MARVALEE H. WAKE
Morphology and development are in many ways both the oldest and the newest facets of biology. The morphology of organisms is what we perceive—it is the phenotype, with all its variation and different expressions. Morphology is shape, and size, and color, and ornamentation; it is body parts—stems and leaves, limbs and jaws, sperm, neurons. Morphology is the structural basis for organismal function (i.e., physiology), including respiration, metabolism, and reproduction, and in many organisms—animal, plant, and “microorganism”—for locomotion and other forms of behavior. It is both the proximal contact and the facilitator of an organism’s interaction with its environment. Morphology is the structural composition of organisms at many levels of the hierarchy of biological organization—molecules to cells to tissues to organs to individuals. Morphology is also ontogeny, that is, the physical representation of the changes in structural and functional arrangements that organisms undergo during their lifetimes—from fertilization and cleavage to an adult state, often the reproductive phase, and then the changes of adulthood through senescence to death. The study of morphology can be descriptive, comparative, mechanistic, experimental, and evolutionary.
Development is the way morphology comes to be, in both ontogenetic time and evolutionary time. Development is the process by which morphology is achieved; it changes over ontogenetic time in individuals and over evolutionary time in species and higher taxa; it is shaped genetically and ecologically; and it provides both variation and constraint. Analysis of development, too, includes many levels of the biological hierarchy, ranging from the signaling molecules and proteins that initiate development to the interaction of development with physical factors such as temperature and light. The scientific study of development today is largely experimental and mechanistic, but it is founded in description and can incorporate comparison and even evolutionary frameworks.
One of the great revelations of later twentieth-century biology is of the common genetic basis for much of the morphological organization of organisms—microbial, plant, or animal—and at the same time the basis for the interactions that produce organismal diversity through development and over ontogenetic and evolutionary time scales. Therefore bodies and body plans will loosely be the theme of this chapter—morphology is the pattern, the expression, the phenotype; development is the process by which it is achieved, through genetic and environmental mediation. Bodies and body plans, how they come to be, developmentally and evolutionarily, how they change, and especially what understanding these and related questions will contribute to resolution of some of the scientific challenges of the next century are the questions I will address.
The recently celebrated wedding of development and evolution has attracted much attention because of the insights that developmental biology provides on micro- and macroevolution. (However, given my biases, I believe that more explicit attention should be paid to the way morphology has contributed to that wedding, and the way the wedding is modifying morphology.) Development and evolution are currently enjoying a splendid honeymoon, with major scientific advances daily and the approbation of scientists and the public, after a long courtship that was at times divisive and difficult. But I expect that in the near future the marriage will become polygamous, recognizing morphology and incorporating ecology, behavior, and so on in order to understand and appreciate the bases of complexity and change of organisms and of ecosystems. An integrative approach to the study of morphology and development will contribute to a more synthetic paradigm for the study of biology as a science. That study promises new contributions to society, including the abilities to repair developmental abnormalities, manufacture new body parts from biomemetic materials, make robots that can go places living organisms cannot, understand biodiversity and its maintenance and value, and commit to the social and economic importance of understanding the basis of life.
The last century has witnessed great change in the science of morphology and development. The century opened with developmental biology being largely descriptive embryology, practiced by some of the great morphologists of all time (e.g., Weismann, von Baer, and Haeckel)—those who raised the curtains on the plays of experimental biology and evolutionary biology. But shortly after the turn of the century, morphology and development diverged intellectually and theoretically, as morphology became a major player in systematic biology and evolutionary theory, and development became an experimental field. Both, in various ways and at various times, incorporated new discoveries, in theory and techniques, from genetics, biochemistry, and the emerging fields of cell and molecular biology. By the end of the twentieth century morphology had also become an experimental field, while retaining its descriptive and analytical framework; it now incorporates techniques from other scientific arenas, including genetics, ecology, physics, and engineering. We have a new synthesis of development and morphology in many ways, as questions about the evolution of developmental patterns and processes, the ways morphologies arise and change, and the ways morphologies interact with one another and the environment are attracting the attention of many biologists. The new polygamy is generating new directions for research and discussion.
I will give just a few examples of contributions in morphology and development that illustrate the progression of our understanding from the turn of the nineteenth to the twentieth century to the present, in the context of our understanding of bodies and body plans, and that open a window on the future of the science and its contributions. Then I will conjecture about the future—where I hope development and morphology will progress, how I hope they will be major parts of a new, integrative approach to questions of reductionism and complexity, and how they will contribute to new science and to society as a whole, with an ethic that emphasizes development, morphology, variation, structure-function relationships, and their conservation and potential in the broadest sense. My examples are of animals, mostly vertebrates, because I know them best, but the general principles I will develop obtain for other organisms as well, and my perspective is that of an organismal, evolutionary, and integrative biologist.
Developmental biology became an experimental field as it diverged from descriptive morphology and embryology at the beginning of the twentieth century. The stage had been set by the emergence of questions that could not be answered by inference from dissections and histological examination. The sequence of development (not the stages) of many animals, invertebrate and vertebrate, and plants had been described in detail. Significant similarities in early development, followed by great morphological divergence, had been mapped. Weismann’s theory of the continuity of the germ plasm was being explored, and von Baer, Haeckel, and others had promulgated various versions of the biogenetic theory (Nordenskiöld 1928; Hall 1999)—summarized in the notion that ontogeny recapitulates phylogeny, or that stages of development within and across lineages are more similar in early development than in later. These ideas reinforced the concept of the Bauplan, or body plan, and illustrated that much of variation is modification of an overall body plan.
