390 CHAPTER 15All organisms are subject to physical constraints. For example, the maximal
possible size of insects is thought to be limited by the rate at which oxygen and
carbon dioxide can diffuse through the narrow tubes, or tracheae, that conduct
gases throughout the body. John Maynard Smith and colleagues defined devel-
opmental constraint as “a bias on the production of various phenotypes caused by
the structure, character, composition, or dynamics of the developmental system”
[43]. The two most common suggested causes of developmental constraint are (1)
absence or paucity of phenotypic variation, including the absence of morphoge-
netic capacity (i.e., lack of required genes, proteins, or developmental pathways),
and (2) strong correlations among characters, caused by pleiotropy. Thus, develop-
mental constraints can be considered a form of genetic constraint (see Chapter 6).
Developmental constraints can be revealed by embryological manipulations in
the laboratory. In a classic experiment, Pere Alberch and Emily Gale used the mito-
sis-inhibiting chemical colchicine to inhibit digit development in the limb buds of
frogs (Xenopus) and salamanders (Ambystoma; FIGURE 15.25A,B) [2]. The treatment
consistently caused preaxial (front) digits to be missing in the frogs, and postaxial
(rear) digits to be lost in the salamanders. These results correspond to the different
order of digit differentiation in the two taxa: the last digits to form tended to beFutuyma Kirkpatrick Evolution, 4e
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Evolution4e_15.24.ai Date 11-02-2016FIGURE 15.24 In a work both amus- (A) (B)
ing and well informed by evolu-
tionary principles, Harald Stümpke
imagined an adaptive radiation of
“Rhinogradentia,” or “snouters,”
mammals with noses elaborated for
diverse functions. Otopteryx flies
backward, using its ears as wings
and its nose as rudder. Orchidiop-
sis feeds on insects attracted to its
petal-like nose and ears. Stümpke’s
fantasy illustrates some of the many
conceivable phenotypes that have
never evolved. (From [74].)FIGURE 15.25 Evidence for develop-
mental constraints. (A) X-ray of the right
hind foot of an axolotl salamander (Am-
bystoma mexicanum), showing the nor-
mal five-toed condition. (B) The left hind
foot of the same individual, which was
treated with an inhibitor of mitosis during
the limb bud stage. The foot lacks the
postaxial toe and some toe segments,
and is smaller than the control foot.
(C) A normal left hind foot of the four-
toed salamander (Hemidactylium
scutatum) has the same features as the
experimentally treated foot of the axolotl.
(From [2]; photos courtesy of P. Alberch.)Futuyma Kirkpatrick Evolution, 4e
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Evolution4e_15.25.ai Date 11-02-2016(A) (B) (C)15_EVOL4E_CH15.indd 390 3/22/17 1:30 PM