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Kunstformen der Natur (1904) by Ernst Haeckel
Kunstformen der Natur (1904) by Ernst Haeckel

Neo-Darwinism is a term used to describe the 'modern synthesis' of Darwinian evolution through natural selection with Mendelian genetics, the latter being a set of primary tenets specifying that evolution involves the transmission of characteristics from parent to child through the mechanism of genetic transfer, rather than the 'blending process' of pre-Mendelian evolutionary science. Neo-Darwinism also separates Darwin's ideas of natural selection from his hypothesis of Pangenesis as a Lamarckian source of variation involving blending inheritance.

As part of the disagreement about whether natural selection alone was sufficient to explain speciation, George Romanes coined the term neo-Darwinism to refer to the version of evolution advocated by Alfred Russel Wallace and August Weismann with its heavy dependence on natural selection. Weismann and Wallace rejected the Lamarckian idea of inheritance of acquired characteristics, something that Darwin had not ruled out. The term was first used in 1895 to explain that evolution occurs solely through natural selection, in other words, without any mechanism involving the inheritance of acquired characteristics resulting from use or disuse. These two scientists' complete rejection of Lamarckism came from Weismann's germ plasm theory. Weismann realised that the cells that produce the germ plasm, or gametes (such as sperm and egg in animals), separate from the somatic cells that go on to make other body tissues at an early stage in development. Since he could see no obvious means of communication between the two he asserted that the inheritance of acquired characteristics was therefore impossible; a conclusion now known as Weismann's barrier.

From the 1880s to the 1930s the term continued to be applied to the panselectionist school of thought, which argued that natural selection was the main and perhaps sole cause of all evolution. From then until around 1947 the term was used for the panselectionist followers of R. A. Fisher.

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