Ernst Mayr (1982) has provided a thought-provoking analysis of the recent history of morphology and development, emphasizing the century that followed the publication of Darwin’s Origin of Species in 1859 but pertinent to the most recent forty years as well. He commented that at the beginning of the twentieth century zoologists continued the search for homology of all components of anatomy, with an emphasis on phylogeny and the reconstruction of common ancestors. This search produced extensive descriptive treatises, often with analyses of transitions in morphologies, such as fins to limbs and reptile jaw components to mammalian middle ear bones. But Mayr noted that post-Darwinian morphologists ignored a major tenet of Darwinian biology—the explanation of adaptation. They were dealing with archetypes, or common body plans, and their explanations.
Mayr attributed a change in paradigm to the development of evolutionary morphology in the 1950s, when morphologists stopped looking backward to the common ancestor but used the common ancestor as the starting point to ask what evolutionary processes were responsible for the divergence of the descendants. The questions became how new morphologies arise, how selection pressures are involved, what are the characteristics of populations in which changes occur. As Mayr put it, “Evolution … [is] the totality of the processes that are involved in evolutionary change.” He saw evolutionary morphology building bridges to ecology and behavior in order to assess those processes, but he stated that “the solution to perhaps the greatest problem of morphology requires a bridge to genetics, a bridge which at this time cannot yet be built.” He was referring to the origin and the meaning of the great anatomical types, already known to Buffon under the name “unity of plan.” And he cited the mammalian Bauplan, which includes types as different as whales, bats, moles, horses, and humans “without any essential change of the mammalian plan.”
In several of his more recent books Mayr has developed the idea of the domains of the genotype, especially the somatic domain and what happens to it during evolution. He has asked a series of questions (Mayr 1988): “What happens to the genotype during speciation? What happens in the genotype during … evolutionary innovations of the phenotype? What structures of the genotype are responsible for long-time stasis, including the preservation in ontogeny of ancestral developmental stages (such as gill arches in tetrapods) and the stability of the Bauplan (common body plan) of the major types of organisms?” He asserted in his book One Long Argument (1991) that “development involves highly complex interactions between different domains of the genotype and different somatic programs,” and that study of those interactions would be a holistic approach of great potential to understanding the bases of evolution, but he indicated that such work had not yet begun.
B. K. Hall in Evolutionary Developmental Biology (1999) asked similar questions about the developmental processes that initiate and maintain Baupläne (body plans). Hall defines body plans as common, basic organizational plans that reflect community of descent, and then modification of structure within the body plan (concepts going back to Darwin and before). For most workers the Bauplan is common to higher taxa, but the idea that it is a nested set of body plans incorporating and characterizing the evolution of lineages is now receiving attention. As Hall (1999) elucidated, any Bauplan beyond the level of the species represents a nest; for example, snakes have a Bauplan different from those of lizards and turtles but still share a reptilian Bauplan; reptiles and mammals have individual body plans but share the vertebrate Bauplan. This concept permits analysis of the origins of similar structures at any taxonomic level, the search for mechanisms of homology, and the evolution of the origin of characters. The characters of Baupläne can include physiological and ecological as well as morphological features, so Baupläne are “functional types” (Kaufman 1995).
At the same time investigation of the nature of Baupläne is coupled with phylogenetic analysis, so taxonomic levels and the nature of monophyletic and polyphyletic groups are essential to comparative analysis. Hall posed six major questions about body plans: whether Baupläne constrain development and/or evolution, how Baupläne arose in evolution, how rapidly they were assembled, why there are so few Baupläne, why no new Baupläne have arisen in the last half billion years, and whether the existence of body plans necessitates macroevolutionary processes. He noted that this is the opportunity to ask how similar structures arise at any taxonomic level. He anticipated a search for the mechanisms of homology, the unraveling of the roles of key innovations and constraints in the origins of body plans, and determination of whether characters, features of morphologies, arise sequentially or in a coordinated manner. These questions are fundamental to much current study of morphology and development in a broad sense and allow the integration of many fields of biological investigation. Hall’s Evolutionary Developmental Biology presented an erudite history of experimental developmental biology and the recent incorporation of interest in the evolution of development and development in evolution.
Another notable contribution to the field is Wallace Arthur’s Origin of Animal Body Plans (1997). Arthur introduced his examination of the origins of body plans and the evolution of animal development by noting that neo-Darwinian theory is incomplete because it lacks the component of individual development or ontogeny, which is obvious when one reads attempts to explain the origins of the approximately thirty-five animal body plans, and especially the developmental patterns that generate them. Arthur examined development and the morphologies it generates in terms of paleontology, selection theory, and especially developmental genetics. Developmental genetics to him includes the coevolution of interacting genes, gene expression patterns in time and space, and comparisons of patterns (and perhaps processes) across lineages of animals. (Arthur’s table of the rules for naming and abbreviating genes and proteins for those organismal biologists who may not have assimilated the acronyms is a welcome contribution.) His discussion of body plans and phylogeny is synthetic; it argues for “bringing back morphology” in that context.
Both Hall and Arthur review developmental mechanisms thoroughly but from different perspectives. However, the arena of developmental genetics is exploding, and new information accumulates daily. As already noted, eight years ago Ernst Mayr proclaimed his ideas about the interactions of genotypic domains and somatic programs “only words.” Since then a substantial amount of work has accumulated that has built the missing bridge to genetics. Researchers have begun to work through those “highly complex interactions between different domains of the genotype and different somatic programs” and to elucidate answers to some of the questions that Mayr, Hall, and those before (and after) them have posed, as shown in the following examples.
I will present four examples to elucidate the progress in understanding what development and morphology are, as both pattern and process, and to illustrate their contributions to the science of the next century. I have chosen these particular examples because they illustrate novel approaches, are far-reaching in implications, and are potentially or actually integrative approaches to the study of morphology and development, and in some cases ecology and evolution. Remember, my perspective is that of an organismal biologist but one who believes in a transhierarchical approach to biological complexity.
