Editing Thomas Hunt Morgan (section)

===Columbia University===
Morgan worked at Columbia University for 24 years, from 1904 until 1928 when he left for a position at the California Institute of Technology.{{cn|date=May 2024}}

In 1904, his friend, Jofi Joseph died of tuberculosis, and he felt he ought to mourn her, though E. B. Wilson—still blazing the path for his younger friend—invited Morgan to join him at [[Columbia University]]. This move freed him to focus fully on experimental work.<ref>Allen, ''Thomas Hunt Morgan'', pp. 68–70</ref>

[[File:Drosophila in the lab.jpg|right|thumb|200px|In a typical ''Drosophila'' genetics experiment, male and female flies with known [[phenotype]]s are put in a jar to mate; females must be virgins. Eggs are laid in porridge which the larvae feed on; when the life cycle is complete, the progeny are scored for the inheritance of the trait of interest.]]
When Morgan took the professorship in experimental zoology, he became increasingly focused on the mechanisms of heredity and evolution. He published ''Evolution and Adaptation'' (1903); like many biologists at the time, he saw 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 [[Lamarckism|Lamarckian]] mechanism of [[inheritance of acquired characters]], which featured prominently in Darwin's theory, was increasingly rejected by biologists. According to Morgan's 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 Morgan was skeptical of natural selection for many years, his theories of heredity and variation were radically transformed through his conversion to Mendelism.<ref>Allen, ''Thomas Hunt Morgan: The Man and His Science'', pp. 105–116</ref>

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 proposed that new species were created by mutation, bypassing the need for either Lamarckism or Darwinism. As Morgan had dismissed both evolutionary theories, he was seeking to prove De Vries' [[mutation theory]] with his experimental heredity work. He was initially 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.

[[File:Sexlinked inheritance white.jpg|right|thumb|250px|Sex linked inheritance of the white eyed mutation.]]
Following [[Charles W. Woodworth|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.<ref name="Kohler"/><ref name="Hamilton">{{cite journal|last1=Hamilton|first1=Vivien|title=The Secrets of Life: Historian Luis Campos resurrects radium's role in early genetics research|journal=Distillations|date=2016|volume=2|issue=2 |pages=44–45 |url=https://www.sciencehistory.org/distillations/magazine/the-secrets-of-life|access-date=22 March 2018}}</ref> He began cross-breeding experiments to find heritable mutations, but they had no significant success for two years.<ref name="Kohler">Kohler, ''Lords of the Fly'', pp. 37–43</ref>  Castle had also had difficulty identifying mutations in ''Drosophila'', which were tiny. 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 type]]s. When white-eyed flies were bred with a red-eyed female, their progeny were all red-eyed. A second-generation cross produced white-eyed males—a sex-linked recessive trait, the gene for which Morgan named ''[[White (mutation)|white]]''. Morgan also discovered a pink-eyed mutant that showed a different pattern of inheritance. In a paper published in ''[[Science (journal)|Science]]'' in 1911, he concluded that (1) some traits were [[sex-linked]], (2) the trait was probably carried on one of the [[sex chromosome]]s, and (3) other genes were probably carried on specific chromosomes as well.

[[File:Morgan crossover 1.jpg|right|thumb|Morgan's illustration of [[chromosomal crossover|crossing over]], from his 1916 ''A Critique of the Theory of Evolution'']]
[[File:First genetic map (Sturtevant, 1913).png|thumb|First genetic map (Sturtevant, 1913). It shows 6 sex-linked genes.]]
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 [[Chromosomal crossover|crossing over]]. He relied on the discovery of [[Frans Alfons Janssens]], a Belgian professor at the [[Catholic University of Leuven (1834–1968)|University of Leuven]], who described the phenomenon in 1909 and had called it ''chiasmatypy''. 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. The later English geneticist [[J. B. S. Haldane]] suggested that the unit of measurement for linkage be called the [[centimorgan|morgan]]. Morgan's student [[Alfred Sturtevant]] developed the first [[genetic linkage#Linkage map|genetic map]] in 1913.<ref>{{cite journal |last1=Sturtevant |first1=A. H. |date=1913 |title=The linear arrangement of six sex-linked factors in Drosophila, as shown by their mode of association |url=http://www.esp.org/foundations/genetics/classical/holdings/s/ahs-13.pdf |journal=Journal of Experimental Zoology |volume=14 |issue=1 |pages=43–59 |bibcode=1913JEZ....14...43S |doi=10.1002/jez.1400140104 |s2cid=82583173}}</ref>

