The original source of all genetic variation is the original source of all genetic variation is sexual reproduction independent assortment mutation natural selection recombination

The answer for the above question is Mutation. 
Mutations are random spontaneous changes that occur suddenly in the DNA. A single mutation can have a large effect, however in may cases evolutionary changes are based on the accumulation of many mutations. The gene flow is any movement of genes from one generation to another and is an important source of mutation 

When partially digested carbs reach the colon, bacteria convert part of that food into hydrogen and carbon dioxide gasses