Which phenomenon contributes least to genetic diversity?

Cloning contributes least to genetic diversity because it involves the process of producing genetically identical organisms from a single parent. This method relies on asexual reproduction, where the genetic material is copied without the introduction of variation that occurs in sexual reproduction or through processes such as mutation. As a result, individuals produced through cloning have the same genetic makeup as the original organism, leading to a lack of variability within the population.

In contrast, mutation introduces new genetic variations into a population's gene pool. Gene flow enhances genetic diversity through the exchange of alleles between populations, while genetic drift can lead to changes in allele frequencies due to random sampling of organisms, especially in small populations. Therefore, while other processes contribute to increasing genetic diversity, cloning does not, making it the least contributing factor.