Neo-Darwinism is the modern blending of Darwin's ideas regarding natural selection with Mendelian genetics, the system of genetic inheritance developed by Gregor Mendl. Although Darwin first published his On the Origin of the Species in 1859, by the end of the 19th century there was considerable debate as to whether natural selection by itself was sufficient to explain the various, diverse species. In 1895, George Romanes created the term Neo-Darwinism.
In modern parlance, the term Neo-Darwinism is used as an umbrella term for all contemporary evolutionary theory. Darwin himself was said to be unaware of much genetic theory. The famous monk Gregor Mendl, who was working with his bean plants roughly about the same time that Darwin was distilling his experiences on the Beagle, created the basic understanding of genetics still taught in high school biology classes today. Neo-Darwinism combines both men's theories into a holistic understanding of how traits are passed down, and the role that natural selection plays in providing for the diversity of life on Earth.
One of the hallmarks of science is the modification of theories in light of new evidence. Neither Darwin nor Mendl seem to have known much of each other's work. Yet the scientific breakthroughs of one clearly support the other, as if they were two parts of a whole system. Under a Neo-Darwinism framework, natural selection chooses the genes passed on.