Why is crossing-over considered to be a driving force of natural selection?

Crossover is the first way that genes are shuffled to give rise to genetic diversity. Crossover takes place in sexual reproduction. Chromosomes line up side by side and break off pieces of themselves, then trade those pieces with each other. When they break at the same place (locus) in the sequence of base pairs, the result is an exchange of genes called genetic recombination. That is the normal way for crossover to occur. Genetic recombination ensures that the daughter cells produced have a different genetic makeup from the parent cell and thus diversity is created.