Abstract
The paper analyzes the early theory building process of Thomas Hunt Morgan (1866–1945) from the 1910s to the 1930s and the introduction of the invisible gene as a main explanatory unit of heredity. Morgan’s work marks the transition between two different styles of thought. In the early 1900s, he shifted from an embryological study of the development of the organism to a study of the mechanism of genetic inheritance and gene action. According to his contemporaries as well as to historiography, Morgan separated genetics from embryology, and the gene from the whole organism. Other scholars identified an underlying embryological focus in Morgan’s work throughout his career. Our paper aims to clarify the debate by concentrating on Morgan’s theory building—characterized by his confidence in the power of experimental methods, and carefully avoiding any ontological commitment towards the gene—and on the continuity of the questions to be addressed by both embryology and genetics.
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Notes
Morgan’s prolific output as an author, which documents his changes of opinion about several matters (including Darwinian natural selection and Mendelian inheritance), has prompted a similar divergence of opinion among commentators, with some authors highlighting internal inconsistencies. See for instance Gilbert (1978, 1991). See also below the comments on Falk (2007) and Maienschein (1992), notes 13 and 28 respectively. For Morgan’s “theoretical pluralism” as a sign of his “particular understanding of the experimental method” see Manier (1969, p. 201 and note 12) and Kingsland (2007).
Maienschein uses the term “(w)holism” to identify an ontological position whereby: “what exists in the world is complex, interactive wholes” (p. 135). Holism, by contrast, is instead considered an epistemological position.
The reconstruction of the context of this debate is beyond the scope of this article, but there is a wealth of available literature on these themes. See e.g. Sturtevant (1965), Gilbert (1978, 1998), Benson (2001) and Falk (2007), on Mendel and Mendelism see the classic Olby (1979), see also Olby (1997), Müller-Wille and Orel (2007).
An example of this kind of controversy, as described by Vorms (2013), is the debate opposing Morgan and William E. Castle (1867–1962) about linkage maps. The two scientists agreed at a general level (chromosome theory), but their views differed at the local level (physical explanation of the structure of the chromosome).
Note the comment by Allen (2003, p. 67): “For Morgan and others, the Mendelian ‘factor’ or ‘gene’ smacked too much of the embryologists' old bugbear, preformationism […]. Epigenesis, the alternative view, had replaced preformationism by the mid-19th century, so that Mendel's work in the form presented by Mendelians, seemed like an outdated throwback to a long-discarded idea”.
Eventually, this approach became known as Morgan’s chromosome theory of heredity. Morgan’s early anti-nuclear and anti-Mendelian approach has been studied variously, by Allen (1966, 1985), Manier (1969), Gilbert (1978), Benson (2001) and Falk (2007). For a detailed analysis of the reception of the chromosome theory, see Brush (2002).
See Manier’s (1969, p. 202) distinction in Morgan’s work between an alleged empiric approach and a true experimental one: his “confidence in […] the epigenetic theories forced him to submit Mendelian preformationism to such a wide variety of experimental tests that his own final version of Mendelism was considerably strengthened in the process”.
Falk (2007) starts from our same point of view, “[Morgan] saw in Johannsen’s conception of disparate hereditary factors and traits a ‘developmental’ way out of the de Vriesian notion of hereditary preformationism, allowing him to maintain an organismic approach to development while accepting a particulate theory of inheritance”. Yet, he explains this by reference to Morgan’s reductionist approach: “Morgan and his students reduced causation to patterns conjunction” (Falk 2007, p. 258) and that “from its early steps, genetic analysis was paradigmatically reductionist” (Falk 2007, p. 250). As explained later, our perspective on Morgan’s theoretical commitment towards reductionism is different.
To several students this model seems too limited for understanding emergent properties such as the formation of gametic frequencies (Wimsatt 2006).
