Mutations are any changes in genetic material.   Mutations can be caused by such agents as radiation and chemicals. When they occur in sex cells, the mutations can be passed on to offspring.
Mutations occurring in other cells can be passed on to other body cells only. The experiences an organism has during its lifetime can affect its offspring only if the genes in its own sex cells are changed by the experience.

Some Types of Chromosome Mutations

Inversion: chromosome pieces are attached upside down Duplication: involves copying an extra section of chromosome Translocation: chromosome pieces moved Addition and deletion: chromosome is added or removed

Either changes in chromosomes or genes on chromosomes changes the genetic which contributes to sources of variation.

Some Other Sources of Genetic Variability
In addition to mutation, other sources of the variation seen in sexually reproducing offspring include crossing over and genetic recombination during fertilization (union of egg and sperm).  In crossing over which occurs in the production of sex cells or gametes in meiosis, there is an exchange of chromosome pieces between the chromosome pairs associated with each other in this process.

Crossing Over and Genetic Variation

Mutations, crossing over, and genetic recombination ensure that no two gametes formed as the result of sexual reproduction will be exactly the same.   As a consequence, the offspring formed as a result of sexual reproduction will exhibit variations.   Some of these variations will be better suited for survival than others, thus driving the process of biological evolution.