The force in nature which distinguishes between unfit and fit individuals is called natural selection.
What is natural selection?
Natural selection is the process in which nature chooses which organisms will survive and which will perish.
Variations are present in all the organisms in a population. Some have better traits than the others.
The organisms which adapt and change according to the environment have better chances of survival.
Darwin gave the term ‘Survival of the fittest’ that operates under natural selection and drives the evolution of a species.
Organisms which have genes better suited to the environment survive and pass these genes to their next generation. Eventually the population consists of only those organisms having that gene.
An example of natural selection is Darwin's finches.