What does 'Survival of the Fittest' mean?
Survival of the fittest is a phrase used as a way of describing the mechanism of natural selection. It is more commonly used to describe the greater probability that fit individuals will survive a certain test while the unfit won't. The fit meaning the individuals that are more adapted to that environment.
How is it different from natural selection?
'Survival of the Fittest' is a phrase used to describe the mechanism of Natural selection itself. Although they are similar, natural selection is more focused on reproduction while 'Survival of the Fittest' is more focused on survival. Natural selection is more accurate because although a 'fit' organism with good traits may live longer, it may not reproduce and pass it's genes.