What is the central idea of evolution?
The basis of evolution as explained by Darwin, is natural selection. In other words, nature permits the survival of those organisms that have an edge over their counterparts of the same species. This extra edge may be a long neck, as in the case of a giraffe or even bright coloration for mating purposes.
These variations are the result of random mutations occurring in the genetic material of the organism. When these variations turn useful, they get passed down to the next generations thus getting established in the species in a larger scale. (Remember, the individuals with these mutations survive well - survival of the fittest)
These mutations accumulate over time, in around millions of years, to EVOLVE into a new species very different from its parent million years ago.