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How is it that an organism like this can remain almost wholly unchanged for 140 million years? Is it not subject to genetic drift like everything else on the planet?

Sure it is, and of course the modern coelacanth is not the same species as the ancient fish - it has changed slightly since then.

Why not more change? If you're already well-adapted to your environment, and the environment doesn't change, then it becomes increasingly likely that any change will make you less well-adapted, and such changes will be selected against. So therefore, creatures that stick with the tried-and-true body plan will preferentially survive over radical newcomers who are less well-adapted, resulting in organisms that don't appear to change much over time.

It's not just coelacanths, either - sharks, cockroaches, dragonflies, ferns, and lots of other things haven't changed much in the last hundred million years either. If you have something that works, there's a strong incentive not to change.

Sure it is, and of course the modern coelacanth is not the same species as the ancient fish - it has changed slightly since then.

Proof of that? I thought that the main criterion for speciation was the ability of creatures to mate and produce viable offspring. As there is no way for a modern coelacanth to be paired with one that's 140 million years old, such an observation is speculative at best. By appearance alone, one is forced to conclude that the species has remained practically identical to its fossilized progenitors.

Why not more change? If you're already well-adapted to your environment, and the environment doesn't change, then it becomes increasingly likely that any change will make you less well-adapted, and such changes will be selected against. So therefore, creatures that stick with the tried-and-true body plan will preferentially survive over radical newcomers who are less well-adapted, resulting in organisms that don't appear to change much over time.

I've heard that argument before, but it doesn't answer the question of why this creature in particular, was not subject to the genetic drift that seems natural in almost every other species.

It's not just coelacanths, either - sharks, cockroaches, dragonflies, ferns, and lots of other things haven't changed much in the last hundred million years either. If you have something that works, there's a strong incentive not to change.

I appreciate that argument, and it may be true in the cases of all the creatures you mention above. However, the coelacanth is not exactly a model of evolutionary success that may be found spread over the whole world in vast numbers. In fact, they don't seem to be very successful at all, consigned as they are to a few tiny colonies.

In this case, you must argue either a.) the coelacanth has managed to exist into modern times because it is perfectly adapted to the few tiny environments in which it is currently found and has been able to maintain this for over 140 million years without any competitors or predators forcing it out (actually 400 million, but what's a few hundred million years between friends?). Or b.) there are actually large undiscovered populations of coelacanths scattered throughout the world, making it a fantastically successful species similar to sharks and cockroaches.