Question

Who determines whether the two organism of a species will be exactly similar or not

Answer 1

Logically, never. Only twins and clones have the potential to have the exact same genome, and research has shown that point mutations occur at during every doubling of a new embryo from the two-cell stage forward - so by birth, they are not exactly identical either.

Emerging research suggests that some mutations acquired during very early development create variants that increase an individual’s cancer risk, and that the morphological location of the cell that acquired the mutation influences the organ that will later develop cancer.