Question

What are the sources of inheritance variation?

Answer 1

@chumphrey Hmm. At what level are you asking? There two ways to think of this. First, let us look at mechanisms of evolution:

Mutation—random change in one or more of the base pairs in DNA
Genetic Drift—Changes in allele frequencies due to random factors (look up "drunkard's walk"
Migration—individuals leaving/going to a population that is not the population they were born in so to speak.
Natural Selection—A force that causes differential fitness in individuals of a population or species.

You may have noticed that even though I did not explicitly state this, genetic drift, migration, and selection require preexisting differences in individual genomes. This means that all* heritable variation must have originated via a mutation before the others can function.

*I am ignoring epigenetic factors. Epigenetics refers to factors that alter the expression of replication of DNA without changing the DNA sequence. Humans are a good example of this. Female sex chromosomes are XX and males are XY. That means females must have 2x as many X chromosome genes are males. In order to not have too many genes working, one X chromosome is "deactivated" so early in our development that all the cells in a human females body have the same X chromosome deactivated. This is accomplished through means that do not alter DNA sequences.

Now, back to epigenetics. A few of these epigenetic things can be inherited.