[trimethylpurine]

>I've heard it said that the tagline for the theory of evolution should really be "survival of the most adaptable"

That is the tagline. In the phrase, "survival of the fittest," "fittest" means "most adaptable," at least according to the Merriam Webster English dictionary.

[MattPalmer1086]

I don't think that is what was meant by the phrase. Ultimately it refers to how natural selection works.

The adaptability of an organism might be an important fitness factor, or it might not. There are organisms which have survived largely unchanged for millions of years.

[_nalply]

Perhaps because they were lucky? What they needed was available for millions of years. Note the survivorship bias there. I am sure you already know it, so please excuse me pointing out this obvious thing.

[MattPalmer1086]

Sure, could be their environment simply didn't change. In which case, a simpler, less adaptable organism may be fitter than a more adaptable one (if the cost of adaptability was higher).

[Jensson]

Species go extinct all the time, there are always very unadaptable species around and most of them go extinct pretty soon after they become unadaptable because environments change all the time in the grander scale of things.

[MattPalmer1086]

Right, fitness can only be understood in the context an organism is in at the time. There is no long term plan.

An adaptable organism might stay around for longer - or it might be outcompeted in the short term by less adaptable organisms.

[tejasvi88]

Over time, being adaptable is what really matters for survival. If something could adapt perfectly, it’d always come out on top.

Sure, “survival of the fittest” in the short term is obvious, but it’s hardly worth pointing out.

[trimethylpurine]

I might be missing it.

>or it might not

To determine that it is not, would by definition imply an observation that it is no longer fit.

In other words, how do we know that it is not adaptable? We'd see that it no longer propagates, and is therefore unfit.

Can you think of a case where these two synonyms are not inseparable in meaning as pertaining to evolution?

I can't. I think Darwin meant exactly this.