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Biomimicry of forests or trees?

A colleague is teaching a class on biomimicry and asked whether I had a short summary of evolution as background. I gave him part of "Evolution 101", Chapter 3 of my book on Darwinian Agriculture, but also some material the editor made me leave out. Here it is:

A brief analogy with engineering may be useful. Imagine that a pair of ideas-from-nature experts were asked to help engineers improve a bicycle safety helmet. Ideally, they would like to increase fracture resistance, while decreasing weight. Here's how the discussion might go.

Ideas-from-nature expert #1: "Consider mimicking the structure of conch shells. I don't know anything about their structure, but I do know that sting rays have been eating conchs for millions of years, by crushing their shells. Whichever conchs had the toughest shells were most likely to survive and reproduce, all else being equal, because the rays would give up on them and go crack a weaker conch. Whenever mutations arose that made shells stronger, those mutants would have tended to survive longer, so their descendants have displaced those who made slightly weaker shells. This process has been repeated for millions of years, so the structure of conch shells is probably pretty good by now."

Engineer (one week later): "We tested conch shells and their fracture-resistance is indeed amazing (Kamat,S. 2000). Microscopy reveals that conch shells have alternate layers of hard minerals and stretchy organic material. We have some man-made materials with similar properties, but I think we can improve on the conch-shell design. In particular, we could make each of the two kinds of layers thicker. Thicker layers will probably make the material even stronger."

Ideas-from-nature expert #1: "Maybe, but surely mutant conchs with thicker layers must have arisen repeatedly. If thicker layers are better, why didn't conchs with thicker layers in their shells displace conchs with thinner layers?"

Engineer: "Interesting question. Maybe thinner layers actually give better interaction between the two materials. Also, thicker layers would be heavier, for a given number of layers."

Ideas-from-nature expert #1: "And a conch with a heavier shell couldn't swim as fast. Natural selection balanced the tradeoff between strength and weight. Evolution is all about such tradeoffs. We can't be sure that conchs have evolved the best possible compromise between shell strength and weight, but it's probably better than most slightly different alternatives. That's because alternatives that are only slightly different have probably arisen via mutation, and tested through indirect, sting-ray-imposed competition."

Engineer: "OK, but our bike helmets will also be exposed to ultraviolet light from the sun. Conch shells didn't have that additional challenge."

Ideas-from-nature expert #1: "Right. Some of the challenges that shaped conch-shell evolution are the same as those we're facing in designing a bike helmet. Other challenges are different. I can see that you recognize the difference, so I'll leave you to your work."

Ideas-from-nature expert #2 (arriving out of breath): "Sorry I'm late. I just heard the last part of that. I have a different idea. Instead of copying conch shells, what about copying rocks? They've been tested over thousands of years. I know where there's some obsidian that's been exposed to ultraviolet light and erosion for millennia, and it's still there. Maybe your helmet shells should mimic the glass-like structure of obsidian."

Engineer (one week later): "We tested the obsidian. The conch shell is hundreds of times stronger."

Ideas-from-nature expert #2: "Well, how about this sedimentary rock? It's been around for millions of years, so it must be really strong. It has particles of different sizes; maybe you could copy that structure."

Engineer (one week later): "We tested the sedimentary rock, and it's also much weaker than the conch shells."

Ideas-from-nature expert #2: "Hmmm... Why not combine our best ideas? Here's a sedimentary rock that contains conch shells, embedded in inorganic materials. It's been around for millions of years, so it's probably great. We could make bike helmets with little bits of conch-like material, embedded in some rock-like material."

Ideas-from-nature expert #1: "This illustrates two different kinds of testing over time. The structure of the conch shells in the sedimentary rock has been tested and improved by millions of years of indirect competition among conchs having different shell structures. But the organization of the sedimentary rock (the ratio of conch shell to other materials, or whether the conch shell is uniformly distributed or clumped) hasn't been improved by competition with other rocks. By simply persisting over millions of years, the rock passed a weaker kind of test than that imposed on conch shells through competition over millions of years. If reasonable durability were all we needed, it would be safe to conclude that the rock meets that minimum standard. But, in the absence of data, we have no reason to assume that the rock is more durable than some manmade material, like concrete. We would have to run lots of expensive tests to find out. The conch shells, on the other hand, have already been subject to millions of years of comparative tests by sting rays."

Ideas-from-nature expert #2: "OK, then what about land-snail shells? Their structure has been tested by competition (like conch shells), yet they're sometimes exposed to ultraviolet light (like rocks)."

Ideas-from-nature expert #1: "Good idea. Let's explore that option."

Natural selection has tested the adaptations of trees competitively, like conchs. Forests have only been tested by their ability to persist, like rocks. Nature's wisdom may be found more in trees than in forests.

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