Evolution is change in the heritable characteristics of biological populations over successive generations

The scientific theory of evolution by natural selection was conceived independently by Charles Darwin and Alfred Russel Wallace in the mid-19th century and was set out in detail in Darwin's book On the Origin of Species (1859). Evolution by natural selection was first demonstrated by the observation that more offspring are often produced than can possibly survive. 
This is followed by three observable facts about living organisms: 
(1) traits vary among individuals with respect to their morphology, physiology and behaviour (phenotypic variation), 
(2) different traits confer different rates of survival and reproduction (differential fitness) and 

(3) traits can be passed from generation to generation (heritability of fitness).
Thus, in successive generations members of a population are more likely to be replaced by the progenies of parents with favourable characteristics that have enabled them to survive and reproduce in their respective environments.
 In the early 20th century, other competing ideas of evolution such as mutationism and orthogenesis were refuted as the modern synthesis reconciled Darwinian evolution

 with classical genetics, which established adaptive evolution as being caused by natural selection acting on Mendelian genetic variation.