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Spandrels/Exaptations vs Adaptations

by bohm2
Tags: adaptations
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Jul16-13, 10:35 AM
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Some scientists question the use of the term "natural selection" arguing that its use (really over-use) has become a vacuous tautology. They argue that other forces such as physical and chemical laws/constraints are at work shaping evolution. From my understanding, it's like why Helium came after Hydrogen during the evolution of our universe: there are serious constraints based on physical laws that shape evolution that often don't have much to do with natural selection/adaptation. In a classic paper, Gould and Lewontin warned against "naive adaptationism," the inappropriate use of adaptive theorizing to explain traits that have emerged for other reasons. The argument is illustrated by an analogy with the mosaics on the dome and spandrels of the San Marco basilica in Venice:
Spandrels-the tapering triangular spaces formed by the intersection of two rounded arches at right angles...are necessary architectural by-products of mounting a dome on rounded arches. Each spandrel contains a design admirably fitted into its tapering space. An evangelist sits in the upper part flanked by the heavenly cities. Below, a man representing one of the four biblical rivers ... pours water from a pitcher in the narrowing space below his feet. The design is so elaborate, harmonious, and purposeful that we are tempted to view it as the starting point of any analysis, as the cause in some sense of the surrounding architecture. But this would invert the proper path of analysis. The system begins with an architectural constraint: the necessary four spandrels and their tapering triangular form. They provide a space in which the mosaicists worked; they set the quadripartite symmetry of the dome above. Such architectural constraints abound, and we find them easy to understand because we do not impose our biological biases upon them....Anyone who tried to argue that the structure [spandrels] exists because of [the designs laid upon them] would be inviting the same ridicule that Voltaire heaped on Dr. Pangloss: "Things cannot be other than they are ... Everything is made for the best purpose. Our noses were made to carry spectacles, so we have spectacles. Legs were clearly intended for breeches, and we wear them." ... Yet evolutionary biologists, in their tendency to focus exclusively on immediate adaptation to local conditions, do tend to ignore architectural constraints and perform just such an inversion of explanation.
The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist program

Now a recent paper just published in Nature seems to support their criticism of adaptationism:
The question of how often adaptive traits have non-adaptive origins has profound implications for evolutionary biology, but is difficult to address systematically. Here we consider this issue in metabolism, one of the most ancient biological systems that is central to all life...Our observations suggest that many more metabolic traits may have non-adaptive origins than is appreciated at present. They also challenge our ability to distinguish adaptive from non-adaptive traits.
A latent capacity for evolutionary innovation through exaptation in metabolic systems

Other write-ups on that paper:
The findings underscore the idea that traits we see now-even complex ones, like color vision-may have had neutral origins that sat latent for generations before spreading through populations, Wagner says...If exaptations are pervasive in evolution, he adds, it becomes difficult to distinguish adaptation from exaptation, and it could change the way evolutionary biologists think about selective advantage as the primary driver of natural selection.
Great Exaptations: Most Traits Emerge for No Crucial Reason, Scientists Find

Simulated metabolic networks show exaptations far outnumber adaptations

Gould along with some other scientists like Lewontin, Chomsky, etc. have even suggested that many of our mental systems (e.g. language) may have also arose as nonadaptations:
The human brain is the most complicated device for reasoning and calculating, and for expressing emotion, ever evolved on earth. Natural selection made the human brain big, but most of our mental properties and potentials may be spandrels—that is, nonadaptive side consequences of building a device with such structural complexity. If I put a small computer (no match for a brain) in my factory, my adaptive reasons for so doing (to keep accounts and issue paychecks) represent a tiny subset of what the computer, by virtue of inherent structure, can do (factor-analyze my data on land snails, beat or tie anyone perpetually in tic-tac-toe). In pure numbers, the spandrels overwhelm the adaptations. The human brain must be bursting with spandrels that are essential to human nature and vital to our self-understanding but that arose as nonadaptations, and are therefore outside the compass of evolutionary psychology, or any other ultra-Darwinian theory.
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Greg Bernhardt
Jul31-13, 01:32 PM
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Wow, some really great ideas here! It will take me some time to look through them. One thing I've always thought was that biology/evolution should be a required class in high school. There is so much misconception about even the basic fundamentals.
Jul31-13, 06:30 PM
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I think one of the more annoying and simultaneously intriguing examples of over-use is evolutionary psychology. Without a concrete molecular story, you can make up anything you like to explain human behaviors as an evolutionary adaptation.

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