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The Crux of Crucial Experiments: Confirmation in Molecular Biology

Paper presented at the conference Confirmation, Induction and Science

London School of Economics, 8-10 March 2007

Marcel Weber

University of Basel

Philosophy Department and Science Studies Program



I defend the view that single experiments can provide a sufficient reason for preferring one among a group of hypotheses against the widely held belief that “crucial experiments” are impossible. My argument is based on the examination of a historical case from molecular biology, namely the Meselson-Stahl experiment. “The most beautiful experiment in biology”, as it is known, provided the first experimental evidence for the operation of a semi-conservative mechanism of DNA replication, as predicted by Watson and Crick in 1953. I use a mechanistic account of explanation to show that this case is best construed as an inference to the best explanation (IBE). Furthermore, I show how such an account can deal with Duhem's well-known arguments against crucial experiments as well as Van Fraassen's “bad lot” argument against IBE.

1. Introduction

Some of the major discoveries in the history of molecular biology are associated with an alleged “crucial experiment” that is thought to have provided decisive evidence for one among a group of hypotheses. Well-known examples include the Hershey-Chase experiment (1952), which showed that viral DNA, not protein, enters a bacterial cell to reprogram it to make virus particles, or the “PaJaMo” experiment (1958), which showed that a certain bacterial gene makes a substance that represses the activity of other genes.1 In both cases, there were two major hypotheses that could explain the facts known beforehand: Either viral protein or viral DNA contains the information for making new virus particles (Hershey-Chase). Similarly, either “generalized induction” (in the molecular biological, not logical sense!) or suppression of a repressor (the “double bluff” theory of Leo Szilard) was thought to be responsible for the regulation of sugar metabolism in bacteria. In both cases, a single experiment seems to have enabled a choice between the competing hypotheses at hand, thus strongly resembling Bacon’s “instances of the fingerpost” or Newton’s “experimentum crucis”.

Philosophers of science, of course, have been less than enthusiastic about the possibility of crucial experiments. Following Duhem (1954), many seem to think that a single experiment, as a matter of principle, is not able choose among a group of hypotheses. However, as I will show, Duhem made extremely strong assumptions concerning the kind of inferences that are to be permitted. Namely, he allowed only deductive inferences to be used. The main goal of this paper is to show that when crucial experiments are construed along the lines of inductive (ampliative) inference, Duhem’s arguments don’t go through.2

I want to demonstrate the possibility of crucial experiments on a concrete historical example, namely the Meselson-Stahl experiment (1957) in molecular biology. Even though there is an extremely detailed historical study of this experiment available (Holmes 2001), it has to my knowledge never been subjected to a thorough methodological analysis. “The most beautiful experiment in biology,” as it has been called, is widely thought to have demonstrated semi-conservative replication of DNA as predicted by Watson and Crick in 1953. But it remains to be shown that this experiment was actually decisive from a methodological point of view.

In Section 2, I will discuss Duhem’s infamous argument against crucial experiments. Section 3 provides a brief account of the Meselson-Stahl experiment and some of the theoretical controversies that preceded it. In Section 4, I argue that the case is best construed in terms of a mechanistic version of inference to the best explanation (IBE). Section 5 analyzes what I would like to call the “theory of the instrument”, which showed that the experimental method used by Meselson and Stahl was reliable. In Section 6, I show how my approach can handle Van Fraassen’s (1989) “bad lot” argument against the soundness of IBE. This will lead me to the conclusion that crucial evidence and inductive inferences in molecular biology are underwritten by complex systems of material assumptions and background knowledge.

2. Duhem on the Logic of Crucial Experiments

Duhem characterized crucial experiments as follows:

Do you wish to obtain from a group of phenomena a theoretically certain and indisputable explanation? Enumerate al the hypotheses that can be made to account for this group of phenomena; then, by experimental contradiction eliminate all except one; the latter will no longer by a hypothesis, but will become a certainty (Duhem 1954, 188).

