Neo-Darwinism versus Darwinism

What is Neo-Darwinism about?

When Darwin proposed his theory, he did not account for 'why are there variations in a population?'. His theory was based on observations that he made and with the underlying clause that there is always variation in a population.

In Neo-Darwinism, with the knowledge of molecular biology, Darwin's theory was expanded to account for 'why are there variation in a population' for a complete picture which we have already done so in tutorials:
1) heritable differences: meiosis (crossing over + independent assortment) + random fusion of gametes
2) mutations (gene and chromosomal)