During which process does genetic variation most commonly increase due to the introduction of new genes?

Genetic variation increases most significantly through the process of migration, which involves the movement of individuals from one population to another. When individuals migrate, they can introduce new alleles and genetic traits into a different gene pool. This influx of new genetic material enhances the overall genetic diversity within the receiving population.

Migration can occur through various means, such as animals moving to new habitats or plants' seeds being dispersed by wind or water. As a result, this process plays a crucial role in evolution, as the introduction of new genes can lead to adaptations and variations that may increase the chances of survival and reproduction for the organisms involved.

In contrast to migration, natural selection works with existing genetic variation by favoring certain traits that enhance survival; mitosis primarily results in identical cellular copies and does not promote genetic diversity; and apoptosis is programmed cell death, which does not contribute to the introduction of new genetic material. Thus, migration stands out as the key mechanism by which genetic variation is increased through the introduction of new genes in populations.