Darwin’s theory of natural selection of small, inherited variations that increase an individual’s ability to compete for resources, survive and reproduce. The problems for Darwin were how such genetic variations arose and how they were transmitted from one generation to the next. The answer to the first problem was random mutation, with chromosomal rearrangement and genetic recombination being added at a later date. As an answer to the second problem, Darwin invented a unit of inheritance that he termed ‘gemmules‘ (small, invisible carriers of inherited features) in his theory of pangenesis that was later replaced by Mendelian genetics in which the gene became the unit of heredity.
See Gene, Genetic (or DNA) recombination, Ethology, Mutation (biology), Mendelian genetics, Modern synthesis, Natural selection, Neo-Darwinism, Phyletic gradualism and punctuated equilibrium, Theory of molecular evolution, Theory of natural selection, Theory of sexual selection