Moss - What genes cant do - 2003, страница 8

Exemplars are modelproblem-solutions. They provide a kind of perceptual and methodological template for how to solve some new class of problems by learning tosee that class as being similar to the one that has been solved.The idea of particulate factors responsible for the inheritance of characteristics was not unique to Mendel; however, Mendel’s paper illustratesan exemplar for how to set up an empirical practice which makes goodon the concept through the ongoing production of data. In the light ofan exemplar, nature shows up a certain way, its joints construed in justthat manner which allows a certain kind of problem solving to ensue.

Inorder to clarify Mendel’s role in the history of the gene-concept one mustlook back beyond the interpretations of de Vries and Bateson to Mendelhimself. Mendel, unlike his mentor C. F. Gärtner, who was principallyaddressing a professional audience, maintained a close relationship withthe practical concerns of his Moravian countrymen. Where Gärtner andother professional breeders were occupied with questions of fertility andthe changeability of species associated with the breeding of interspecific24Chapter 1hybrids, Mendel’s innovation as a breeder was to turn his attentiontoward intraspecific hybrids. Mendel crossed intraspecific varieties thatdiffered in a few discrete properties in order to clarify breeders’ problems with the constancy of introgressed traits (Falk, 1995).Gärtner, who hybridized thousands of interspecific plants, did notobtain uniform types adequate for abstracting quantifiable results.

Withpractical motivations, Mendel established an exemplar which allowedfor the appearance of discrete traits, that is, a paradigmatic approachwhich simultaneously defined what could count as a proper trait and disclosed its existence. He chose to work with the garden pea (Phaseoluspisum), in which hybrids from the mix of red and white flowering plantsresulted in only red and white progeny, as opposed to working with thesweet pea (Phaseolus lathyrus), in which hybrids of red and white flowering plants produced red, white, and pink progeny (Falk 1991). Of theseven traits of the pea plant that Mendel worked with, two of these, podcolor and seed coat color, were always associated with other characteristics of the plant.

In constructing his exemplar, Mendel chose to classifythese as “pleiotropic” effects of the same “unit-trait” and circularly todefine a unit-trait as that which is transmitted as a unit in hybridizationexperiments (Falk 1991). What Mendel’s construction of the “unitcharacter” provided, what his exemplar—i.e., the inheritance of unitcharacter traits of the garden pea—offered was a problem-solvingapproach by which plant breeders could make instrumentally tractablethe cultivation of properly selected discrete traits.

Mendel was proferring neither a universal theory of the constitution of the organism noran account of the relationship of the genotype to the phenotype. Indeed,Mendel did not distinguish between a genotype and phenotype, nor didhe have to. Rather, he adhered to a simple preformationist view of theunit-character which obviated any need for a more elaborate developmental story. As the geneticist and historian Raphael Falk has argued:Mendel’s reductionist perception is basically that of a preformationist, so thatthe hidden Factoren were for him identical with Anlagen—that is, preformedorgans or traits.

It was the preformed Anlagen of the traits that were transmitted in the ovules and pollen cells. On empirical as well as conceptual grounds,Mendel consciously selected from the traits available to him only those that“truthfully” represented the Anlagen, since he, like many of the early studentsof Mendelism thirty-five years later, did not conceive of traits as entities that aredistinct from the preformed potential for the traits (Falk 1995).Genesis of the Gene25Mendel’s reduction of the Anlagen to a preformationist concept of heritable unit-characters can be understood in its practical context. His wasa form of “instrumental preformationism.” Where Weismann’s far moreelaborate model of corpuscularized Anlagen, being pure speculation,could not provide biologists with systematic empirical support, theappeal of Mendel’s work was precisely its exemplary character.The one tension that Mendel had to face in identifying the trait withthe Anlagen for the trait was the disappearance of traits in the case ofheterozygotes.

He handled this, in the absence of any theory of development, by attributing to the unit-characters (Anlagen) the properties ofdominance and recessivity. Again, the sense in which Mendel’s solutioncan be characterized as a kind of instrumental preformationism comesto the fore. By attributing the determination of expression to the preformed Anlagen, i.e., to the unit-character itself, Mendel could sidestepquestions of developmental interactions which were outside of his scopeof interest and irrelevant to the practical applications of his exemplar.The scope and ambitions of Mendel’s exemplar with its rhetoricalresources became greatly expanded, however, when appropriated for useby evolutionary theorists.While the recognized champions and proximate conduits ofMendelism, such as Hugo de Vries, William Bateson, and WilhelmJohannsen, were themselves plant breeders and naturalists, the ultimatepath by which a Mendelian gene concept became the twentieth-centurysuccessor to the nineteenth century’s Keime und Anlagen followed theconversion of certain leading embryologists to the newly constitutedscience of genetics (Gilbert 1978).

The classical gene concept emergedlargely out of debates between embyologists, which culminated in thegenetic studies of T. H. Morgan at Columbia University. The impact ofMendel’s work on Morgan and other embryologists, however, was notdirect but rather mediated by the first generation of Mendelians.Enjoining Mendel’s DiscretionAt the turn of the century, Darwinism was lacking a consensual theoryof heritable variation.

The banner of orthodox Darwinism was held bythe biometrician and eugenicist Frances Galton (also Darwin’s cousin)and his student Karl Pearson. The biometrical approach to heredity and26Chapter 1variation was largely statistical in nature and represented differences inorganisms along the continuum of a bell curve. The notion of variationalong a continuum, however, presented certain problems for a Darwinian theory of evolution. The classification of life-forms into species recognizes discontinuities. Heritable variations along a continuum are likelyto be quickly assimilated to the common form through blending. TheDutch botanist and plant breeder Hugo De Vries, whose pangenesismodel of inheritance followed the particulate tradition of Weismann,attempted to address this problem through a theory of evolutionarysaltations based on large mutations.

De Vries’s mutations would be thecause of new species and not susceptible to assimilation by blending(Allen 1975). For de Vries the role of natural selection would be onlythat of eliminating unsuitable novelties and not the design of biologicalinnovations by means of progressive incremental processes.In England, William Bateson found that organisms distributed alongan environmental gradient are prone to vary in a discontinuous manner.He too concluded that whereas the environment provided the basis forvariation along a continuum, variation based directly on inheritance wasdiscontinuous (Allen 1975).

It was this vociferous debate between theadvocates of continuous variation and those of discrete variation whichset up and structured the rediscovery of Mendel’s paper in 1900 (Carlson1966), with the latter—i.e., de Vries, Bateson, et al.—finding in Mendela critical resource with which to bolster their cause.Evolutionary Mendelism and the New Particulate PreformationismIf Mendel’s preformationist model can be conceived as instrumental, thatis, if Mendel’s work was born of “an effort to select those specific traitsthat segregate as distinct units, in order to achieve the empirical simplification of the problem of inheritance-as-transmission” (Falk 1995), thende Vries’s intentions were of a much broader theoretical scope:de Vries mobilized Mendel’s heuristic of selecting appropriate traits that wouldmake his experiments intelligible, and turned it into a basic conception of histheory of “intracellular pangenesis,” namely, that organisms are composed ofunit-characters that are distinct in transmission as much as in development (Falk1995).Genesis of the Gene27For de Vries the point of Mendelism was to vindicate a particulatepreformationist general theory of the organism which supported amacromutational12 theory of variation and evolutionary speciation:According to pangenesis the total character of a plant is built up of distinct units.These so-called elements of the species, or its elementary characters, are conceived as tied to bearers of matter, a special form of material bearer corresponding to each individual character.