One of the major accomplishments in developmental biology was Hilde Mangold and Otto Spemann’s identification of the dorsal lip of the blastopore as the “organizer” region that somehow controls or facilitates development and differentiation in frogs and salamanders. With a series of experimental implantations of dorsal lip tissue into various regions of the gastrula at slightly different stages of development and differentiation, they demonstrated that the tissue caused the formation of particular structures, especially a second body axis and all its derivatives (e.g., notochord, neural tube, somites), no matter where it was placed (Spemann and Mangold 1924). The dorsal lip tissue as the organizer of vertebrate development quickly became dogma, and the experiments were repeated in different animals, so that a general principle was assumed. However, for many years following the Spemann-Mangold reports there was no information about what the organizer really was and how it implemented and influenced development. In fact, Ernst Mayr noted as recently as 1991, paraphrasing his colleague June Goodfield (1968), that one must avoid the acceptance of unanalyzed terms, such as organizer, which only inhibit analysis.
In the decade since that cautionary comment, understanding of the organizer has dramatically increased. Richard Harland identified the gene noggin about ten years ago; its product is expressed in the cells of the dorsal lip of the blastopore. Since then he and others have identified a set of genes that are so expressed and have begun working out how they influence organization, induction, and differentiation (see Harland and Gerhart 1997; Gerhart and Kirschner 1997). These genes determine various aspects of head and body development, including the differentiation of neural structures (Table 3.1), as well as those of mesodermal derivatives. For example, Daniel Bachiller and colleagues (2000) showed that noggin and chordin are responsible for forebrain development in mice. These genes are present in several animals and presumed to be in all vertebrates and probably invertebrates as well; it is becoming clear that their interactions vary among species. It was initially thought that the products directly induce, but recent research shows that the gene products are not directors or inducers of development; rather they are antagonists, or derepressors, so they “permit” development. Such inducers act by forming complexes with signaling molecules to block the binding sites with the receptors of the signals, so that the signals are inhibited (Brunet et al.1998; Mariani and Harland 1998; McMahon et al.1998; Zimmerman, Jesus-Escobar, and Harland 1996).
However, as experiments and data proliferate it is becoming apparent that the timing of turn-on of the genes, the nature of the receptors of the gene products, and the kinds of interactions among the commonly maintained genes vary considerably among taxa of animals, and, therefore, the developmental and morphological variation long observed is being supplied a mechanistic explanation. Although that explanation is not fully complete, developmental biologists are pursuing reductionistic, fine-grained exploration of these mechanisms. At the same time developmental biologists are acquiring a new sense of their potential contributions to questions of the evolution of form and function, and the evolution of organisms, which in conjunction with the work of morphologists, evolutionists, systematists, and ecologists should begin to elucidate the nature and effect of variation and change in organisms—that is, the nature of natural selection (e.g., Northcutt 1993, 1995, 1996).
Table 3.1
Genetic Products and Proteins and Their Induction Effects in the Organizer
Product/Protein |
Induction Effect |
Chordin |
Neuralizing, dorsalizing |
| Follistatin | Neuralizing, dorsalizing |
| Nodal | Neuralizing, dorsalizing |
| Noggin | Neuralizing, dorsalizing |
Sonic hedgehog |
Neuralizing |
The search for the ancestors of vertebrates is a long-standing example of efforts to understand bodies and body plans. In the early twentieth century there were several competing hypotheses about what invertebrate taxon the vertebrate ancestor might have been: Geoffroy St.-Hilaire had advocated insects, Kovalevsky and Garstang tunicate tadpoles, and Dohrn annelid worms (as the ancestors of both arthropods and vertebrates) (summarized in Hall 1999). The annelid and arthropod theories were based on the nature of segmentation, either embryological or adult. Most workers ruled out insects and annelids as vertebrate ancestors simply because they were invertebrates and therefore “separate” (Hall 1999, 84–85). However, Geoffroy St.-Hilaire had had a significant insight in 1822—that insects are upside-down versions of vertebrates, with ventral nerve cords, dorsal hearts, and so on. The idea of the dorsoventral reversal became part and parcel of the annelid ancestor theory as well, despite the difficulty of explaining the development of the notochord, building a new mouth and anus, and so on in vertebrates. The idea was laid to rest for a while as evidence accumulated for the relationship of ascidians to vertebrates, and both to hemi-chordates, and of these taxa to echinoderms as the nearest “invertebrate” relatives, all based on the key features of deuterostome development—the patterns of the mouth and anus, the coelom, and so on.
Figure 3.1. The annelid-chordate dorsoventral axis body plans. Is the chordate plan that of an annelid turned upside down? Genetic analysis of axis patterning is shedding new light on an old question (modified after Romer 1960; Hall 1999).
The notion of the upside-down annelid persisted, though, at least as a historical perspective, through the 1960s, as illustrated in the great comparative anatomy textbook by A. S. Romer (1960) and by Hall (1999). There has recently been an interesting resurrection of that idea (see Figure 3.1), based on the homology of the genes that regulate dorsoventral patterning in Drosophila and in Xenopus (see Arthur 1997). In Drosophila, decapentaplegic (dpp) is expressed in a broad dorsal band, and short gastrulation (sog) is in a broad ventrolateral band. Similar to Hox genes, these genes are involved in the cascade of gene-switching events that determine the spatial arrangement of morphological structures as they develop (François et al. 1994). Further, some of the genes downstream of those are involved in specification of dorsal versus ventral structures, such as tinman, downstream of dpp, in dorsal heart formation (Azpiazu and Frasch 1993).
