What is Neo-Darwinism?

After publication of On the Origin of Species, the fact of organic evolution began to be accepted in the scientific community. But only certain elements of Darwin's thesis gained anything like a wide acceptance for the rest of the 19th and early part of the 20th century. Transmutation (speciation) and the decent of all organisms from a common ancestor were more or less accepted, but perhaps the primary mechanism of evolution, natural selection, was not necessarily thought to be a main factor in evolution as well as the gradualistic nature of evolution that Darwin envisioned. Darwin, however, did not understand the nature of genetic material or the source of the variation on which selection acts. After the turn of the 20th century, Mendel's research was rediscovered after being virtually ignored for close to fifty years. Mendel showed through experiments with pea plants how traits were passed from one generation to the next. Early geneticists and embryologists began to explain how variation in eukaryotic organisms can arise and affect phenotypic expression. Ernst Mayr, G. G. Simpson and others towards the mid-20th century recognized that a new synthesis should be made between Mendelian genetics and theory of natural selection to which Darwin adhered. This became to be known as the Modern Synthesis or Neo-Darwinism. Over the last six decades no theoretical challenge to the Modern Synthesis has gained a wide consensus.