What is the source of genetic variation? in other words, where do the tiny changes in individual organisms come from, that allow a trait to be passed on to the next generation?

The right answer is it comes from mutations that occur in the genome.

Genetic variation is due to changes in genetic composition, to differences from environmental or endogenous factors.

A mutation is a modification of the nucleotide sequence of  DNA, due to endogenous (physiological) or exogenous (physical or chemical) factors . These mutations can be:

*Substitutions,

*Insertions or deletions.

Mutations can take place on stem cells, and in this case,  we talk about germline mutation.

The individual will then carry the mutation while his parents do not have it in their genetic inheritance. Its mutations are then transmissible to future generations: they are hereditary.

They can also take place on somatic cells.

i believe it would be the lungs.