Scott Holley and his colleagues (1995) examined the accumulating genetic evidence for patterning in Drosophila and Xenopus. They suggested that a reversal of the dorsal-ventral axis occurred after the divergence of the common ancestor of insects and vertebrates, based on the patterns of genes with suggestive sequence homology and expression patterns in Drosophila and Xenopus. Homologues of dpp and sog in Drosophila are the genes chordin and bmp-4 (bone morphogenetic protein) in Xenopus; bmp-4 is expressed ventrally and chordin dorsally, so the pattern is indeed opposite that of Drosophila. Holley et al. (1995) confirmed the functional equivalence of the two systems by a series of interspecific injections of the mRNAs for the respective genes. Drosophila dpp and sog mRNAs were injected into Xenopus embryos, chordin mRNA from Xenopus into Drosophila. The Drosophila gene products mimicked the effects of the Xenopus homologues when injected into the tadpoles (i.e., dpp induced ventral rather than dorsal structures, and sog dorsalized instead of ventralizing). Chordin, when tagged to the N-terminal region of sog mRNA, ventralized in Drosophila.
Therefore the developmental mechanics of the host embryo are induced by gene products either of their own genes or of homologues from distantly related species. This supports the evolutionary interpretation of Geoffroy St.-Hilaire’s theory that axis formation reversed after the divergence of either insects or vertebrates from their common ancestor. Not all workers consider the case for homology and axis inversion conclusive, but many are persuaded. These results do, though, open new questions. For instance, What are the mechanisms by which taxa maintain their evolved pattern responses to genes rather than reverting to the presumed ancestral condition when homologues are introduced?
Lennert Olsson and B. K. Hall (1999) stated that the first great period of understanding of body organization was the late nineteenth century; the second is now. The drivers of that new understanding include molecular phylogenies of the Metazoa that challenge the view of a Cambrian explosion of body plans and indicate that their origins are much earlier in evolutionary time; new ideas about the origin and distribution of patterns of early development; and demonstrations of the sharing of highly conserved homeobox genes, now known to deal not only with specific patterns of segmentation but with much of body plan organization, and of other genes and of signaling molecules that participate in the development of the body plans of many animals (and their homologues in plants).
I add two further components: (1) investigation of new paleontological data, which is changing the way we think about the timing of origins of major taxa and their body plans, and the relationships of major groups of organisms; and (2) biogeography, with emphasis on the processes by which geology and geography change, and thereby affect the evolution and distribution of organisms. Doug Erwin (1999) pointed out that paleontologists have documented the origins of metazoan body plans beginning about 610 million years ago; major environmental changes seem to have triggered the burst of new forms; developmental innovations can be bracketed in time using molecular evidence. He has concluded that as particular developmental patterns are established, they limit subsequent evolutionary trajectories. Still, developmental mechanisms are highly conserved over evolutionary time and taxonomic distances, but taxonomic variation is great.
Ernst Mayr, with his usual perspicacity, anticipated the template for current research on such questions with his idea of the study of the interaction of the domains of the genotype and of whole somatic programs. In One Long Argument (1991) he emphasized that stages of development are targets of selection and that embryonic stages serve as “somatic programs” in development, tending to become highly conserved in evolution. (And, though he did not say so, such conservation might contribute to the limited numbers of body plans that have developed.) Mayr went on to say that conserved stages aid in reconstruction of phylogeny, but evolutionary interpretations are constrained by the extent to which causations of development have been elucidated by embryologists. He explained that “somatic programs” may underlie most instances of recapitulation; if a structure of an ancestor is retained, it may have been preserved by natural selection because it serves as the somatic program for subsequent ontogenetic stages. This retention imposes constraints on evolution, thereby providing resistance to evolutionary change. He asserted that study of domains of the genotype and of whole somatic programs is a holistic approach that demands attention. Mayr foresaw that there would be progress through further analysis, because “development involves highly complex interactions between different domains of the genotype and different somatic programs.” Since 1991 there has been extensive progress in understanding the complexity of the interaction of the domains of the genotype, but his prediction continues to generate new and exciting questions. In particular, more comparative assessment of nonmodel organisms, in a phylogenetic context, is beginning to occur, and those approaches should shed light on questions of development, morphology, the environment, and evolution.
The analysis of biomechanics, using morphology as the basis for examining how structures function and organisms work to produce behaviors, such as feeding and locomotion in animals and water transport in plants, has been practiced for a long time, but recently the coupling of new equipment and new approaches has revolutionized the field. For most of the twentieth century the ways organisms function were studied usually for single species in an idiographic approach and usually at a particular level of biological organization—mostly whole organisms but often at the bone-muscle interaction level. Models and general principles of several aspects of structure-function relationships have been developed, mostly during the last fifty, or even ten, years. Two examples summarize progress in the understanding of biomechanics—in these cases of animal locomotion—during this past century.
The gifted Eadweard Muybridge, working in the mid- to late nineteenth and early twentieth centuries, pioneered analysis of animal locomotion (Muybridge [1887] 1957). It was his insight that a series of photographs made in rapid succession at regulated time intervals would provide the data that would resolve conflicting opinions about animal locomotion, including long-debated questions such as whether a horse ever has all four feet off of the ground, how birds run and fly, how animals shift from walks to trots to gallops—that is, gait analysis. Muybridge used wet collodion plates but was able to get well-resolved images at high frequency and at distance; he developed an automatic exposure system that used motor clocks for making and breaking electrical circuits and the zoopraxiscope, an instrument that demonstrated movements that had been analytically photographed. His apparatus presaged all the image analysis instruments available today, and the same basic principles of photography and analysis guide current work.
Figure 3.2. Footfall patterns of elephants as an example of the analysis of the biomechanics of locomotion (after Muybridge [1887] 1957).
