Which evolutionary process involves the slow addition of allelic variation that increases the hereditary diversity of populations, ultimately leading to evolutionary change?

Mutation

Explanation:

Mutation is a change in the nucleotide sequence by addition, deletion, substitution, etc which results in the slow addition of allelic variation that increases the diversity in a population and leads to evolutionary change.

The right mutation in the allele or gene is favored by nature and it is passed in the next generation through reproduction. Mutation helps in the survival of a population in the continuously changing environment by adding new alleles that increase the fitness of the organism. So the correct answer is mutation.

it creates acid rain