Thomas Hunt Morgan

Thomas Hunt Morgan was an American evolutionary biologist, geneticist and embryologist. He is known for his discoveries relating the role the chromosome plays in heredity.

Biography
Morgan received his PhD from Johns Hopkins University in zoology in 1890 and researched embryology during his tenure at Bryn Mawr. Following the rediscovery of Mendelian inheritance in 1900, Morgan's research moved to the study of mutation in the fruit fly Drosophila melanogaster. In his famous Fly Room at Columbia University, Morgan demonstrated that genes are carried on chromosomes and are the mechanical basis of heredity. These discoveries formed the basis of the modern science of genetics.

Evolution
In Evolution and Adaptation (1903), Morgan argued the non-Darwinian position that selection could never produce new species by acting on slight individual differences. He also rejected Darwin's theory of sexual selection and the Neo-Lamarckian theory of the inheritance of acquired characters. Morgan was not the only scientist who rejected natural selection. This period from 1875 - 1925 has been called The eclipse of Darwinism. The mutation theory of evolution first denied natural selection, but this was shown to be incorrect due to discoveries in Mendelian genetics.

Natural selection
Morgan, like most scientists of his time, believed in evolution but was skeptical of natural selection. He became a critic of natural selection which he equated as a tautology. He wrote that natural selection could not produce novel variations or determine their continued production to form new species. He wrote that selection acted negatively to weed out the unfit, but had no creative power to shape original adaptations. Morgan proposed that mutation was the mechanism that drove evolution.

Morgan was a proponent of De Vries mutation theory of evolution but converted to Mendelian genetics in 1910 and later on in his career embraced natural selection after discovering many small stable heritable mutations in Drosophila. Morgan discovered the relevance of mutations for evolution is that only characters that are inherited can have an effect in evolution. Scientists of this period including Morgan began to reconcilise Mendelian genetics with natural selection. Morgan contributed to the neo-Darwinian synthesis, despite his criticism of natural selection at the beginning of his career.

Sexual selection
Morgan came to accept natural selection, but remained a critic of sexual selection throughout his career. In 1903 he ridiculed sexual selection by writing:

Shall we assume that... those females whose taste has soared a little higher than that of the average (a variation of this sort having appeared) select males to correspond, and thus the two continue heaping up the ornaments on one side and the appreciation of the ornaments on the other side? No doubt an interesting fiction should be built up along these lines, but would anyone believe it, and if he did, could he prove it?

Morgan (1932) considered sexual dimorphism to be explained by a proximate mechanism - hormonal difference between males and females and denied that it needed an evolutionary explanation by sexual selection.

Selected publications

 * The Scientific Basis of Evolution (1932)
 * The Physical Basis of Heredity (1919)
 * A Critique of the Theory of Evolution (1916)
 * Evolution and Adaptation (1903)