In this paper, I trace the historical changes in the concept of gene and show that the changes are in accordance with David Hull's evolutionaly view. This approach seems to give an interesting "research programme" in this case .
2. Historical changes of the concept of the gene
The first direct ancestor of the concept, which we now call "gene," can be found in Mendel's classical paper, "Experiments in Plant Hybridisation" in 1865. In the first part of his paper, Mendel proposes a law about the inheritance of characters. This law shows that the characters do not blend with each other. To explain these phenomena, Mendel assumed that there are certain differences in "internal composition" (Mendel 1965, p. 21) in germ cells. He called them "elements" (p. 37). Actually Mendel's own terminology and notation are not clear. He used the word "character" to refer to both the apparent differences of peas (color, height and so on) and the differences of germ cells (p. 21). He introduced the letters "A" "a" "B" "b" and so on to refer to apparent characters (p. 14), thus his expression for F2 generation was "A+2Aa+a" (p. 14), not "AA+2Aa+aa." But he used these same letters to refer to the differences between germ cells (p. 21), and he used these two different meanings even in one and the same equation (p. 26). These facts indicate that Mendel did not have the clear distinction between "genotype" and "phenotype," , or the unit and the character that it produces in later terminology.
Mendel's work was ignored for a while and rediscovered in 1900. Soon after that Bateson adopted Mendelism and proposed a name for the new field, "genetics" (Darden 1991, p. 40). He also inherited the ambiguity of the terminology from Mendel. Bateson used the word "unit-character" or "allelomorph" to refer to both the unit of character and the corresponding genetic element (pp.178-179)(note 1). However, geneticists, including Bateson, soon became aware of the distinction between "genotype" and "phenotype" (note 2), and started to use the words "factor" or "unit-factor" to denote the genetic composition which causes the character (this causal relation was also unclear in Mendel) . This clarification was necessary to introduce the ideas that (1) several factors can affect one character and (2) one factor can affect several characters.
T. H. Morgan adopted the word "gene" in place of "factor" in his 1917 paper, "The theory of the gene." The term "gene" was coined by Johannsen in 1909. In this original usage, this word was supposed to be free of all hypotheses(note 3) (Darden 1991, p. 180). Morgan borrowed this word, but he added a lot of content to it based upon accumulated evidence. He defines "hereditary material (genes)" (Morgan 1917, p. 514) as follows: first he explained Mendel's two laws: the first law is that the characters do not blend with each other, and the second law is that characters are inherited independently. Then he continues, "the germ plasm must, therefore, be made up of independent elements of some kind. It is these elements that we call genetic factors or more briefly genes" (Morgan 1917, p.515). He also adopted the chromosome theory of heredity, so he assumed that genes have "a real basis" and are "located in the chromosomes" (Morgan 1917, p. 520). Genes are supposed to make incomplete linear linkage groups on chromosomes like "beads along a thread" (Darden 1991, p. 145). He also added the notion of the mutation of Mendelian factors (Darden 1991, p. 160).
Morgan's contribution added rich content to the concept of the gene. But its chemical nature remained unknown, and so were the mechanisms for how it works --such as the duplication of gene, the mutation, causation of characters, and so on. We should wait for the clue for the answer to these questions until the double helix structure of DNA, which was put forward by Watson and Crick (1953a). They explained the implication of the structure for the genetics in the next paper (Watson & Crick 1953b). A DNA molecule consists of two long helical chains of bases, and two sequences of bases linked complementary by hydrogen bonds with adenine bonding to thymine and cytosine to guanine. Watson and Crick suggest that "the precise sequence of the bases is the code which carries the genetical information" (Watson & Crick 1953b, p. 244). In this view, the "beads along a thread" model and mechanism of the duplication are nicely explained. They also put forward an explanation of mutation (Watson & Crick 1953b, p. 246), and Watson had already a vague idea about how DNA produces protein (Watson 1980, pp. 89-90). In this model, "gene" got an entirely new meaning, that is, correspondence to a segment of bases on DNA molecules.
3. Hull's evolutionary view of concept change
Before this paper starts to analyze the history, it will briefly state the view to be used. This is the view David Hull explained in his (1988a) and (1988b). He tries to explain concept change in terms of the analogy to the evolution of species. For this purpose, he abstracts essential parts of evolutionary theories (Hull, 1988b, pp. 134-135). First, he distinguishes replicators and interactors. A replicator passes on its structure to successive generations. An interactor interacts with its environment and this interaction causes differential replication of replicators. Selection is a process in which the difference in success of interactors causes the difference in replication. As a result of selection, some replicators pass on their structure through the time with or without small changes; this temporal succession is called a lineage.
