Editing Thomas Hunt Morgan (section)

==Career and research==
===Bryn Mawr===
In 1890, Morgan was appointed associate professor (and head of the biology department) at Johns Hopkins' sister school [[Bryn Mawr College]], replacing his colleague [[Edmund Beecher Wilson]].<ref>{{Cite journal | last1 = Morgan | first1 = T. H. | author-link = Thomas Hunt Morgan| title = Edmund Beecher Wilson. 1856–1939 | doi = 10.1098/rsbm.1940.0012 | journal = [[Obituary Notices of Fellows of the Royal Society]] | volume = 3 | issue = 8 | pages = 123–126 | year = 1940 | title-link = Edmund Beecher Wilson | s2cid = 161395714 | doi-access = free }}</ref> Morgan taught all morphology-related courses, while the other member of the department, [[Jacques Loeb]], taught the physiological courses. Although Loeb stayed for only one year, it was the beginning of their lifelong friendship.<ref>Allen, ''Thomas Hunt Morgan'', pp. 50–53</ref>  Morgan lectured in biology five days a week, giving two lectures a day. He frequently included his recent research in his lectures. Although an enthusiastic teacher, he was most interested in research in the laboratory. During the first few years at Bryn Mawr, he produced descriptive studies of [[sea acorn]]s, ascidian worms, and frogs.{{cn|date=May 2024}}

In 1894 Morgan was granted a year's absence to conduct research in the laboratories of ''[[Stazione Zoologica]]'' in [[Naples]], where Wilson had worked two years earlier. There he worked with German biologist [[Hans Driesch]], whose research in the experimental study of development piqued Morgan's interest. Among other projects that year, Morgan completed an experimental study of [[ctenophore]] embryology. In Naples and through Loeb, he became familiar with the ''Entwicklungsmechanik'' (roughly, "developmental mechanics") school of experimental biology. It was a reaction to the vitalistic ''[[Naturphilosophie]]'', which was extremely influential in 19th-century morphology. Morgan changed his work from traditional, largely descriptive morphology to experimental embryology that sought physical and chemical explanations for organismal development.<ref>Allen, ''Thomas Hunt Morgan'', pp. 55–59, 72–80</ref>

At the time, there was considerable scientific debate over the question of how an embryo developed. Following [[Wilhelm Roux]]'s mosaic theory of development, some believed that hereditary material was divided among embryonic cells, which were predestined to form particular parts of a mature organism. Driesch and others thought that development was due to epigenetic factors, where interactions between the protoplasm and the nucleus of the egg and the environment could affect development. Morgan was in the latter camp; his work with Driesch demonstrated that [[blastomeres]] isolated from [[sea urchin]] and ctenophore eggs could develop into complete larvae, contrary to the predictions (and experimental evidence) of Roux's supporters.<ref>Allen, ''Thomas Hunt Morgan'', pp. 55–59, 80–82</ref>  A related debate involved the role of [[epigenetic]] and environmental factors in development; on this front Morgan showed that [[sea urchin]] eggs could be induced to divide without fertilization by adding [[magnesium chloride]]. Loeb continued this work and became well known for creating fatherless frogs using the method.<ref>{{cite journal
|author=Loeb, Jacques|author-link=Jacques Loeb
|journal=American Journal of Physiology | volume=31 | pages=135–138 | year=1899
|title=On the Nature of the Process of Fertilization and the Artificial Production of Normal Larvae (Plutei) from the Unfertilized Eggs of the Sea Urchin|issue=3
|url=http://www.stanford.edu/group/Urchin/loeb.htm|doi=10.1152/ajplegacy.1899.3.3.135
|hdl=2027/hvd.32044107304297
|hdl-access=free}}</ref>
<ref>{{cite book|title=Artificial parthenogenesis and fertilization|author=Loeb, Jacques |publisher=University of Chicago Press | year=1913 | url=https://archive.org/details/artificialparth00loebgoog|quote=jacques loeb sea urchin.}}</ref>

When Morgan returned to Bryn Mawr in 1895, he was promoted to full professor. Morgan's main lines of experimental work involved [[regeneration (biology)|regeneration]] and larval development; in each case, his goal was to distinguish internal and external causes to shed light on the Roux-Driesch debate. He wrote his first book, ''The Development of the Frog's Egg'' (1897). He began a series of studies on different organisms' ability to regenerate. He looked at grafting and regeneration in tadpoles, fish, and earthworms; in 1901 he published his research as ''Regeneration''.

Beginning in 1900, Morgan started working on the problem of [[Sex-determination system|sex determination]], which he had previously dismissed when [[Nettie Stevens]] discovered the impact of the Y chromosome on sex. He also continued to study the evolutionary problems that had been the focus of his earliest work.<ref>Allen, ''Thomas Hunt Morgan'', pp. 84–96</ref>

===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.

===Caltech===
[[File:Thomas Hunt Morgan sketch 1931.png|thumb|1931 drawing of Thomas Hunt Morgan]]
In 1928 Morgan joined the faculty of the [[California Institute of Technology]] where he remained until his retirement 14 years later in 1942.

Morgan moved to California to head the Division of Biology at the [[California Institute of Technology]] in 1928. In establishing the biology division, Morgan wanted to distinguish his program from those offered by Johns Hopkins and Columbia, with research focused on genetics and evolution; experimental embryology; physiology; biophysics, and biochemistry. He was also instrumental in the establishment of the [[Kerckhoff marine laboratory|Marine Laboratory]]  at [[Corona del Mar]]. He wanted to attract the best people to the Division at Caltech, so he took Bridges, Sturtevant, [[Jack Shultz]] and [[Albert Tyler (biologist)|Albert Tyler]] from Columbia and took on [[Theodosius Dobzhansky]] as an international research fellow. More scientists came to work in the Division including [[George Beadle]], [[Boris Ephrussi]], [[Edward L. Tatum]], [[Linus Pauling]], [[Frits Went]], [[Edward B. Lewis]], and Sidney W. Byance with his reputation, Morgan held numerous prestigious positions in American science organizations. From 1927 to 1931 Morgan served as the President of the National Academy of Sciences; in 1930 he was the President of the [[American Association for the Advancement of Science]]; and in 1932 he chaired the Sixth [[International Congress of Genetics]] in [[Ithaca, New York]]. In 1933 Morgan was awarded the [[Nobel Prize in Physiology or Medicine]]; he had been nominated in 1919 and 1930 for the same work. As an acknowledgment of the group nature of his discovery, he gave his prize money to Bridges, Sturtevant, and his own children. Morgan declined to attend the awards ceremony in 1933, instead attending in 1934. The 1933 rediscovery of the giant [[polytene chromosome]]s in the salivary gland of ''Drosophila'' may have influenced his choice. Until that point, the lab's results had been inferred from phenotypic results, the visible polytene chromosome enabled them to confirm their results on a physical basis. Morgan's Nobel acceptance speech entitled "The Contribution of Genetics to Physiology and Medicine" downplayed the contribution genetics could make to medicine beyond [[genetic counseling]]. In 1939 he was awarded the [[Copley Medal]] by the Royal Society.{{cn|date=May 2024}}

He received two extensions of his contract at Caltech, but eventually retired in 1942, becoming a professor and chairman emeritus. George Beadle returned to Caltech to replace Morgan as chairman of the department in 1946. Although he had retired, Morgan kept offices across the road from the Division and continued laboratory work. In his retirement, he returned to the questions of sexual differentiation, regeneration, and embryology.