It is often difficult to tell weather variation is environmental or genetic. In almost all cases it is a mixture of both. The prime display of this being that two twins with exactly the same genes have the potential to end up very different.
There are three things that cause genetic variation:
- The random fusion of gametes: the specific sperm and egg that happened to meet at the specific moment that a offspring is created are completely random and have a massive effect on the genes inherited.
- Mutations: DNA can be changed by mistakes in copying or caused by things like radiation.
- Meiosis: this form of cell division that creates gametes has crossing over (genetic recombination) and independent segregation which both vary the genes inherited.