Muybridge employed a level of technological innovation paralleled to some degree today in his use of the relatively new field of photography, coupled with a structure that facilitated quantitative analysis, but he also explored the questions of interest with a kind of scholarship rarely pursued today. For example, in his consideration of the gaits of the horse, he compared the depictions in Greek and Byzantine pottery, paintings, and statuary with Roman and later efforts. He made the point that there were many stylistic renditions, most with considerable accuracy regarding the limbs of the horse, but they usually depicted the rider as immovable, unresponsive to the horse’s gait—truly inaccurate, as his ([1887] 1957) photographic sequences of horse and rider show. His photographs of animals of diverse sorts (Figure 3.2; elephants), and of humans in different movements and both healthy and infirm children and adults, inform analysis of locomotion to this day. For nearly one hundred years there was little progress in kinematic analysis of locomotion, save that provided by major advances in photographic technique. The movie camera, for example, coupled with the development of strobes and image analysis programs, improved technique but introduced few new principles.
However, during the last twenty or so years analysis of the kinematics and biomechanics of animal locomotion has made use of new technology and ideas. Michael Dickinson and colleagues (2000) have summarized the complexity of locomotion that emerges from the synergistic interactions among the nervous, muscular, and skeletal systems as well as the physical environment. They note that studies of different locomotor behaviors in a diversity of species are resulting in the elucidation of a few general principles common to most forms of animal locomotion. Integrative approaches that include the reductionistic through the holistic show not only how each component of a locomotor system works but also how they function as a collective whole.
Biomechanics and its practitioners are making major contributions to the emerging field of robotics, and the integration of biological and engineering principles is resulting in new instruments that meet significant social needs. For example, my colleague Robert Full and his associates have looked at the locomotion of cockroaches, centipedes, crabs, salamanders, and other animals (e.g., Martinez, Full, and Koehl 1998; Kubow and Full 1999). They have found that many animals use an alternating tripod gait and have analyzed the mechanics of the system. They have also studied gait changes, direction changes, and the differences in intermittent and sustained locomotion. Understanding of how animals locomote is allowing the development of “walking” robots to investigate uneven substrates, including the bottoms of lakes and oceans. The tripod gait that characterizes most animal locomotion and the “revelation” that animals do not move in straight lines at constant speeds but must adjust to compensate for both external and internal factors are principles that are revolutionizing robotics. Further, the adjustment can be simply a physical property of the appendages of a crab or a robot—neural feedback is not required. Big and small robots are being developed that can explore oceans and go into terrestrial areas where humans cannot (or should not) venture, and miniaturized robots are being developed that can potentially be employed in blood vessels—but making them able to move is the key.
Similarly, Dickinson’s work with flight in flies has given us whole new ways of looking at morphology (see Dickinson 1999). For example, it was long thought that a key feature of the evolution of flies, members of the order Diptera, was evolutionary reduction of their pair of hind wings and that the remaining rudiments, the halteres, were vestigial structures. However, Dickinson and his colleagues have shown that the halteres are equilibrium organs that detect angular rotation of the body during flight. They have mapped the haltere-mediated reflexes and determined that haltere afferent nerves provide direct input to a steering motor neuron (Dickinson 1999; Fayyazuddin and Dickinson 1996). Dickinson and his colleagues have developed “robofly” to test models of neural input to control of flight.
Functional morphology and biomechanics are informing engineering, and engineering and the physical sciences are informing morphology. The instruments now available to functional morphologists are far beyond those Muybridge had: much better cameras, computer-aided devices and analysis, treadmills, running tracks, flumes, wind tunnels, and others. Many functional morphologists are becoming highly integrative as they look at the feedback from the skeleton to the nervous system, and muscle fiber dynamics at one level of understanding locomotion, and the mechanical properties provided by the environment at another. As I will discuss, these advances in understanding and application of biological principles can make major contributions to instruments that may work for the good of humanity and of nature—but also possibly increase the rate of nature’s destruction.
A final example deals with the progress in this century from descriptive morphology and development to the integration of studies of development and morphology with ecological and behavioral information in order to understand aspects of the evolution of taxa, including their extinction. Through much of the twentieth century morphologists, embryologists, natural historians, ecologists, and systematists were engaged in the description of the adult morphology of many species, the embryology and development of a few species, and the ecology of even fewer species, mostly in single-species approaches. Only relatively recently has focus on the comparative method employed in a phylogenetic context given morphologists and systematists, and other biologists, new insight into the nature of characters and the relationships among taxa. (Again this is something Ernst Mayr predicted and recommended, as he has kept abreast of the technical and theoretical advances that have made the work possible.) Of course, community ecologists were examining multispecies systems, but usually not with an eye to the evolution of the systems and the relationships of the taxa in them, or to broad-based comparisons. This, too, has changed recently. We gained a splendid base for more complex analysis in all the information gathered about the structure, function, and development of the individuals that compose populations and species. I offer an example of research during the last ten years that I hope might serve as a model for the research of the next century as it explores the complexity of morphology and development in environmental and evolutionary contexts.
Developmental biology, coupled with ecology and life history studies, can tell us about changes in biodiversity. In 1990 two scientists reported finding multilegged frogs and salamanders in and around a small set of ponds in California and suggested a possible mechanism for the development of extra limbs (Sessions and Ruth 1990). The ponds had been monitored from 1974 until 1986 because the salamander species, Ambystoma macrodactylum croceum, was an endangered taxon; the frogs, Hyla regilla, were highly abundant. There were no individuals with extra limbs during the initial twelve years of monitoring. Then in 1987, and massively in 1988, numbers of individuals of both the frog and the salamander were found with extra limbs (Figure 3.3). Among the salamanders, 39 percent of late larvae, 38.5 percent of juveniles, and 4.6 percent of adults had extra limbs; of the frogs, 72 percent of adults had hind limb anomalies and fully 50 percent had extra full limbs, ranging from one extra on one side to nine on the left side and three on the right. There were few front limb anomalies.
