Spontaneous mutations can result in the appearance of new traits. Characteristics acquired during an organism's life are not passed on through genes. Darwin was able to formulate his theory of evolution based on several facts. All populations have the potential to increase in size. Individual organisms in a population are not alike.
Most populations are stable in size.
Characteristics are inherited as genes on chromosomes. Natural resources are limited. The fittest individuals leave the most offspring. There is differential reproductive success within populations. Variation exists within populations.