Does mutation decrease or increase genetic variation

Mutations are changes to an organism's DNA and are an important driver of diversity in populations. Species evolve because of the accumulation of mutations that occur over time. ... This mutation has introduce a new allele into the population that increases genetic variation and may be passed on to the next generation

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.

this is more of a statement than a question. is there answers to go along with this, or is it opinion based?