Figure 3.3. Limb duplication in a frog, likely a consequence of interactions of parasites, frog developmental physiology, and stress to the frogs caused by environmental changes that outstrip the compensatory ability of processes of evolution.
Chemical analysis of the pond water revealed nothing unusual. The ecologist S. B. Ruth, who had discovered the multilegged animals, enlisted the aid of a developmental biologist, S. K. Sessions. Sessions quickly recognized that all the specimens of both species were infected with metacercarial cysts of a trematode flatworm parasite, and that there were concentrations of cysts in the tissues at the bases of the hind limbs but also elsewhere. The cysts were in close association with the extra limbs in both the frogs and the salamanders. But the question remained whether the cysts caused the extra limbs or whether they were just infections of already anomalous or sick animals.
Based on knowledge of limb development, Sessions developed a straightforward experiment on lab populations of the frog Xenopus laevis and the salamander Ambystoma mexicanum, the axolotl, both “model organisms” in developmental biology. He implanted resin beads the size of the metacercarial cysts in the developing limb buds of larvae of the two species; extra limb structures occurred in 20 percent of the experimental animals. Sessions and Ruth (1990) concluded that mechanical disruption of tissue interactions in the limb bud resulted in bifurcation of primordia and developmental duplication of limb structures. This is consistent with what is known of the pattern of extension and bifurcation as limb cartilages aggregate and develop, and the fact that tissue damage during development causes duplication events in a number of animals, including vertebrates and insects. The case is an example of a morphological and developmental perturbation caused by an environmental factor—parasitic flatworms.
Recently populations of frogs with multiple limbs have been found worldwide. There are alternative hypotheses for multiple limbs, such as stimulation or mimicry of the developmental inducer retinoic acid in the animals (Gardiner and Hoppe 1999), but the parasite hypothesis remains persuasive (Johnson et al. 1999; Sessions, Franssen, and Horner 1999). (Parasites are not always present when multiple limbs develop.) None of the hypotheses explains why the amphibians suddenly became susceptible to the parasites—were they more vulnerable because of other environmental stresses? Did the flatworm population suddenly increase? This is an instance of developmental biology shedding light on a morphological and ecological phenomenon that may be a significant warning to all of us about the condition of our environment.
There are some general challenges to biology in the next century that are so obvious they almost defy articulation: (1) answering the unanswered questions, and posing new ones; (2) integrating the study of development and morphology, and other subfields, into a framework that generates clear principles of biology; and (3) making the contribution of biology to society clear and relevant. How does the study of development and morphology, and the application of the data and analyses that emerge from the study, relate to these general challenges? As I have illustrated with the preceding examples, morphology and development are alive and well as fields of biology participating in a new synthesis and integration so that issues of biological complexity can be understood. The study of morphology and development, at any and all levels but especially in an integrative manner that looks at multiple parts, multiple effects, and multiple associations of structure and function, has unlimited potential to contribute to understanding of ourselves and the world around us.
Developmental biology is opening up whole new vistas of science. Its integration of genetics with biochemistry is providing new understanding of the structural elements and the morphologies and functions that are part of development and ontogenetic change. Developmental biology is opening the black box of how morphologies unfold and are maintained in individuals, and to some degree in species, by providing a mechanistic approach to the iteration of morphology. Developmental biologists are giving evolutionary biologists new ways to consider the relationships of taxa to one another by revealing the genetic and developmental patterns and processes that are common to many taxa.
Major unanswered questions, though, remain, including those with which I started this discussion: Why are there so few body plans? What provides variation among taxa and individuals, given that much of the regulation of development depends on a common substrate? How does the genome respond to the environment through development? As Hall (1999) noted, there is not a one-to-one correspondence of genotype to phenotype. Therefore developmental biologists must integrate environmental factors in order to understand what might alter the developmental pattern to provide variation. We still do not know how new morphologies, including “key innovations,” arise, and we do not know why distantly related organisms respond with similar morphologies to certain environmental parameters (homoplasy, or convergent and parallel evolution; see Sanderson and Huffard 1996). An understanding of how current body forms are produced and how they vary should yield new knowledge of how to produce new body forms—that is, speciation—and also an understanding of why old body forms no longer “fit” their environments—that is, extinction. Lineages do not just die off all at once, but developmental biology might give us answers about why the genotype and the variation expressed are not sufficient to allow organisms to adapt to change in the environment.
Developmental biology also has the potential to make major contributions to the understanding of biodiversity generation and maintenance, and of the current decline in biodiversity. An example of such research is the work of Andrew Blaustein and his colleagues (Blaustein et al. 1994, 1998, 1999; Fite et al. 1998; Hayes et al. 1996; Kiesecker and Blaustein 1995, 1999). They have demonstrated that excess UVB dramatically affects hatching success of the embryos of several species of frogs and toads in Oregon; they propose that UVB is destroying the embryos’ DNA, and that their levels of the repair enzyme photolyase cannot cope with the rate of destruction. The increase in UVB levels, presumably caused by human effects that have altered the ozone layer, has happened so rapidly that the repair system cannot evolve in response, and the populations of the animals are in serious decline. Blaustein and his colleagues are also investigating the synergistic effects of pathogens, particularly species of fungi, in these vulnerable animals.
Blaustein’s integrative approach is a model, because he continues to expand his combination of ecology, biochemistry, and development to try to understand the phenomenon of population declines. New appreciation for the interaction of mechanisms of development and environmental cues and signals should give insight into interactions and maintenance, as well as the effects of disruption of the environment on developmental patterns and potentially on evolution and extinction.
