At any given point in time it will with great certainty NOT be the most fit that are the most successful.
This is a manifestation of the difference between optimization and survival.
When an individual or agent (that be an institution, country or animal) optimizes he or she does so given the prevailing environment in order to do well. Given that such agents solely use current circumstances as information to guide their decisions, such agents will in all likelihood do very well. They will do so because they fully utilize all resources available in an "efficient" manner where no redundancies exist in their behavior or bodies.
But; when circumstances change dramatically over a short period of time such optimizing agents die. They die because they are fragile to rapid change that creates environments that in no way resembles the ones they optimized over. To rephrase this: when the world rapidly changes around them they die because they are not ready for the changes.
This hints at why many people misunderstand the concept of "survival of the fittest". When this term is thrown around is it often used to in relation to success. However, there is a great difference between survival and success (!). Success is a reference to someone or -thing doing well. Survival is someone or -thing existing. The latter has to do with being fit and surviving adverse shocks, i.e. surviving a spurt of rapid change, the former has to do with doing well under current circumstances.
Now; a necessary condition of having success is having survived so long it is possible to have it. That is one needs to not have died before success is even possible. But in order to be the MOST successful at any point in time one needs to take full advantage of current circumstances - i.e. reduce redundancies that are irrelevant to the current environment. Reducing the redundancies leads to two things: success and fragility. Success because one now uses available resources without "wasting" them on things that are irrelevant to current circumstances, and fragility because smaller changes now can kill. As increased fragility decrease the chance of surviving adverse shocks it is clear how the most successful at any given point in time with great certainty is NOT the most fit.
The implication for economics is this: at any given point in time it will not be the most popular theories that stand the greatest chance of surviving the test of time. Learning what is not mainstream thus possibly have a much higher value than is currently thought.