1238 THE STRUCTURE OF EVOLUTIONARY THEORY
diving as directly adaptive (that is, first evolved in conjunction with, and presumably
for, the behavior) in two cases, but as exaptive in the other four sequences.
Interestingly, the exaptive lineages coopted their sand-diving movements from two
different functional sources in ancestral lineages: from "part of the drilling
mechanism used in firm substrates" (p. 161) by ancestral lineages that hide
themselves in harder grounds, and "from the burial pattern adopted before periods of
inactivity" (p. 161) in other ancestral lineages (that is, from movements carried out
much more slowly, in a clandestine fashion, and for purposes of dormancy rather than
escape from predators).
Two additional criteria might be cited as evidence for both the use and
usefulness of exaptation as a concept in evolutionary biology, and in other forms of
historical study as well.
- Utility in fields distant from evolutionary biology. Markey (1997) invoked
our concept of exaptation to explain the peculiar history of the letter perth in the runic
alphabet of futhark. (The word "futhark" is an acronym for the first six letters of the
runic alphabet, just as "alphabet" itself combines the first two letters of the Greek
sequence, alpha and beta.) Perth must have had ordinary phonemic value in a still
earlier system, but the letter is never glossed in futhark texts and has left no
descendants in any Germanic language. (Actually, the letter occurs only one time in
all runic literature—in the English Rune Poem.)
Markey (1997) argues that, having lost its original phonemic use, "the p-rune
appears to have been a redundant luxury" (p. 10). Some versions of later futhark
alphabets simply eliminated the symbol, but others retained the p-rune, apparently for
an interesting structural reason with excellent literary analogy to quirky functional
shift in biological features. The p-rune happened to stand right at the middle of the
futhark alphabet, where its phonemic suppression encouraged a different use as a
place marker or mnemonic guide, at the halfway point of a long sequence more easily
recalled in two divided halves. (Markey shows that several versions of futhark added
letters, but always kept the p-rune right in the middle of the sequence.)
Markey argues that the p-rune, phonemically extinct in Germanic tongues, then
lost its exapted function as a place marker when these languages replaced the runic
alphabet with our current Latin system. At this point, the p-rune again resisted
extinction by another exaptation, this time for spelling Latin loan words with an
initial p sound before a vowel (as in papa for "pope," or pater for "priest"), a
phonemic combination not found in Germanic words. (Runic-p served the same
function for some English loan words of non Indo-European origin, as in "pebble.")
In any case, Markey (p. 11) found our biological concept of exaptation useful in
describing this complex double quirky functional shift from ordinary phonemic value
in a hypothetical ancestor, to place marking in futhark, to renewed phonemic value
for loan words when the Latin alphabet replaced futhark: "Exaptation is manifested
by functional bifurcation. The primary function of feathers was warmth, the
secondary function flight. The primary function of runic-p appears to have