Natural selection and survival of the fittest are concepts on which Darwin's theory of evolution is based.
There is a difference in the genetic makeup of every organism which makes its physical characteristics distinct from other organisms, even of the same species. It is the genetic makeup that defines the physical traits that an organism has.
During sexual reproduction, the genes of those organisms that have a better ability to survive have a higher probability of being passed on to the next generation than those of organisms that are less likely to survive.
Evolution is based on the fact that the off-spring which survives is that which has inherited the genes of the fittest organisms. In this way the genetic traits of a species are constantly being altered and improved, to increase the chance of survival in the environment its lives in.