From: Damien Broderick (thespike@earthlink.net)
Date: Thu Jan 16 2003 - 23:24:53 MST
> Spandrelized? Is that one of those scanner errors? {8^D
Check out S. J. Gould, who coined it (and in my comment I extended it as a
general metaphor, as others have done). Hell, let's just google:
http://www.ecs.soton.ac.uk/~harnad/Hypermail/Thinking.Psychologically96/0105
.html
The concept of an evolutionary "spandrel" has come about from an
architectural term relating to a necessary by-product of creating
something else. The exact analogy quoted by Pinker and Bloom (1990)
relates to Gould and Lewontin"s (1979) reference to the arches and
spandrels in the dome 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." This feature has been elaborately
decorated with mosaics. An easy mistake would be to assume that the
triangular spaces had been put there intentionally as part of the
detailed decoration. In fact, they were there because of something
else. This argument is used by Gould and Lewontin to explain some
aspects of evolution which they do not feel can be accounted for by
natural selection. Epiphenomenal spandrels are seen as those which exist
but are of no real interest, such as the redness of blood, the V-shaped
space between a pair of fingers or the fact that there are a
prime-number of digits on each limb. Such spandrels have no direct
relevance to any behaviour or function and give no clues as to whether
the structures that they are associated with were shaped through natural
selection.
The next rung up the ladder is to look at those spandrels which have
been modified and put to better use. This cannot be done alone and
requires the intervention of natural selection. It may be seen as the
process responsible for the modified design of an already existing part
in order to put it to more effective use.. This is where nature makes
the best possible use of what already exists to suit its own needs and
purposes; remembering that it does not have an empty page on which to
begin its creativity.. These are usually a single, unique part or
repeated shapes or processes involving simple physical or geometric
laws, such as hexagonal honeycombs and spiral markings. These are
modified and arranged alongside other more complex processes in order to
perform a function. Ultimately, they can only be explained by natural
selection.
The nonselectionist aspect of evolution concerns unmodified spandrels.
This is used to describe a structure that already has a useful purpose
but that original function is usurped by another. Gould (1987a) (as
cited by Pinker & Bloom (1990)) talks about a wading bird that uses its
wings primarily as a sun visor when looking for fish in the water. It
is difficult to argue successfully that something with such a complex
engineering design as the wing would have evolved through natural
selection for the primary purpose of blocking out the sun. A simpler
structure would have done the job just as well.
This explanation of spandrels attempts to clarify what is being
discussed in terms of nonselectionism. It is also important when
looking at evolution to be aware of two other possibilities which exist
outside of selectionism. [tc]
===
Damien Broderick
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