"Mendel's 'Hybridism': An Analysis of Two of His Theories and of Their Relationships"

LORENZANO, PABLO

Congreso; 25th International Congress of History of Science and Technology; 2017

Division of History and Philosophy of Science and Technology

It might be said that Johann Gregor Mendel worked in a similar way to members of the school (or tradition) of "horticulturalists" (or "plant breeders"), but with the intention to answer questions raised by members of the school (or tradition) of "hybridists". The main problem Mendel faced and tried to solve was the problem of "hybridism" ("Can new species be originated by means of hybridization of preexisting species?") and not the problem of inheritance, a problem that may be related to the former, but cannot be identified with it. Based on a statistical analysis of his experiments, which was a novelty for both traditions, and seeking a "generally applicable law governing the formation and development of hybrids" (Mendel 1865: 3), Mendel states "the law of development/evolution found for Pisum" (Mendel 1865: 32), which decomposes in a restricted version, "the law of simple combination of characters" (Mendel 1865: 32), and in a generalized form, "the law of combination of different characters" (Mendel 1865: 32). But neither of them is identical to the laws usually attributed to Mendel, namely, the "Law of Segregation of Genes" and the "Law of Independent Assortment of Genes" of Classical Genetics. The law (or laws) stated by Mendel is formulated in terms of characters, and not in terms of "factors", "alleles" or "genes", i.e. in terms of those entities responsible for the expression of the characters. When Mendel tries to provide the "foundation and explanation" (Mendel 1865: 32) of the law of formation and development of hybrids, he does it in terms of the production and behavior of egg cells and pollen cells, and, ultimately, in terms of the nature and behavior of what he calls "elements" (Mendel 1865, p. 41) or "cell elements" (Mendel 1865, p. 42). But Mendel's concept of cell element is different from the Classical Genetics' concept of factor, allele or gen. Mendel's elements are of a different nature of, and behave in a different way as, factors, alleles or genes. Furthermore, Mendel recognizes the existence not just of hybrids that behave like those of Pisum ‒ i.e. of "variable hybrids" ‒ but also of hybrids that "remain perfectly like the hybrid and continue constant in their offspring" (Mendel 1865: 38) and "acquire the status of new species" (Mendel 1865: 40) ‒ i.e. of "constant hybrids" (for this distinction see also Mendel 1869: 27-28, 31; Correns 1905: 1270). In that way, Mendel supports "hybridism (in the narrow sense)", i.e., hybridism conceived as establishing a mechanism of speciation, that is, of evolution. And the law that would govern the behavior of constant hybrids would also find its foundation and explanation in terms of the nature and behavior of elements (or cell elements). For all these reasons, it can hardly be said that Mendel had been a proponent, even less the first proponent, of Classical Genetics. In fact, it might be said that Mendel was an excellent "hybridist". His hybridism consists of two theories: a first theory that moves on a level, let's say, more "empirical" or "phenomenological", according to Schleiden's first "special guiding maxim", the "Maxim of the history of development/evolution [Entwicklungsgeschichte]", which can be called "Mendel's theory of the development/evolution of hybrids" (DEH), and a second  theory that moves on a level, let's say, more "theoretical", according to Schleiden's second "special guiding maxim", the "Maxim of the autonomy of cells in plants", which can be called "Mendel's theory on the cellular foundation of the development/evolution of hybrids" (CFH).The aim of this communication is to present a conceptual analysis of these two theories ‒ "Mendel's theory of the development/evolution of hybrids" and "Mendel's theory on the cellular foundation of the development/evolution of hybrids" ‒ and of their relationships.