Survival of the fittest was Darwin's legacy and fault. The strongest shall prevail and survive in the jungle out there and knock out the competition. They are the ones who get the girls, lots of prestige and money - or the equivalent thereof.
However, this train of thought has been twisted and contorted over the last centuries. For example, Nietzsche drew his own conclusions by claiming that one should avoid compassion for the weak and needy as it was a waste of time and energy to help the poor and mediocre. And all of this derailed into its chilling culmination in the Nazi quest for a superior race and the brutal and horrifying atrocities made under the guise of this deeply flawed and inhumane ideology.
Such ideas, however, had already existed even long before Darwin's observations. The ancient Greek philosopher Plato preferred a republic of strong and resourceful people with healthy offspring, while the deformed, feeble-minded and the ones with a short attention span should be "removed" from the eyes of society so as to ensure that they did not propagate their so-called "mediocrity".
To modern ears these are horrible claims, but we have to keep in mind that we have evolved greatly since, especially when it comes to the exploration of individuality, freedom and equality. We still may believe in the survival of the fittest, but the game has changed and the stakes are different.
In today's world it is not so much about physical strength nor beauty (though that still makes a difference), but what seems to be more important is strength of mind, character and personality.
Fit nowadays is not a simple statement about physical condition. The fact that you can run a marathon or swim a mile does not necessarily make you a superior person. For one, your athletic ability won't necessarily land you a job or give you an important position in society. True, we all have a certain admiration - not to mention pride if they happen to be from our own country competing in the Olympics - for the athlete's prowess, but athletes are not the highest echelon of society.
Similarly, jocks may be popular during high school especially among the girls but no one in their right mind would give them the highest and best paid positions in their company.
Intelligence doesn't always cut it either, at least not on its own merit. People with philosophy majors are still caught bussing tables, much more so during the global crisis. This is something that existed even in Plato's times when philosophers lacked the respect they deserved and which is why Plato envisioned an ideal state with philosopher-kings at its top.
So what are the most important characteristics for being considered "fit" in the eyes of the modern world? A most recommendable skill is being street-savvy. Those who are street smart know how to get by, know how to approach and talk to people, they instinctively know how to network actively and how to sell and market their skills, they know how to win friends and influence their uncles: Those are the ones who end up doing well by society's standards.
If this skill is backed up with an educational degree and official certification, then so much better for you. If it is followed by lack of scruples, the paths of success become even wider.
It's a competitive cruel world; it's a jungle out there. It has been so since the beginning of mankind's history. Cain killed his own brother Abel. Neanderthals fought off each other with rocks. It's just that the game for survival gets tweaked and adjusted with each new generation and the meaning of "fit" depends on the context and the times.