For morphologists the new insights into developmental constraint and mechanism should allow new paths for analysis of major questions such as the way adaptation, in both evolutionary and physiological senses, occurs; whether behavioral and ecological changes drive morphological change or morphological variation drives behavioral and ecological changes (and the mechanisms by which either might occur). Recent developments in systematic and evolutionary biology, and in engineering and biophysics, allow new ways of examining questions of structure-function relationships and how organisms really work, ranging from the subcellular mechanisms through integrated examinations of neural control and environmental mediations.
Morphologists have more to say now about the nature of characters for use in systematic analysis, and the issues of functional units, character independence, and so on take on new meaning. But I do urge them to view their research more reciprocally than most now do—what functional morphology tells us about structure and thereby characters should be communicated to systematists and evolutionary biologists so their database is increased and ever more robust phylogenetic hypotheses can be generated. Similarly, the understanding of phylogeny developed by systematists and evolutionists should be better communicated to other biologists. Use of the comparative method is enjoying a renaissance (Harvey and Pagel 1991; Brooks and McLennan 1991), and new methods of phylogenetic analysis—for the production of robust hypotheses on the relationships of taxa to determine the direction of evolutionary change—are facilitating that renaissance.
New awareness of the patterns of morphology and the processes of development that underlie the behavior and ecology of organisms is arising. That recognition occurs, for example, as ecologists learn that the species composition, rather than just the activity, of “functional groups” or guilds matters (see Tilman 1999; Symstad et al. 1998) and behaviorists see the interplay of structure at all levels, such as the biochemistry of color of a feather, the structure of its parts, and the neuromusculoskeletal basis for waving feathers to produce a behavior that elicits a response from a viewer or potential mate.
Integrative analysis of development and morphology, when meshed with other subfields of biology and with information from the physical and social sciences, can lead to understanding of the origin and evolution of biological complexity at several levels. We need to know how complexity arises, functions, and is maintained at subcellular through organismal through ecological and behavioral levels; we need to know how complexity, including patterns and processes of development and morphologies, or organisms, can change. Developmental mechanisms provide some of that understanding. Analysis of developmental and morphological mechanisms will provide answers to other kinds of questions of the evolution of complexity. For example, it is becoming apparent that small genetic changes direct major morphological changes of various sorts, ranging from sexual dimorphism to insect castes. Understanding of how such change is directed is likely to facilitate understanding of the evolution of complex behaviors and even social systems.
But I have so far alluded only to some of the general research questions that remain unresolved. Developmental biology and morphology are also making significant contributions to society, ranging from medical to ecological applications, and their potential for further contribution is ever expanding. Understanding of developmental mechanisms, especially their genetic basis, is facilitating the initiation of new treatments and even corrections of heritable diseases and deformities. Craniofacial development is a case in point—understanding the biological basis of the development (and evolution) of the head is providing new understanding of the bases for anomalies of the teeth, the jaws, the musculature, and so on, and therefore means of modulating those anomalies either developmentally or surgically. Research is also determining that some developmental anomalies can be prevented by a healthy diet, and therefore might not require genetic manipulation or surgical intervention—the reduced incidence of spina bifida, or the failure of completion of the neural arches of vertebrae in humans and other vertebrates, by including high levels of biotin in the maternal diet is an example. The biological basis of such structural modification needs more extensive exploration.
Bioengineering is dealing with materials for repair—the skeleton of corals is now being used to assist bone repair; functional morphologists and biomechanicians are providing the principles and the empiricisms that lead to development of better prosthetic devices. I have already mentioned advances in robotics—devices that can go where humans cannot, whether on ocean substrates or in blood vessels, are being developed, and only the imagination limits the possibilities for their employment. Other uses of morphology—devices that can read the retinal image of the eye or the print of the hand to be used as an identification marker, for example—are being developed. The potential contributions of developmental biology and morphology to the aid of society are unlimited—if a focus on the quality of life of the donor species, as well as the recipients, is maintained.
We are doing less well in conveying the potential applications of our advances in developmental biology and morphology to other socially important questions. For example, we constantly observe ontogenies—children growing up, tadpoles becoming frogs, caterpillars becoming butterflies, seedlings becoming flowers or herbs or trees. But we concentrate too often on single species, and think in terms of stages, not continua; we forget about aging too—it is the intergenerational interaction of all these components that generates our environments. The species composition of the “stages” of ecological succession is in effect ontogeny at several levels—that of the individual organisms as well as that of the ecosystem over time. We are literally seeing the trees but not the forest.
To carry that allusion further, I think of ecology as the study of the interaction of morphologies of different species with one another and with the physical environment. Even the functional groups so popular in ecological analysis today could profitably be characterized by the morphological features that provide the functional contribution to the interactions of the species involved. Such a perspective might further elucidate the composition of functional groups in like habitats in different regions and shed light on the concept. Morphological features such as tree height, leaf form, or the morphologies of top carnivores are major features of the organisms’ function in their habitats—morphology is what they are and where they are, and development is how they got that form.
Getting back to a more holistic view, I suggest that a more integrative, transdisciplinary, multihierarchical approach to hypothesis formation, testing, and analysis of major questions and problems will provide a much better understanding of biology and its contribution to society. We need to develop our research enterprise in terms of significant, synthetic themes that will affect many parameters of science and society.
We must recognize that many elements of the progress in development and morphology have been aided immeasurably by advances in technology, as illustrated by the examples I have cited. Computer-aided data generation and analysis, modeling and simulations, search engines, and so on have made science work faster and more innovatively. Instrumentation of all sorts is making new analyses possible. Technology clearly will continue to provide new devices and new applications, but all of them will depend on the ideas of the inventors who devise them and the scientists who use them to make progress, and both scientists and designers must be responsible for wise application of technology.
