16) Developmental Constraints and Morphological Evolution (by: Rey J. Rosa Morales)

        Traditional neo-Darwinian theory explains how natural selection, genetic drift, and gene flow, acting on the raw material of genetic variation, have produced the astonishing variety of organisms. But does it explain why organisms have not evolved certain features, or in certain directions? Does it explain why there are no live-bearing turtles, for instance, or why frogs have no more than four digits on their forelimbs? Such questions have led evolutionary biologists to ask what the constraints on evolution might be.

        Several kinds of constraints on evolution have been distinguished. Some are universal, in that they affect all organisms; an example is the constant presence of gravity during morphogenesis. Others, referred to as Phylogenetic Constraints, are more local, affecting only a group of related organisms. There are several types of constraints on evolution:

1. Physical constraints. Some structures do not evolve because the properties of biological materials (e.g., bones, epidermis, DNA, RNA, etc.) do not permit them.
2. Selective (or functional) constraints. Some features do not appear in particular lineages because they are always disadvantageous, or because they might interfere with the function of an existing trait.
3. Genetic constraints. Genetic variation in a particular phenotype may not be present. Developmental pathways are expected to have varying degrees of tolerance for variation in their components, and their limits of tolerance may limit variation in the resulting traits. Genetic constraints, such as paucity of variation and genetic correlation, are closely related to developmental constraints. 
4. Developmental constraints. Maynard Smith et at. (1985) defined a developmental constraint as "a bias on the production of various phenotypes caused by the structure, character, composition, or dynamics of the developmental system."  The two most common phenomena attributed to developmental constraints are absence or paucity of variation, including the absence of morphogenetic capacity (i.e., lack of cells, proteins, or genes required for the development of a structure), and strong correlations among characters, which may result from interaction between tissues during development or the involvement of the same genes or developmental pathways in multiple morphogenetic processes.

     Developmental constraints can be revealed by embryological and genetic manipulations in the laboratory. For example, in a classic experiment, Pere Alberch and Emily Gale (1985) used the mitosis-inhibiting chemical colchicine to inhibit digit development in the limb buds of salamanders and frogs (Figure 1 below). The treatment consistently caused specific digits to be missing in each species, and the missing digits were the preaxial ones in frogs and the postaxial ones in salamanders. These results reflected the different order of digit differentiation in the two species; the last digits to form tended to be the most sensitive to the colchicine treatment. Furthermore, the results strongly reflected evolutionary trends: salamanders have often lost postaxial digits, and frogs have repeatedly experienced preaxial digit reduction, during evolution. Although the digit number variation in the study bias produced artificially, the results suggest that naturally occurring variation in developmental systems may be constrained by intrinsic, species-specific developmental programs.

Figure 1. Evidence for developmental constraints.  (A) X-ray of the right hind foot of an axolotl salamander (Ambystoma mexicanun), showing the normal five-toed condition. (B) The left hind foot of the same individual, 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 treated foot of the axolotl in B. (From Alberch and Gale 1985; photos courtesy of the late P. Alberch.)

References:

1. Alberch, P., S. J. Gould, G. F. Oster, and D. B. Wake. 1979. Size and shape in ontogeny and phylogeny. Paleobiology 5: 296-317.
2. Futuyma, D.J. (2005). Evolution. Sinauer Associates, Inc. Publishers Sunderland, Massachusetts U.S.A.
3. Palmer, RA (2004). "Symmetry breaking and the evolution of development". Science 306 (5697): 828–833.