Genetic variation between organisms of the same species is caused by mutations in genes. Mutations can occur in either germ cells (sex cells like eggs and sperm) or somatic cells (non-sex cells), but only mutations in germ cells can be passed on to offspring. Mutations may be harmful, neutral, or beneficial to an organism's survival and ability to reproduce. Harmful mutations reduce chances of reproduction and survival, so they are less likely to be passed on. Neutral mutations do not affect survival or reproduction. Beneficial mutations increase chances of survival and reproduction, so future generations are also likely to have them.