have no evidence at all about whether it happens in crocodiles.)

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I think the "fixed jaws" argument is a weak one: if they evolved jaws which are more fixed then they can evolve ones that are less fixed, and you're whole answer hinges on that argument. You also have no sources and say nothing direct about genetic drift or consistent/variable selection (niche stability). As it stands this is a very opinion based answer and has questionable scientific content. – rg255 Mar 2 at 8:09
@rg255 to answer your first point, it's typically much easier to lose complex features than to evolve them in the first place. E.g. vertebrate eyes evolved only once but have been lost but have been lost many times. (I don't know if a species exists that is known to have lost its eyes and then un-lost them, but I very much doubt it exists.) If the developmental pathways responsible for the jaw hinge mechanism have been lost they are very unlikely to reappear. On your second point I can only plead guilty to the lack of references, but I did mention the other two things, albeit briefly. – Nathaniel Mar 2 at 8:31
(The reason I only covered them briefly is that you already covered them in your answer. This was only intended as a complement to yours.) – Nathaniel Mar 2 at 8:36

I think rg255 and Troyseph pretty much nailed it, but another thing to consider is the crocodilian's habitat. All surviving forms are aquatic, with at least one species - the marine crocodile - at home in the sea. In addition, most, if not all, crocodilians live in tropical or subtropical regions.

In fact, many "living fossils" are associated with tropical forests or the sea. The marine coelacanth is one of the most celebrated living fossils, for example. The most primitive living fishes also include the lungfishes, which are generally freshwater tropical species.

The tropical monotremes are considered the most primitive living mammals (and the platypus is semi-aquatic to boot).

Their niche and "biology" also make it relatively difficult for crocodiles to exploit other niches. Even if they were't tied to water, one could hardly imagine a crocodile climbing over a high, cold mountain pass to reach a lush, tropical forest on the other side.

It's worth noting that all the terrestrial crocodilians have died out, leaving only their aquatic or semi-aquatic relatives.

Here's one interesting source -- 12 of the most astounding 'living fossils' known to science.

Most of the species listed (including the coelacanth) are marine organisms. It also lists crocodiles, and I was surprised to learn that crocs evolved from marine organisms themselves. (See This handsome sea creature is where crocodiles came from).

Similar lists include tropical forest creatures, like the African okapi and Australasian cassowary. Note that rhinoceroses and tapirs now occur only in the tropics (and perhaps subtropical regions for African rhinos), now that the more recently evolved woolly rhinoceros is extinct.

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Do you have any sources to add to this? – rg255 Mar 3 at 8:21
So you're saying that living fossils tend to inhabit large niches that are unlikely to disappear completely at any point in history? – Troyseph Mar 10 at 10:00
I wouldn't call them "large niches" so much as consistent niches. For example, the temperature in the sea doesn't vary nearly as much as it does on land. It really can't get any colder than 32 degrees fahrenheit, or the water simply freezes and floats to the surface. During the Ice Age, much of Eurasia and North America were scoured by glaciers. But tropical forests remained tropical forests, even if the temperature was a little cooler. I think such places are sometimes termed refugia ("refuges"). – David Blomstrom Mar 10 at 13:22

Well, two things.

First the assumption that creatures that are living fossils are strong as a species isn't strictly true. Evolution; especially in relatively long-lived animals like crocodiles and sharks (compared to dogs, houseflies, j-walkers, etc.) is such a long process. It isn't factually verifiable that they are doing well as a species, only that they are in a stable food chain.

Second; and User23715 raises all these point, animals that aren't exposed to evolutionary motivators (mutagens, predation, and habitant change) won't have as much genetic divergence. A crocodile won't benefit from a Darwinian advantage if it doesn't provide enough of a survival advantage to eventually fuel a divergent evolution. Any dominant trait needs to be aggressive enough to stay dominant. In a simple dominant-recessive model it would need to be twice as potent for hundreds of generations (close to 2000 years for crocodilian life spans and birth rates).

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Welcome to Bio. Could you add sources to your claims? – Christiaan Feb 27 at 9:15

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