150 INTELLIGENT DEVELOPMENTDevelopmental Plasticity
Canalization of development is obviously advantageous when a single
form or function fi ts a predictable environment— one that recurs from
parents to off spring across many generations. Having initially appeared
accidentally, such a single, well adapted, form would have been hugely
favorable to survival. It and any gene variants supporting it would have
been targets of strong natu ral se lection.
However, off spring can sometimes experience signifi cant aspects of
environments not experienced by immediate parents. Th is demands
a diff er ent kind of development: an adaptability to changed circumstance
in a way that could not have been foreseen in the information in genes.
Th e phenomenon came to be called developmental plasticity.
A popu lar example is that of the water fl ea, Daphnia. If the juveniles
develop in the same body of water as a predatory midge larvae (Chao-
borus), they develop a protective neck spine or helmet and extended tail
spine. Th ese defensive structures allow the Daphnia to escape from their
predators more eff ectively. Th is structure is completely absent in their
parents, who have developed in predator- free water.^11
Numerous other cases of intelligent, predator- induced, plasticity have
been studied. Tadpoles of the wood frog (Rana sylvatica) that grow in
water previously containing predatory dragonfl y larvae— and, presum-
ably, some chemical produced by them— develop bigger tails that allow
faster swimming and turning. A species of barnacle reacts to the presence
of predatory snails in its environment by developing a bent shell form that
is more resistant to predation compared with the more typical fl at form.
Th e logic of such pro cesses is fairly obvious. It would be wasteful of
developmental resources if the defensive structures were produced when
they are not needed. It makes sense to leave the direction of growth and
reproduction until aft er the adult habitat has been reached. So they are
induced on the spot by a chemical substance released by the predator.
Sometimes gross changes in morphology are involved. Some of the most
striking examples are the diff er ent castes in bees and ants. Here, devel-
opmental plasticity radically alters be hav ior and physiology as well as
anatomy and is unrelated to ge ne tic variation. Locusts also develop
physiologically and behaviorally distinct morphs in response to current
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