[[File:Drosophila Gene Linkage Map.svg|right|thumb|250px| Thomas Hunt Morgan's ''[[Drosophila melanogaster]]'' [[genetic linkage]] map. This was the first successful [[gene mapping]] work and provides important evidence for the [[chromosome theory of inheritance]]. The map shows the relative positions of [[allele|allelic]] characteristics on the second ''Drosophila'' chromosome. The distance between the genes (map units) is equal to the percentage of [[Chromosomal crossover|crossing-over]] events that occurs between different alleles.<ref name="image">{{cite book |last=Mader |first=Sylvia |date=2007 |title=Biology Ninth Edition |location= New York |publisher= McGraw-Hill |page= 209 |isbn= 978-0-07-325839-3 }}</ref>]]

In 1915 Morgan, Sturtevant, [[Calvin Bridges]] and [[H. J. Muller]] wrote the seminal book ''The Mechanism of Mendelian Heredity''.<ref>{{cite book|author=Morgan, Thomas Hunt; Alfred H. Sturtevant, H. J. Muller and C. B. Bridges|title=The Mechanism of Mendelian Heredity|publisher=Henry Holt|location=New York| url=https://books.google.com/books?id=GZEEAAAAYAAJ&q=mechanism+of+mendelian+heredity+morgan|year=1915}}</ref> Geneticist [[Curt Stern]] called the book "the fundamental textbook of the new genetics".<ref name="Stern">{{cite journal |last1=Stern |first1=Curt |title=The Continuity of Genetics |journal=Daedalus |date=1970 |volume=99 |issue=4 |page=899 |jstor=20023976 |pmid=11609639 |url=https://www.jstor.org/stable/20023976 |issn=0011-5266}}</ref>
[[File:Genetic_map_of_drosophila,_as_of_1926.png|thumb|Genetic map of drosophila, published in ''The theory of the gene'' 1926 edition.<ref>{{Cite book |last=Morgan |first=Thomas Hunt |url=http://archive.org/details/theoryofgene00morg |title=The theory of the gene |date=1926 |publisher=New Haven, Yale University Press; [etc., etc.] |others=MBLWHOI Library}}</ref>]]
In the following years, most biologists came to accept the [[Boveri-Sutton chromosome theory|Mendelian-chromosome theory]], which was independently proposed by [[Walter Sutton]] and [[Theodor Boveri]] in 1902/1903, and elaborated and expanded 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."  But, the details of the increasingly complex theory, as well as the 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 [[Richard Goldschmidt]] and others thought there was no compelling reason to view genes as discrete units residing on chromosomes.<ref>Allen, ''Thomas Hunt Morgan'', pp. 208–213, 257–278. Quotation from p. 213.</ref>

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 organism]]s.<ref>Kohler, ''Lords of the Fly'', chapter 5</ref>  Morgan's group remained highly productive, but Morgan largely withdrew from doing fly work and gave his lab members considerable freedom in designing and carrying out their own experiments.

He returned to embryology and worked to encourage the spread of genetics research to other organisms and the spread of mechanistic experimental approach (''Enwicklungsmechanik'') to all biological fields.<ref>Allen, ''Thomas Hunt Morgan'', pp. 214–215, 285</ref>  After 1915, he also became a strong critic of the growing [[eugenics]] movement, which adopted genetic approaches in support of [[racism|racist]] views of "improving" humanity.<ref>Allen, ''Thomas Hunt Morgan'', pp. 227–234</ref>

Morgan's '''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.