Thursday, May 23

Is there really such a thing as a spandrel?

While engaging in a bit of background research for last week's post, I stumbled upon a discussion of spandrels on one of my favorite blogs. When discussing the validity of evolutionary theories and evolutionary psychology in general, spandrels come up fairly often because they are usually red herrings. If a trait is a spandrel, and did not develop in an adaptive way, applying adaptive logic to it in order to justify your theory could undermine your work.

The article I linked to mentions the spandrel of blood color. As far as anyone knows blood doesn't appear red for any adaptive reason, it's just the color imparted by the iron element in hemoglobin. But hold on: the redness of inflamed or swollen tissues is absolutely used adaptively, in primate reproductive swelling, which I can't seem to stop referring back to.

The swellings themselves are certainly not spandrels; in baboons the swellings appear on their rumps, but in geladas swellings appear on their chests. Geladas are perhaps best knows by this trait, having been nicknamed the "bleeding heart baboons" (another of my favorite blogs). In each species the swellings appear in the most visible location. If these traits are adaptive and rely upon the redness of blood, can blood color still be a spandrel?

The real answer is that it doesn't really matter. Adaptive traits and spandrels are all evolving in concert with each other over the course of thousands and millions of years. None of these traits are independent, they are all inexorably intertwined (which is exactly why there are so many adaptive traits that rely on the bright redness of blood function). This is one of the most important lessons of ethology.

Gould and Lewontin, the the creators of the spandrel theory, wished to classify traits as spandrels or not spandrels. Spandrel traits can be found along all avenues of evolutionary biology, not just psychology. For instance, reproductive swellings in primates rely on both biology and psychology. Given the previously discussed risks of misapplying evolutionary psychology, focusing on spandrels just isn't a particularly useful or efficient way for an evolutionary psychologist to spend their time.

1 comment:

  1. Distinctions between adaptive and non-adaptive features of organisms are not qualitatively meaningful. It's really a scale of the level of detailed information that's required to determine whether an organism with a particular phenotype is going to produce more offspring. In many cases, it is useful to think of things as either selection-driven or products of genetic drift, but it's a mistake to think there's a clear-cut distinction between the two.