How does sexual reproduction generate genetic variation?

The process of sexual reproduction involves two parents, each contributing one gamete. Gametes are produced by a process called meiosis, which starts by the duplication of the chromosomes, followed by two rounds of cell divisions and halving of the chromosome number. Gametes have half the chromosome number of other adult cells  of an organism. A key feature of meiosis is the exchange of chromosome pieces which occurs in the first division of this process, called recombination or crossing over. Recombination is a very important source of genetic variation between individuals of sexually reproducing species, and the driving force for the process of natural selection.