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Biologists Are Getting to Be Less Reticent About Using the Phrase "Design Principles"

We’re seeing more instances of biologists (especially cell biologists) writing about “design principles” in their papers. Phrases like that are hard to square with blind evolutionary mechanisms. Yes, they still believe that natural selection finds ways to optimize things, but that gets harder to justify the more the focus is on design. Here’s a recent example in a PNAS paper about signaling networks.

Cells continually have to sense their environments to make decisions — to stay put or move, to differentiate or proliferate, or even to live or die. However, they are thwarted by noise at the cellular scale. Cells use signaling networks to filter this noise as much as possible and sense accurately. To operate these networks, resources are required: time, protein copies, and energy. We present a theory for the optimal design of cellular sensing systems that maximize sensing precision given these resources. It reveals a new design principle, namely that of optimal resource allocation. It describes how these resources must be allocated so that none are wasted. We show that the chemotaxis network of Escherichia coli obeys this principle. (Emphasis added.)

This sounds like an intelligent design paper, but the authors, Christopher C. Govern and Pieter Rein ten Wolde from the Netherlands, put in the obligatory nod to natural selection:

Our analysis reveals that this network obeys the principle of optimal resource allocation. This indicates that there is a selective pressure on not only the topology of sensing networks that enhances robustness of adaptation, but also on the efficient allocation of cellular resources for precise sensing.

Let’s count the mentions of “design” and see if they explain how “selective pressure” gets the signal to the top of Mt. Improbable, where “optimal resource allocation” prevails. The word “design” appears 24 times in the paper. “Selection” appears twice, in the phrase “selective pressure” (one of them is just a repetition from the Abstract). Any form of the word “evolution” appears just once:

To the extent that all resources affect growth, evolutionary pressure should tend to drive systems so that no resource is wasted, which occurs when all are equally limiting.

We see, therefore, that “design” references outnumber evolutionary references eight to one. We also find “machine” or “machinery” four times, “coding” or “encoding” 15 times, “information” (in terms of information to be processed) five times, “accurate” (in terms of sensing accuracy) 11 times, “precision” 29 times, “efficient” four times, and “optimal” or “optimum” 28 times. Taken together, these design words outnumber evolution words 40 to 1.

Do the three passing references to evolution/selection add anything to the paper? One would expect to see it in the final Discussion section, but instead, we find these references to design:

These design principles are in marked contrast to those of equilibrium sensing systems, which are not driven out of equilibrium via fuel turnover: the sensing precision of these systems is limited by the number of receptors; downstream networks can never improve the accuracy of sensing….

Finally, the process of sampling a time series, like the receptor state over time, defines a specific, familiar computation that could be conducted by any machine; it is instantiated in the biochemical system by the readout-receptor pair.

In other words, the cell meets the stringent “design principles” for optimal signaling (high accuracy at lowest cost) that could be expected from any well-designed system of information-processing machines. It doesn’t float downstream to equilibrium; it fights upstream to the prize.

In their one reference to evolution, the authors merely state that “evolutionary pressure should tend to drive systems” to the optimum. Did anyone tell the cell that?

Nothing about fitness, survival, adaptation, or the environment can make any organism do what it “should” do. It’s much easier to roll over and die when blind, purposeless process are the only things around to “drive systems” along. Will a salmon evolve precision navigation systems because it “should” swim upstream to spawn? Of course not. The fish doesn’t care. The river doesn’t care. Nothing is cheering the salmon on to fight its way miles up against the current. It would just as soon float downstream and go extinct. Same with E. coli.

The paper would lose nothing if its three passing references to evolution/selection were left on the cutting-room floor. All these scientists could do was look at the end product and decide, “Yep, it’s fit. It’s optimal.” Seemingly, they would just as soon let somebody else tell the evolutionary story, if one is needed. They said their pledge of allegiance to Darwin, now leave them alone.

Image: Scanning electron micrograph of Escherichia coli/Wikipedia.

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