For plants and other organisms to develop differences among their respective species the alleles of their genes must contain variations. What is the original source of these variations?

mutation

The original source of these variations is Random Mutation

Explanation:

Evolution of new species in plants and organisms happen due to the variation happening in the alleles. Without genetic variation formation of new species is highly impossible.

Random mutation happens in the alleles when the mating happens between two different species. This out-breeding enables formation of new genetic characteristics which causes genetic variations.

The mutation happens In a way that the newly formed variant is able to possess two dominant characters inherited from the parents.

The resulting gene if perfect gets carried away to the subsequent generations leading to the formation of new species. If the hybrid genotype is not perfectly suitable for the living conditions it dies.