Exaptation and cookie cutters

Elaine Morgan (Elaine@desco.demon.co.uk)
Mon, 25 Sep 1995 12:13:11 GMT

"Exaptation" was a term introduced bt E. Vrba and Stephen Jay
Gould to describe evolutionary features that seem not to be
advantageous to the species in which they occur.The term has been
widely taken up and is sometimes loosely used as if it simply meant
"it is no good looking fior an explanation of this; there isn't obne,
and there doesn't need to be."

One of Gould's metaphors for these apparently functionless features is
"spandrels". A spandrel is an architectural feature found in
association with domes and arches. For non-architects it is not
necessarily enlightening, especially when the real experts get down to
debating the difference between a spandrel, a pendentive, and a
squinch. I'd like to propose a humbler metaphor from home baking which
I think makes the same point.

If you have a shhet of pastry and a circular cookie-cutter and cut out
and remove 16 contiguous pastry circles arranged in rows of four, there
will remain nine diamond-shaped segments with concave sides. Anyone who
hadn't seen the first operation might think: those diamonds, so uniform
and regularly arranged, must have a purpose. What was it? But they were
what Gould calls spandrels. They were the accidental result of some
other process of which the purpose is easier to discern.

Okay, I can think of possible evolutionary examples. Face-to-face sex,
for instance. People used to think of a rewason why this was adaptive.
e.g. as a way of cementing the pair bond, because a mating couple can
look into one another's eyes during coition,,and this was supposed to
make them more aware of, or more likely to remember later, who they
were doing it with. My own first thought was (guess what!) that it must
be an aquatic adaptation because the ventro-ventral approach is very
common in aquatic mammals and very rare in land ones. I now concede
that it was a spandrel, an inevitable result of the spine and the hind
limbs being habitually arranged in a straight line, thus altering the
angle of the vagina. To my mind that was indirectly aquatic because I
believe bipedalism was a result of wading; but I that if anyone comes
up with a convincing non-aquatic explanation of bipedalism, they will
have explained ventro-ventral sex also. The aquatics are frequently
ventro-ventral of course because swimming also arranges the limbes and
spione in a straight line.

BUT. Important but. I think you can only legitimnately use the term
"spandrel" as a way of dismissing apparently non-adaptive features
if you can offer some kind of idea about what the spandrel was the
side-effect OF : if you can point to the dome or the arch, or to the
pile of cookies, or to the bipedal or natatory locomotion, and say
*that* was adaptive, and it led secondarily to this other thing.

Gould's definition of exaptive is that it describes "any organ not
evolved under natural selection for its current use - either because it
performed a different function in ancestors (classic preadaptation) or
because it represented a non-functional part available for later
co-aptation" (Bully for Brontosaurus, p. 144n)

I am quite happy for Alex to say maybe the descended larynx or any
other feature was exaptive in the first of these two senses - i.e. it
performed a different function in our ancestors ( possibly during the
semi-aquatic phase). But I am not happy for anyone to say they are
exaptive in the second, fuzzier, "non-functional"
spandrel sense, without giving some idea of what the spandrel was a
spin-off from. The pastry diamonds may not have been the object of the
exercise, but they weren't there by accident either. "Non-functional"
is a cop-out word that collects undertones of pure chance.