During asexual reproduction, living things copy their Instruction Books. Sometimes, point mutations occur, producing small changes in the Instruction Book.

Sexual reproduction is more complicated than asexual reproduction. First, an organism recombines its two Instruction Books to produce new Instruction Books. It then produces gametes, each carrying one of these new Instruction Books. Two sexual organisms combine gametes to produce offspring. Eukaryotes reproducing sexually also shuffle Instructions to produce new ones via exon shuffling. Because of all this shuffling, eukaryotes have a lot of variety in their Instruction Books.

Prokaryotes pass around Instructions via conjugation. Even though conjugation looks like a form of reproduction, it isn't.

There are lots of things in the environment that produce mutations. Mutations occur frequently, and living things have repair systems that fix the changes. Still, some changes are unavoidable.

Whether a change is good or bad depends on the the environment the organism is in.