By 1904, when Morgan took a professorship in experimental zoology at
Columbia University, he was becoming increasingly focused on the mechanisms of heredity and evolution. The previous year, he had published
Evolution and Adaption; like many biologists at that time, he saw clear evidence for biological evolution (as in the
common descent of similar species) but rejected Darwin's proposed mechanism of
natural selection acting on small, constantly-produced variations. Extensive work in
biometry seemed to indicate that continuous natural variation had distinct limits and did not represent heritable changes. Embryological development posed an additional problem in Morgan's view, as selection could not act on the early, incomplete stages of highly complex organs such as the eye. The common solution of the
Larmarckian mechanism of
inheritance of acquired characters, which featured prominently in Darwin's theory, was increasingly rejected by biologists. According to Morgan biographer Garland Allen, he was also hindered by his views on taxonomy: he thought that species were entirely artificial creations that distorted the continuously variable range of real forms, while he held a "typological" view of larger taxa and could see no way that one such group could transform into another. But while he would remain skeptical of natural selection for many years, his theories of heredity and variation were radically transformed through his conversion to Mendelism.
In 1900 three scientists,
Carl Correns, Erich von Tschermak and
Hugo De Vries had rediscovered the work of
Gregor Mendel, and with it the foundation of
genetics. De Vries had gone on to propose that new species are created by mutation, bypassing the need for either Larmackism or Darwinism. Morgan dismissed both of these evolutionary theories, and was actually seeking to prove
Hugo De Vries' mutation theory with his experimental heredity work. He was initially quite skeptical of
Mendel's laws of heredity (as well as the related chromosomal theory of sex determination), which were being considered as a possible basis for natural selection.
Following
C. W. Woodworth and
William E. Castle, around 1908 Morgan started working on the fruit fly
Drosophila melanogaster, and encouraging students to do so as well. With
Fernandus Payne, he mutated
Drosophila through physical, chemical, and radiational means, and began cross-breeding experiments to find heritable mutations. However, they had no significant success for two years. Castle had also had difficulty identifying mutations in
Drosophila, hardly unusual given the flies' tiny size. Finally in 1909, a series of heritable mutants appeared, some of which displayed Mendelian inheritance patterns; in 1910 Morgan noticed a white-eyed
mutant male among the red-eyed
wild types. When white-eyed flies were bred with a red-eyed female, their progeny were all red-eyed, while a second generation cross produced white-eyed males—a sex-linked recessive trait, the gene for which Morgan named
white. Morgan also discovered a pink-eyed mutant that showed a different pattern of inheritance. In a paper published in
Science in 1911, he concluded that (1) some traits were
sex-linked, (2) the trait was probably carried on one of the
sex chromosomes, and (3) other genes were probably carried on specific chromosomes as well.
Morgan and his students became more successful at finding mutant flies; they counted the mutant characteristics of thousands of fruit flies and studied their inheritance. As they accumulated multiple mutants, they combined them to study more complex inheritance patterns. The observation of a miniature wing mutant which was also on the sex chromosome but sometimes sorted independently to the white eye mutation, led Morgan to the idea of
genetic linkage and to hypothesize the phenomenon of
crossing over. Morgan proposed that the amount of crossing over between linked genes differs and that crossover frequency might indicate the distance separating genes on the chromosome; later English geneticist
J. B. S. Haldane suggested that the unit of measurement for linkage be called the
morgan. Morgan's student
Alfred Sturtevant developed the first
genetic map in 1913.
In 1915 Morgan, Sturtevant,
Calvin Bridges and
H. J. Muller wrote the seminal book
The Mechanism of Mendelian Heredity. Geneticist
Curt Stern called the book "the fundamental textbook of the new genetics" and
C. H. Waddington noted that "Morgan's theory of the chromosome represents a great leap of imagination comparable with Galileo or Newton". In the following years, most biologists came to accept the "Mendelian-chromosome theory" pioneered by Morgan and his students. Garland Allen characterized the post-1915 period as one of
normal science, in which "The activities of 'geneticists' were aimed at further elucidation of the details and implications of the Mendelian-chromosome theory developed between 1910 and 1915." However, the details of the increasingly complex theory, as well as the very concept of the
gene and its physical nature, were still controversial. Critics such as
W. E. Castle pointed to contrary results in other organisms suggesting that genes interact with each other, while to
Richard Goldschmidt and others, there was no compelling reason to view genes as discrete units residing on chromosomes.
Because of Morgan's dramatic success with
Drosophila, many other labs throughout the world took up fruit fly genetics. Columbia became the center of an informal exchange network, through which promising mutant
Drosophila strains were transferred from lab to lab;
Drosophila became one of the first, and for some time the most widely used,
model organisms. Morgan's group remained highly productive, but Morgan largely withdrew from doing fly work himself and gave his lab members considerable freedom in designing and carrying out their own experiments. Instead, Morgan returned to embryology and worked to encourage the spread of genetics research to other organisms and the spread of the mechanistic experimental approach (
Enwicklungsmechanik) to all biological fields. After 1915, he also became a strong critic of the growing
eugenics movement, which frequently co-opted the ideas of genetics in support of racism and worse.
John Hopkins awarded Morgan an honorary LL.D. and the University of Kentucky awarded him an honorary Ph.D. He was elected a member of the
National Academy of Sciences and made a foreign member of the
Royal Society. In 1924 Morgan received the
Darwin Medal. His Fly Room at Columbia became world famous and he found it easy to attract funding and visiting academics. In 1927 after 25 years at Columbia, and nearing the age of retirement he received an offer from
George Ellery Hale to establish a school of biology in California.
