Here's why:

* Crossing over: This is a key process in meiosis where homologous chromosomes exchange genetic material. It's a major driver of genetic variation within a species.

* Independent assortment: During meiosis, chromosomes align randomly, leading to different combinations of maternal and paternal chromosomes in gametes. This contributes significantly to variation.

* Mutation: Mutations are changes in the DNA sequence. They can occur spontaneously and introduce new alleles into a population, increasing variation.

* Gene flow: This refers to the movement of genes between populations. When individuals migrate and breed, they introduce new alleles, increasing variation within the receiving population.

Therefore, all these processes are naturally occurring methods of recombining DNA within a species to increase variation.