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Theoretical Commitments as Guide and Constraint

Disputes of the type discussed in the previous section, concerning novel experimental techniques, are of course not unusual in ‘micro histories’ of scientific developments. What makes the case of membrane models in cell biology (Sect. 2.3) significant, is the way it illustrates the complex relation, across a span of several decades, between theoretical commitments and—old and new—empirical evidence.

In another recent case study of the interplay between model, theory, and evidence in molecular biology, Samuel Schindler argues that Francis Crick and James Watson’s discovery of the structure of DNA would not have been possible, had they not single-mindedly—and in the face of contravening evidence—pursued their double-helix hypothesis. Whereas ‘Crick and Watson stuck to one structural hypothesis throughout’, Rosalind Franklyn ‘conducted experiments and, on the basis of the obtained evidence, she tried to infer the structure that would fit her data best’ (Schindler 2008, 627)—alas, her attempt ‘to use her experimental data as fully as possible’ (as Crick described it; 1988, 68) turned out to be self-defeating, given the diverse range of X-ray photographs, some of which seemed to contradict any helical structure. From the perceived superiority of the ‘top-down’ strategy of sticking with a structural hypothesis, Schindler draws far-reaching conclusions, ruling out both ‘the abductive component of IBE’ and ‘classical hypothetical-deductive confirmation’ as ways of describing this historical episode, and concludes that ‘taking particular evidence too seriously blocked some researchers from making discoveries, which were made by others who ignored the same evidence’ (2008, 651). Sometimes, then, saving models from empirical challenges—as in the case of Watson and Crick sticking with the structurally appealing DNA double helix model, even in the face of seemingly disconfirming X-ray diffraction images—may turn out, by hindsight, to have been a fortuitous shortcut to what we now take to be the best scientific account of the underlying structure of reality.

However, the case of membrane models should give defenders of the ‘top-down’ strategy cause to pause. Unlike in the DNA double helix case, stubborn adherence to an elegant, simple, and universal (i.e., ‘neat’) structural hypothesis—in the form of the unit-membrane model, consisting of a lipid bilayer coated uniformly with proteins—did not advance but, on the contrary, hindered the discovery of the true structure of the cell membrane, which turned out to be a messy mosaic of different kinds of proteins embedded in, and attached to, a lipid bilayer without any discernible long-range order. Importantly, it was not only new phenomena such as the rapid intermixing of proteins in heterokaryons that should have called the ‘elegant’ unit- membrane model into question: recall that, as early as the late 1920s, membrane proteins were known to be amphipathic and not completely polar and, arguably, should never have been considered obvious candidates for forming a uniform protein cap in the first place (see Sect. 2.3). The very theoretical virtues of the unit- membrane model—its simplicity and assumed universality, combined with the cognitive attractiveness of its easy visualizability—shielded it from potentially challenging phenomena, whose evidentiary significance was not realized until new technological developments and experimental techniques created the conditions that made a successful synthesis in the form of the fluid-mosaic model possible. Whenever the ‘beauty’, ‘elegance’, or ‘universality’ of a model are adduced as a reason for saving it from empirical challenges, caution is therefore advised. What matters is that incoming evidence be assessed on the basis of a range of criteria— which must include technological conditions of evidentiary significance that reflect the most current and most reliable techniques available.

 
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