As far as
evolutionary biology goes,
fitness is entirely a measure of your ability to
reproduce and pass on your genes. An
organism that is incredibly
strong, lives for years,
slaughters all that stand in its way and in the process produces two
offspring is less fit than another member of the same
species that's unable to
fight at all, dies when it's half the age of the other but produces four offspring. The
fittest do not necessarily
survive, but their
genes do.