But to claim "nature selects the fittest" is far from explaining where the "fittest" come from.
The fittest comes from natural variation.
Take a look around you next time you walk down the street.
Are all people the same height?
Are all people the same weight?
Do all people walk at the same speed?
No, of course not.
Now, if we lived in a world where there was an ever present death field that caused your head to explode, that was placed exactly six feet off the ground, then people who have a height of less than six feet become the fittest to survive.
Likewise, if there was some virus that caused people to implode into a singularity when the reached a weight of exactly 120 pounds, then people who have weights less than 120 pounds become the fittest to survive.
Finally, if there was some ubiquitous predator whose speed was limited to 3m/s that wasn't afraid of cities, then people capable of moving at speeds in excess of 3m/s would become the fittest to survive.
The other thing is, in all three of these scenarios, it's possible to disprove natural selection, because natural selection predicts that because we're eliminating the herditary traits that lead to tall people, heavy people, or slow people (respectively), that those traits should become rare, or extinct. If, however, in a few hundred generations, Holland still had as many people in talled than six feet as they do today, natural selection would be falsified.
The moral of the story is that fitness is determined by environmental pressure acting on natural statistical variation.