thumb|upright|[[Friedrich Leopold August Weismann, considered the "founder of Neo-Darwinism" for expanding Darwin's theory along genetic lines]]
thumb|upright|[[George John Romanes originally used Neo-Darwinism in 1895 to refer to an early modification of Darwin's theory. Photograph by Elliott & Fry (1896)]]
Neo-Darwinism is generally used to describe any integration of Charles Darwin's theory of evolution by natural selection with Gregor Mendel's theory of genetics. It mostly refers to evolutionary theory from either 1895 (for the combinations of Darwin's and August Weismann's theories of evolution) or 1942 ("modern synthesis"), but it can mean any new Darwinian- and Mendelian-based theory, such as the current evolutionary theory.
Original use
thumb|upright=1.75|Several major ideas about [[evolution came together in the population genetics of the early 20th century to form the so-called modern synthesis, including genetic variation, natural selection, and particulate (Mendelian) inheritance. This was at the time called neo-Darwinism.]]
Darwin's theory of evolution by natural selection, as published in 1859, provided a selection mechanism for evolution, but not a trait transfer mechanism. Lamarckism was still a very popular candidate for this. August Weismann and Alfred Russel Wallace rejected the Lamarckian idea of inheritance of acquired characteristics that Darwin had accepted and later expanded upon in his writings on heredity. -->