Sexually reproducing organisms pass half of their genetic information on to their offspring.

1/2 from the mother (egg), and 1/2 from the father (sperm).

Since 1/2 is from each parent the offspring is thus not identical to the parent but rather resemble both of the parents

Crossing over- is a process in gametes in each parent which cause variation in the offspring.

After crossing over the gametes then combine their instructions = Genetic Recombination

This recombination occurs during fertilization.

By having such genetic recombination natural selection is thee outcome.