This passage strongly suggests that Duhem thought of crucial experiments in terms of eliminative induction, in other words, in terms of the following logical scheme:

(1) H1H2

(2) H1e

(3) H2  e

(4) e

(5) From (3), (4): H2 [by modus tollens]

(6) From (1), (5): H1 [by disjunctive syllogism]

Such a train of inference faces two major problems according to Duhem. The first problem is the one that is today known as “Duhem’s problem”. This is the problem that auxiliary assumptions are needed to secure the deductive relation between hypothesis and evidence. Therefore, (5) will never involve a hypothesis alone; it will always be a conjunction of hypotheses that can be said to be falsified. Famously:

The only thing the experiment teaches us is that among the propositions used to predict the phenomenon and to establish whether it would be produced, there is at least one error; but where this error lies is just what it does not tell us (ibid., 185).

But if the falsity of one of the hypotheses at issue cannot be asserted, the inference (6) does not go through.

As if this weren’t enough, Duhem identifies a second problem:

Between two contradictory theorems of geometry there is no room for a third judgment; if one is false, the other is necessarily true. Do two hypotheses in physics ever constitute such a strict dilemma? Shall we ever dare to assert that no other hypothesis is imaginable? Light may be a swarm of projectiles, or it may a vibratory motion whose waves are propagated in a medium; is it forbidden to be anything else at all? (ibid., 190).

The answer to the latter, rather rhetorical question is clear: Unlike mathematicians, Physicists can never have grounds for assuming that they have exhausted the space of possible truths. In other words, there can be no warrant for premise (1) in the scheme above.

Given what he sets out to prove, Duhem’s arguments are impeccable. But note that Duhem is clearly thinking in terms of deductive inference. What he proves is that experiment conjoined with deductive logic is unable to bring about a decision for one among a group of hypotheses. He is dead right about that. However, Duhem’s arguments do not touch the possibility of inductive or ampliative inference enabling such a choice. An ampliative inference rule might very well be able to mark one hypotheses as the preferable one. The critical question will be if

such a procedure will not run into the same or similar difficulties. I shall save this question for later. Right now, it is time to introduce my historical example.

3. “The Most Beautiful Experiment in Biology”

As is well known, James D. Watson and Francis H.C. Crick closed their landmark paper on the structure of DNA with the short and crisp remark “It has not escaped our notice that the specific base pairing we have postulated immediately suggests a possible copying mechanism for the genetic material” (Watson and Crick 1953). It is fairly obvious what Watson and Crick had in mind: Because of the complementarity of the base sequences of the two nucleotide chains in the double helix, a DNA molecule could be copied by first separating the two strands, followed by the synthesis of two new strands using the two old strands as templates. On this scheme, each newly synthesized DNA molecule will contain one strand that was already present in the parental molecule, and one newly made strand. This scheme is called “semi-conservative replication.” However, as plausible as this scheme might seem, skeptics were quick to notice some theoretical difficulties. Here is the greatest skeptic of them all, Max Delbrück:

I am willing to bet that the complementarity idea is correct, on the basis of the base analysis data and because of the implication regarding replication. Further, I am willing to bet that the plectonemic coiling of the chains in your structure is radically wrong, because (1) The difficulties of untangling the chains do seem, after all, insuperable to me. (2) The X-ray data suggest only coiling but not specifically your kind of coiling (Delbrück to Watson, 12 May 1953, quoted from Holmes 2001, 21-22).

The term “plectonemic” referred to the topological property that, according to Watson and Crick, two DNA strands are twisted about each other so that they cannot be separated without uncoiling. The “base analysis data” refer to the work of Erwin Chargaff, who had shown previously that the building blocks of DNA occur in certain fixed ratios. Delbrück is also pointing out that the double helix was, at the time when Watson and Crick proposed it, strongly underdetermined by the available X-ray diffraction data (i.e., other coiled structures would have been consistent with these data).

But Delbrück not only expressed skepticism about the specific kind of coiling. His point (1) called into question the whole idea of a semi-conservative replication mechanism as suggested by Watson’s and Crick’s model. The problem was that, given the plectonemic topology of the Watson-Crick double helix, untangling the two strands requires the breaking and rejoining of the sugar-phosphate backbone of the molecule. Given the fast rate by which DNA replicates, especially in rapidly dividing bacterial cells, the molecule would have to rotate at mind-boggling velocities.3 This was also known as the “problem of untwiddling”. For a while, it was a major source of skepticism about Watson’s and Crick’s extremely elegant solution. While the structure itself became rapidly accepted thanks to the available of improved X-ray data, the semi-conservative replication mechanism continued to be doubtful for the years to come.

