Changing While Staying the Same: Preservation of Structural Continuity During Limb Evolution by Developmental Integration

Imagine a human arm morphing into the wing of a bat. Each finger grows long and slender. Webbing extends to the tips. Blood vessels and nerves weave out into the spaces between. Long tendons extend to connect muscle to bone across joints that adjust their range of motion to support powered flight. Opponents of Charles Darwin found the limb to be one of the greatest challenges to his theory that natural selection, acting gradually on small variations, could be sufficient to generate as complex a structure as a wing (Darwin 1859; Mivart 1871; Gould 1985). Functional morphologists have long tackled the problem of identifying the adaptive advantage of “incipient”, or partial structures, but criticisms are also grounded in the assumption that morphological restructuring of limbs and other structures requires simultaneous and sometimes massive remodeling of multiple tissues in a coordinated manner (Mivart 1871). From this perspective, it seems almost implausible that limbs acquire qualitatively different morphologies or functions from their ancestral state, and yet the limb has been modified in myriad ways in all vertebrate lineages. The idea that restructuring requires genetic changes independently affecting multiple tissues stems from early ideas that the embryo forms as an assemblage of parts, each manifesting its own intrinsic destiny. We now know that development proceeds by a combination of intrinsic (or autonomous) and extrinsic (or nonautonomous) mechanisms. Component parts are in constant communication, and abnormalities or deficiencies are often compensated by neighboring cells and tissues. Together, autonomous and non-autonomous mechanisms contribute to both the robustness and malleability of the developing organism (Lawrence and Levine 2006). Kirschner and Gerhart (2005) argued that the interdependence of developing tissues confers plasticity of organ morphology, and that if this plasticity is evolutionarily conserved, functionally integrated structures are produced in response to the change in a component tissue. This process, which Kirschner and Gerhart termed “exploratory behavior,” is not only important for preserving function in the face of environmental and genetic variation, but may also have a crucial role in producing novelty by amplifying the effect of change occurring in one tissue or cell type (Muller 1990; West-Eberhard 2003; Kirschner and Gerhart 2005). Tissue interdependence in development and evolution has been perhaps best illustrated by embryonic manipulations to produce chimeric faces of the “quck” and “duail” (Schneider and Helms 2003). Interspecific transplants of neural crest cells from the quail to duck and vice versa showed there is an intrinsic program by which the neural crest cells form the beak or bill of their species of origin. These experiments also showed the ability of neural crest-derived structures to non-autonomously impart donor-species characteristics on host-derived tissues. For example, the gene expression of the overlying ectoderm and morphology of the egg tooth match the donor rather than the host. Together, these outcomes demonstrate both the autonomous and non-autonomous properties of embryo development and illustrate how genetic change in one tissue (the neural crest-derived skeleton) can influence the development of another (the ectoderm-derived egg tooth). At this small scale, neighboring integrated tissues function as the “environment” that mutually influences morphological development. On a grander scale, features of the developing organism are so malleable that behavior can have a great impact on form and function. Mary West-Eberhard termed this effect “phenotypic accommodation” and described it as the “adaptive mutual adjustment among various parts during development without genetic change” (West-Eberhard 2003). She argued that organisms could adjust their physiology or morphology in a single generation in order to adapt to an environmental change. If mutations occur that “hardwire” the phenotypic outcome, these allele frequencies could increase in a population by natural selection in a process called “genetic assimilation” (Waddington 1953; West-Eberhard 2003, 2005). A striking example of phenotypic accommodation occurred when a two-legged goat, born without forelegs, learned to hop upright. The Dutch anatomist, Everhard J. Slijper, acquired this remarkable animal and, after an unfortunate accident resulting in its death, described its unique anatomical features. Slijper’s goat had developed a suite of characteristics that appear convergently in naturally bipedal species, including modifications to its spine, ribcage, hindlimbs, and musculature (Slijper 1942a, 1942b). This example highlights the potential evolutionary role of behavior in shaping the musculoskeletal system through the coordinated change of multiple structures and tissue types. If locomotor changes and their concomitant morphological effects are adaptive, and if they alter an animal’s occupied niche, such changes may also influence and accelerate the trajectory of subsequent evolutionary events (Diogo 2017). Although these examples support the concepts of exploratory behavior and phenotypic accommodation, the mechanisms acting to shape the limb during evolution remain largely unknown. Since developmental integration of multiple tissues in the limb likely underlies evolution of the same complex structures, we can make great strides by considering lessons from experimental embryology. Here, we detail the origins and developmental interdependence of the skeleton, muscle, and connective tissues of the vertebrate limb. While much is known of the molecular pathways intrinsic to the development of each tissue, we limit our discussion to communications between tissues and to the environmental effects of mechanical force and locomotion. We highlight opportunities where tissue integration and responsiveness to mechanical force may maintain structural and functional continuity as the limb is transformed by natural selection. The diverse outcomes of selection for novel limb structures and functions are innumerable. We therefore leave the discussions of specific morphologies to others, including the complementary article on the evolution of avian limbs by Vargas and colleagues that also appears in this issue (citation to be added in press).