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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5$\begingroup$ 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. $\endgroup$– James ElderfieldCommented Feb 26, 2016 at 11:53
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$\begingroup$ Molecular evolution != phenotypic evolution! $\endgroup$– VéraceCommented Feb 27, 2016 at 10:38
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4 Answers
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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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$\begingroup$ 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 $\endgroup$ Commented Feb 26, 2016 at 13:59
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6$\begingroup$ @TheVoid - diseases have basically no effect on mutations. What are you even talking about? $\endgroup$– DavorCommented Feb 26, 2016 at 14:41
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6$\begingroup$ @TheVoid Mutations don't happen on purpose, they just do. They are not trying to make any species better. $\endgroup$ Commented Feb 26, 2016 at 15:35
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5$\begingroup$ 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 $\endgroup$– rg255Commented Feb 26, 2016 at 15:40
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1$\begingroup$ 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. $\endgroup$– MichaelCommented Feb 27, 2016 at 14:05
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Crocodylomorpha were actually once a lot more varied than they are today, so their group isn't immune to change or evolution.
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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1$\begingroup$ Looking at adapting to niches from the other side, we could ask why dolphins look so much like ichthyosaurs. $\endgroup$– jamesqfCommented Feb 26, 2016 at 19:00
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2$\begingroup$ 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? $\endgroup$– rg255Commented Feb 26, 2016 at 23:19
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$\begingroup$ @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? $\endgroup$– TroysephCommented Feb 28, 2016 at 20:22
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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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$\begingroup$ 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. $\endgroup$– rg255Commented Mar 2, 2016 at 8:09
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$\begingroup$ @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. $\endgroup$– N. VirgoCommented Mar 2, 2016 at 8:31
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$\begingroup$ (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.) $\endgroup$– N. VirgoCommented Mar 2, 2016 at 8:36
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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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1$\begingroup$ Welcome to Bio. Could you add sources to your claims? $\endgroup$– AliceD ♦Commented Feb 27, 2016 at 9:15