These “discrete particles” corresponded to specific phenotypic traits, which would be extensively studied in Drosophila during the next 5 years. The results showed a hundred new and inherited traits (clustered in 4 main groups) such as olive, speck, white or fringed and affecting different parts of the body (such as body color, thorax, eye color, wings). For an analysis of how the changing of the nomenclature meant also a changing in the approach in Morgan’s work (from developmental function to simpler phenotypic deviation) see Falk (2007, p. 258). It is worth noting that while Morgan had already started to relate chemical substances to underlying elementary particles, there is not yet any mention of the gene.
The quotes are from Morgan (1917, pp. 513–514).
Morgan rephrased the same concepts—deemed “a priori objections”—2 years later in “The physical basis of heredity” (1919), with slight though significant differences: for example, “molecules” replaced “atoms”.
The process by which “virtual chromosomes of linkage data were finally bolstered by physical reality” is described in Falk (2004). Skopek (2011) has recently put in evidence how the early Mendelians generated a new way of experimental seeing; using pedigrees, linkage-maps and Mendelian ratios they taught students how to “see through”, going beyond the visible appearance of the organism. See also the two volumes edited by Rheinberger and Gaudillière (2004a, b).
When looking at the editor’s announcement of “The physical basis of heredity” (1919), signed by Jacques Loeb, Winthrop John Van Leuven Osterhout and Morgan himself, it is important to remember the context: the “new experimental biology” was struggling to secure a place among the other “real” sciences, trying to shed its reputation as a “purely speculative and descriptive” approach (Morgan 1919, p. 5).
“[The term ‘gene’] should express only the simple conception that the characteristics of an organism are, or can be, caused or co-determined by ‘something’ in the make-up [Konstitution] of the gametes. The word ‘gene’ is thus completely free of any hypothesis” (Johannsen 1909; translated by Burian 2000, pp. 143–144).
For the relevance of development in history of heredity, see Amundson (2005) about Morgan see especially pp. 177 ff.
Maienschein’s position is peculiar because it seems to combine Morgan’s gene-centric perspective with an opposed embryological perspective: “Even in his role as geneticist, Morgan remained at least loyal in principle, if not in practice, to his embryological roots” (1992, p. 124); some lines later, we read: “For Castle, unlike the Morgan group, the gene was not inviolable and sacrosanct”. See also a recent comment by Maienschein: “Morgan’s words here [(Morgan 1924)] and in his many, many other articles and books might mislead readers looking with particular genetics-oriented assumptions to miss the emphasis on development” (Maienschein 2016, p. 592).
At the meeting of the American Society of Naturalists, in December 1906, Morgan said that the biological "question of the hour" was that of preformation vs. epigenesis, and he compared Mendelian inheritance to preformation (Allen 1966).
For an analysis of why Morgan cannot be considered a forerunner of present-day Evo-Devo thinking, despite providing a potential bridge between genetics and developmental issues, see Schwartz (2006).
For example, Waters (2004) account of Morgan’s work supports our view: the distinction between investigation and explanation largely match with our hypothesis that Morgan was mostly pragmatic in his methodology, and was wary of strong ontological commitments.
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Acknowledgements
The authors wish to thank the organizers and the participants of the workshop “The ‘Artificial’ and the ‘Natural’ in the Life Sciences, c. 1850–1950” (University of Exeter, 26–28 June 2014) for the inspiring discussion of our paper, and in particular John Hodges and Staffan Müller-Wille for their insightful comments. We are also grateful to the anonymous referees: the points they raised forced us to make the paper stronger. We thank the Editorial Board for their valued help while reviewing the manuscript.
Funding
Funding was provided by MIUR - Italian Government (Grant No. Futuro in Ricerca 2010-RBFR10Q67A_002).
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Frezza, G., Capocci, M. Thomas Hunt Morgan and the invisible gene: the right tool for the job. HPLS 40, 31 (2018). https://doi.org/10.1007/s40656-018-0196-z
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DOI: https://doi.org/10.1007/s40656-018-0196-z