Hull applies these notions to science itself (Hull, 1988a, p. 434). Replicators in science are beliefs, goals, methodologies, and so on. Interactors are scientists. Scientists acts for their conceptual inclusive fitness, namely, so as to encourage other scientists to use their works. For example, scientists give credit to other scientists in their own work because this increases the credibility of the work. This will in turn increase the possibility that the work is cited by other scientists (p. 310). His view on the mechanism of selection in scientific activities is not clear. A selection takes place in a scientist's mind, when he/she decides which paper to cite. Researchers of science need to do psychological investigation to know exactly what occurs. Despite this vagueness, Hull's evolutionary view provides an interesting method for analysing the history of science.
4. Concept change as an evolution -- the case of the gene
Now it is the time to apply Hull's account to this specific example -- changes in the gene concept. There are several eminent features in this history. First, the history shows the process of clarification -- especially clarification of the distinction between genotype and phenotype. Second, it shows the process of materialization -- at first genes are postulated in germ cells, then located on chromosomes, and finally get a chemical structure on DNA molecules. Third, many meanings are added based upon evidence -- such as, multi factors for a character, linkage group, mutation, and so on.
The first question is whether these changes are evolutionary, as Hull says, or revolutionary, as Kuhn (1962) says. At a glance, revolutions seems to occur. The chromosome theory of heredity changed the meaning of "gene" drastically, and so did the double helix structure of DNA. Mendel's "element" and Watson and Crick's "genetic material" seem to have totally different meanings. But at the same time there is a certain consistency in the usage of the concept. In Hull's term, these concepts form a "lineage." The core of this concept is obvious in Morgan's definition quoted above. Here "gene" is defined as hereditary materials that have some effect on Mendelian characters. This is also what Mendel meant when he used the word "element." Watson and Crick's DNA model seems to change the meaning of the word entirely. However, when we want to define a gene, even now we need to refer to the character the gene causes (Kitcher, 1994)(note 4). This evidence is enough to show that an evolutionary view is more appropriate than a revolutionary view in this case.
The second question is how this evolutionary change occurred. On the one hand, many of the changes seem to have occurred to explain some anomalous empirical data. Morgan's linkage group was devised to explain anomalies to Mendel's second law. Hull may interpret this situation as using empirical data to increase conceptual inclusive fitness, but this answer causes a further question, namely, the question of why empirical data matter so much to scientists. There are many other philosophical theories that give better explanation than Hull's about the importance of the data in concept change.
On the other hand, sometimes a concept seems to have prevailed with little empirical support. The idea of Mendelian factors attracted scientists even though there was no material basis which supported the existence of such factors until Morgan. The double helix DNA model was accepted with enthusiasm even though there was no direct evidence for the relation between the structure and the gene at that time. These phenomena can be interpreted as a result of scientists' efforts for conceptual inclusive fitness; that is, some scientists introduce attractive ideas in order to be cited by others, and other scientists cite these ideas in order to be cited in connection with the ideas. For example, Watson had a strong motivation for a Nobel prize (Watson, 1980), and getting a Nobel Prize is definitely a good standard for a high conceptual inclusive fitness.
Both of these two factors -- empirical data and scientists' motivation -- are indispensable if we want to understand scientific activities. Hull's theory gives us an answer to the former (though other theories do better) and a good insight into the latter. Because of the ambiguity mentioned above, however, his argument is not decisive in either case, and should be supplemented by further psychological investigation.
In this paper, the history of the concept of gene is traced. Then Hull's evolutionary view of concept change is introduced, and examined by applying it to the case of this concept. A couple of conclusions are reached. First, these concept changes are evolutionary rather than revolutionary. Second, Hull's approach may not give a good account for the relation between concept changes and empirical data, but it is promising in taking scientists' motivation into account. To sum up, Hull's argument is not decisive in this case, but his approach is insightful and presents an interesting research programme.(note 5)
(1)Maybe he meant to be ambiguous. This ambiguity was in a sense legitimate, because there was no direct evidence for such an invisible factor According to Maienschein, Castle later used this word purposively to blur the genotype-phenotype distinction. See Maienschein, 1994, p.125.
(2)These terms themselves are introduced by Johannsen in 1909; see Darden, 1991, p.40.
(3)He defined this word as "stood for the "something" in the gametes that determined (bedingt ) or "has some effect on" (mitbestimmt ) a character in the developing organism. See Darden, 1991, p.180.
(4)There is an ongoing debate between reductionists and anti-reductionists on this topic. See Kitcher, 1984, pp.343-346, Waters, 1990, pp.128-130.
(5)I owe a lot of ideas in this paper to the discussion in Prof. Darden's seminar. She also gave me a lot of good suggestions to the first virsion of this paper.
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