Large number of variations arise in natural populations of...

In natural populations of organisms, a large number of variations arise primarily due to gene recombination during sexual reproduction.

While mutations are the ultimate source of new genetic material (alleles), gene recombination shuffles these existing alleles into new combinations in every generation. This occurs through crossing over during prophase I of meiosis, independent assortment of chromosomes during metaphase I, and the random fusion of gametes during fertilization. These processes collectively generate a vast amount of genetic diversity among individuals in a population.

Answer: gene recombination