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More Complexity by Subtraction: The Case of the Gecko

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You may recall the "complexity by subtraction" theory from 2013. It’s back. Three researchers from UC Riverside with two other colleagues have a new way to get animals to evolve: take away a "remarkable innovation" so that they are free of the constraints and costs that the innovation generates. Case in point: subtract the adhesive gecko toe pads that are the envy of biomimicry engineers. That way, the lizards can run faster.

The theory is presented in a paper in the Proceedings of the National Academy of Sciences. Higham et al. studied different genera of geckos and paid special attention to the ones that had lost their adhesive toe pads.

Innovations permit the diversification of lineages, but they may also impose functional constraints on behaviors such as locomotion. Thus, it is not surprising that secondary simplification of novel locomotory traits has occurred several times among vertebrates and could potentially lead to exceptional divergence when constraints are relaxed. For example, the gecko adhesive system is a remarkable innovation that permits locomotion on surfaces unavailable to other animals, but has been lost or simplified in species that have reverted to a terrestrial lifestyle. We examined the functional and morphological consequences of this adaptive simplification in the Pachydactylus radiation of geckos, which exhibits multiple unambiguous losses or bouts of simplification of the adhesive system. We found that the rates of morphological and 3D locomotor kinematic evolution are elevated in those species that have simplified or lost adhesive capabilities. This finding suggests that the constraints associated with adhesion have been circumvented, permitting these species to either run faster or burrow. (Emphasis added.)

How did that "remarkable innovation" arise in the first place? Never mind about that. A look at how they use the word "innovation" in the paper might lead you to think it does so by magic. "Structural innovations involving coordinated changes in multiple anatomical systems" (that almost sounds like irreducible complexity) "…are associated with the diversification of many groups of vertebrates." But how do those innovations, involving coordinated changes in multiple systems, arise? Well, they just do. They occur. They evolve. They are acquired from the "Acquisition" Department:

Many such innovations often occur repeatedly within, as well as between, clades. One example is the evolution of the prehensile tail, which has arisen in primates, nonprimate mammals, seahorses, amphibians, and several groups of lizard, allowing its possessors to move through the environment in novel ways. Although the acquisition of such innovations is often implicated in both diversification and ecological specialization, much less is known about the causes and consequences of their secondary reduction and loss.

Once again, they show they are not blind to the fact that gecko pads are a remarkable innovation: "Morphological innovations like the adhesive system of geckos are complexly integrated modifications," they admit. But that’s the last you hear about "innovation" in this paper.

Let’s look for the word "novel" or "novelty." Maybe that’s where they will tell us about the addition of innovations instead of their subtraction. They talk about "novel ecological circumstances," but those apply to environments, not animals.

If increased competition or the formation of a structurally novel environment (e.g., desertification) results in geckos occupying flatter, more open terrain, then selection likely favored the reduction or loss of the adhesive system.

That doesn’t help.

Furthermore, the origin of locomotory abilities not associated with climbing, such as burrowing in nocturnal species and greater running speeds in diurnal geckos, could lead to the rapid evolution of both morphology and kinematics.

It "could" supposedly, but what kind of evolution are they talking about? Not novelty; not innovation; just different applications of the same equipment. Geckos already have legs; without adhesive toe pads, they could run faster. Geckos already have toes; without the adhesive setae, they could burrow into the sand now without getting their setae clogged. Try to hold your applause, if you possibly can.

That’s all we hear about "novelty." The rest is about subtraction.

We examined the functional and morphological consequences of reduction and simplification of the adhesive apparatus within the Pachydactylus radiation. Using a comparative framework, we predict that species that have reduced or have lost adhesive capabilities: (i) will show different limb kinematics than those that use adhesion, (ii) will exhibit accelerated rates of kinematic and morphological evolution, (iii) will display different kinematic responses to changes in substrate incline, and (iv) will run faster or more slowly than those with a fully expressed adhesive system.

Voil�, their predictions are confirmed! Subtraction of the adhesive toe pads allows some geckos to move faster (or slower), change their stride, and fall on the ground when trying to climb vertically (if that’s a valid example of what they mean by "different kinematic changes to substrate incline").

In accompanying pictures, they show some species have elongated middle toes and webbing between toes. But the machinery for making digits was already there. What was there for natural selection to do besides adjusting the knobs? No new knobs were added.

They also describe in mathematical detail the percentage changes in stride length, stride frequency, and speed on various inclines. One would think those factors are correlated, for one thing, and might fall within the error bars of lizards in general. They certainly are adjustable by epigenetic processes on existing genes. The "morphological evolution" they talk about does not include any really new thing, like wings or novel organs.

It’s hard to see how they could fill six pages of two-column text to support such a vacuous theory. Building to a climax, they try to make evolution by subtraction a universal principle:

Unlike exaptations, which result from the co-option of a structure from an original function to a "new" one, evolution can also favor the reduction or simplification of a previous complex adaptation, especially when the adaptive complex compromises functional abilities in a new environment. This type of adaptive change, resulting in reduction or loss, has been documented for a number of other taxa, such as eye loss in cave fishes, in which the energetic cost of maintaining vision is likely great enough to be detrimental to survival in a cave environment. The reduction and simplification of the adhesive apparatus in geckos is not the only such case exhibited by the locomotor system. Almost every tetrapod group includes cases of limb reduction or loss, and associated body elongation.

Will some critically minded evolutionary biologist please fill in the blanks here? How do true complex innovations originate by aimless Darwinian processes? That’s the question. We understand subtraction, but want to hear more about addition.

Image: Kirk Crawford/Flickr.