The Questions of Developmental Biology

Developmental Constraints
Another consequence of interacting modules is that these interactions limit the possible
phenotypes that can be created, and they also allow change to occur in certain directions more
easily than in others. Collectively, these restraints on phenotype production are called
developmental constraints.
Physical constraints
There are only about three dozen animal phyla, constituting the major body plans of the
animal kingdom. One can easily imagine other types of body plans and animals that do not exist.
(Science fiction writers do it all the time.) Why aren't there more major body types among the
animals? To answer this, we have to consider the constraints that development imposes on
evolution. There are three major classes of constraints on morphogenetic evolution.
First, there are physical constraints on the construction of the organism. The laws of
diffusion, hydraulics, and physical support allow only certain mechanisms of development to
occur. One cannot have a vertebrate on wheeled appendages (of the sort that Dorothy saw in Oz)
because blood cannot circulate to a rotating organ; this entire possibility of evolution has been
closed off. Similarly, structural parameters and fluid dynamics forbid the existence of 5-foot-tall
mosquitoes.
The elasticity and tensile strengths of tissues is also a physical constraint. The six cell
behaviors used in morphogenisis (cell division, growth, shape change, migration, death, and
matrix secretion) are each limited by physical parameters, and thereby provide limits on what
structures animals can form. Interactions between different sets of tissues involves coordinating
the behaviors of cell sheets, rods, and tubes in a limited number of ways (Larsen 1992).
Morphogenetic constraints
There are also constraints involving morphogenetic construction rules (Oster et al. 1988).
Bateson (1894) and Alberch (1989) noted that when organisms depart from their normal
development, they do so in only a limited number of ways. Some of the best examples of these
types of constraints come from the analysis of limb formation in vertebrates. Holder (1983)
pointed out that although there have been many modifications of the vertebrate limb over 300
million years, some modifications (such as a middle digit shorter than its surrounding digits) are
not found. Moreover, analyses of natural populations suggest that there is a relatively small
number of ways in which limb changes can occur (Wake and Larson 1987). If a longer limb is
favorable in a given environment, the humerus may become elongated, but one never sees two
smaller humeri joined together in tandem, although one could imagine the selective advantages
that such an arrangement might have. This observation indicates a construction scheme that has
certain rules.
The rules governing the architecture of the limb may be the rules of the reaction-diffusion
model (outlined in Chapter 1; Newman and Frisch 1979). Oster and colleagues (1988) found that
the reaction-diffusion model can explain the known morphologies of the limb and can explain
why other morphologies are forbidden. The reaction-diffusion equations predict the observed
succession of bones from stylopod (humerus/femur) to zeugopod (ulna-radius/tibia-fibula) to
autopod (hand/foot). If limb morphology is indeed determined by the reaction-diffusion
mechanism, then spatial features that cannot be generated by reaction-diffusion kinetics will not
occur.