Evolution is a change to the genes, or genetic material, of a species that passes from generation to generation over time. Note that this is not changes in genetic material to an individual, but a population of individuals over time. Often this is a long period of time, but it doesn't have to be, as we'll see in a future evolution misconception discussion. In an individual's lifetime, changes to the genetic material may or may not be hereditary. Somatic mutations that occur in some, but not all cells, are not passed to the next generation. Hereditary mutations that are present in the sperm and egg cells do pass to the next generation.
A good example of a somatic mutation not passed on to the offspring is exposure to ultraviolet radiation that results in skin cancer. The individual's genetic makeup has changed, but this change is not passed to the next generation. Lung cancer due to smoking is another example. A parent who smokes and contracts lung cancer does not pass this genetic material on to the child. The child can be affected through second hand smoke, but this is not an evolutionary process.
An example of a hereditary mutation in which the genetic material is passed along to the offspring is hemophilia, a disorder in which blood does not clot. The chances of having hemophilia depend on your parents.
- No sons of a man with hemophilia will have hemophilia.
- All daughters of a man with hemophilia will be carriers (called obligate carriers).
- If a carrier has a son, the son has a 50% chance of having hemophilia.
- If a carrier has a daughter, the daughter has a 50% chance of being a carrier.
I've pulled the above list from How Hemophilia is Inherited.
The point in all of this is that an individual does not evolve. It's a population of individuals that evolve over time. You are not suddenly going to evolve a third arm. Your failure to grow a third arm is not a failure in the Theory of Evolution.