Evolution Darwin's theory Variation Inherited variation
 Variation can only affect future generations if it is inherited. Because of the work of Gregor Mendel, we know that much variation is inherited. Mendel's 'factors' are now called genes. Research has shown that 'almost every individual in a sexually reproducing species is genetically unique'.Futuyma D.J. 2005. 'Evolution'. Sinauer, Sunderland, Massachusetts.