In the years following Watson’s and Crick’s announcement, two alternative replication mechanisms were proposed. Delbrück devised a scheme under which each newly synthesized DNA molecule contains bits of the parental molecule that are interspersed with newly synthesized material:

This became known as the dispersive mechanism.

Gunther Stent proposed that the whole double-stranded DNA molecule could serve as the template for synthesizing a copy. This would not require any untwisting of the parental molecule:

According to this mechanism, which was called the conservative mechanism, the parental molecule emerges unchanged from the replication process while the newly synthesized molecules contain only new material. The three mechanisms thus differ with respect to the distribution of parental and newly synthesized material that end up in the daughter molecules. This is summarized in the following diagram:

Thus, in the mid-1950s there were three different hypotheses concerning the distribution of parental and newly synthesized nucleic acid chains.

Now enter two young experimentalists, Matthew Meselson and Frank Stahl, working at the California Institute of Technology in Pasadena. Using a powerful analytic ultracentrifuge, they performed a remarkable experiment in 1957. In order to convey to the uninitiated reader the basic idea of this experiment, I will first show a highly schematic representation, before I discuss it in more detail.4

Meselson and Stahl grew E. coli bacteria in the presence of a heavy isotope of nitrogen, nitrogen-15. Ordinarily, DNA contains the most common isotope of nitrogen, which is nitrogen-14. But when grown in the presence of nitrogen-15, the bacteria incorporate the heavy nitrogen into their DNA. Now, DNA that contains the ordinary, light nitrogen atoms and the DNA containing heavy nitrogen can be distinguished by their weight. Of course, DNA occurs not in large enough quantities to be weighed by an ordinary balance. But Meselson and Stahl developed a highly precise instrument for determining the weight of DNA. They first dissolved the bacterial cells in a strong detergent. Then they placed the extract on a very dense solution of the salt CsCl. When a CsCl solution is centrifuged at very high speed in an ultracentrifuge for many hours, it will form a density gradient after a while. At equilibrium, the DNA molecules will float in that region of the gradient that corresponds to their own density. They form a band that can be observed with the help of UV light. Thus, they weight of the DNA molecules can be measured by determining the position of the band.

The experiment that Meselson and Stahl now did was to transfer the bacteria from a medium containing heavy nitrogen to a medium containing light medium and allowing the bacteria to multiply further. At regular time intervals after the transfer, they took samples and placed them in the ultracentrifuge. What they observed is that after one generation, a band of intermediate density appeared. After another generation, the intermediate band was still present, but a new band that corresponded to light DNA occurred. This is as the semi-conservative hypothesis predicted: If the intermediate band consists of hybrid DNA molecules that contain one light and one heavy strand, it therefore demonstrates semi-conservative replication.

The picture shown above is actually a highly schematized and in fact somewhat misleading depiction of the experiment, taken from James D. Watson influential textbook Molecular Biology of the Gene from 1965. I am showing it because it makes it easy to grasp the idea of the experiment. In Watson's depiction, the centrifuge buckets look like those of a preparative centrifuge familiar to most biology students. But of course, Meselson and Stahl used an analytical ultracentrifuge, which is a huge and complex machine where the rotor looks very different and is equipped with sophisticated optical equipment in order to monitor the refractive index and UV absorption of the sample.

Meselson and Stahl's original data therefore looked rather different:

These are UV absorption photographs of the ultracentrifuge cell. The bands show where the DNA floats in the CsCl density gradient. What is particularly important about these data is that the band of intermediate density was located exactly in between the heavy and light bands. As both theoretical calculations and measurements showed, the density gradient was very nearly linear in the range were the DNA was floating (see Section 5). This allowed the inference that the intermediate band contained molecules that were composed of heavy and light nitrogen exactly in a 1:1 ratio, as predicted by the semi-conservative hypothesis.

The impact of this experiment on the scientific community at that time was considerable. Almost everyone agreed that the Meselson-Stahl experiment beautifully demonstrates semi-conservative replication. The only exception known to me is Max Delbrück, but his role in the closely knit molecular biology of that time seems to have been that of advocatus diaboli anyway.

In the following section, I shall provide a methodological analysis of this experiment and its evidential support for Watson’s and Crick’s semi-conservative mechanism.

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