The twenty-first century is sure to be an exciting time for developmental biologists and morphologists. The advances in technology will facilitate basic research and its applications to important questions in biology and its social dimensions. Innovative ideas abound. There appears to be vast potential for development of good science. More and more people with both skills and ideas are being trained. However, some of them cannot get jobs, and the distribution of scientists, geographically, nationally, and in kinds of research, is skewed. What do we need to continue and accelerate new technology and new ideas, and to incorporate a view that science operates for the good of society? We need education systems that have a well-grounded philosophy about the teaching of science and then the capacity to do it well. We need teachers confident of their knowledge of the science they are teaching, including efficient means for them to update their information. We need teachers who go beyond the facts, who can be integrative and transdisciplinary, so that their students will be able to understand science broadly.
Such systems and teachers will provide a more scientifically literate and open-minded public. Our constituency would then be well acquainted with not just the facts of science but the way it is done. It would likely press for the kind of science that takes a broad view, and it would understand the contribution of science to the good of society. A public educated with a synthetic perspective will also be aware that the good of society is not simply the good of humankind but is a product of understanding the implications of all the interactions that affect all life on this planet. Advancing the understanding of morphology and of development, preferably together, will be a useful part, but merely a part, of good, integrative, progressive science. We need people with the vision of an Ernst Mayr in classrooms at all levels of instruction, at the bench, and in the field as scientists—the hard part is achieving the uniqueness of Ernst Mayr, but the transmission of his vision, and visions like it, is one way to begin.
I have elucidated many new questions and problems, and some unsolved old ones, in this discussion. As techniques and instrumentation are developed, and as scientists recognize the contribution toward resolution that a synthetic and integrative approach will provide to complex problems, the science of the new century has exceptional promise for dealing with significant problems. Examples of the kinds of complexity that can be understood through new ways of examining problems, including those of the development, morphology, evolution, and ecological interactions of the organisms that inhabit the Earth, include the following:
1. Use of the techniques and approaches of developmental biology and developmental biologists, geneticists, and morphologists to analyze the evolution of new body forms and new species, and contribution of phylogenetic analysis to understanding patterns and processes of development
2. Understanding of the interaction of structure and function, as well as their evolution, through hierarchical and integrative approaches that include multidimensional analyses and the resources of nonbiological (e.g., chemical, physical, engineering) interpretations
3. Examination of the way environmental factors affect development and the morphologies that arise, in terms of “stages” of both life histories and whole ontogenies, especially the consequences to fitness of individuals and populations
4. Application of growing understanding of the interactions of genes, cells, organisms, species, and environments to questions of viability, extinction, conservation, and management (when necessary and appropriate) of life on the planet
5. Education of our intelligent public and policy makers about the nature and contribution of an understanding of biology, including development and morphology, in a computer-literate but increasingly specialized and urban world
We have the resources of technique, innovative colleagues, and a renewed interest in biological science to consider questions of significance that are both intellectual and pragmatic. In fact, an approach to studying questions in biology that meshes the philosophical and the pragmatic has great promise for advances in the new century.
Many students, colleagues, and fellow members of boards and committees have influenced my thinking about development and morphology, the contributions of those fields to biology, and the nature and significance of integrative biology. I appreciate recent discussions with several colleagues, and their loans of reprints and illustrative materials, for the formulation of this contribution. Comments from the reviewers and editors improved the manuscript. I especially thank Adam Summers for preparing several of the slides used in the oral presentation and Karen Klitz of the Museum of Vertebrate Zoology, University of California at Berkeley, for executing the figures. Finally, I am pleased to acknowledge the support of the National Science Foundation for my research in morphology, development, and evolutionary biology.
Arthur, W. 1997. The origin of animal body plans: A study in evolutionary developmental biology. Cambridge: Cambridge Univ. Press.
Azpiazu, N., and M. Frasch. 1993. Tinman and bagpipe: Two homeobox genes that determine cell fates in the dorsal mesoderm of Drosophila. Genes & Dev. 7:1325–40.
Bachiller, D., J. Klingensmith, C. Kemp, J. A. Belo, R. M. Anderson, S. R. May, J. A. McMahon, A. P. McMahon, R. M. Harland, J. Rossant, and E. M. De Robertis. 2000. The organizer factors Chordin and Noggin are required for mouse forebrain development. Nature 403:658–61.
Blaustein, A. R., J. B. Hayes, P. D. Hoffman, D. P. Chivers, J. M. Kiesecker, W. P. Leonard, A. Marco, D. H. Olson, J. K Reaser, and R. H. Anthony. 1999. DNA repair and resistance to UV-B radiation in western spotted frogs. Ecol. Appl. 9:1100–1105.
Blaustein, A. R., P. D. Hoffman, D. G. Hokit, J. M. Kiesecker, S. C. Walls, and J. B. Hays. 1994. UV-B repair and resistance to solar UV-B in amphibian eggs: A link to population declines? Proc. Nat. Acad. Sci. 91:1791–95.
Blaustein, A. R., J. M. Kiesecker, D. P. Chivers, D. G. Hokit, A. Marco, L. K. Belden, and A. Hatch. 1998. Effects of ultraviolet radiation on amphibians: Field experiments. Amer. Zool. 38:799–812.
Brooks, D. R., and D. A. McLennan. 1991. Phylogeny, ecology, and behavior: A research program in comparative biology. Chicago: Univ. of Chicago Press.
Brunet, L. J., J. A. McMahon, A. P. McMahon, and R. M. Harland. 1998. Noggin, cartilage morphogenesis, and joint formation in the mammalian skeleton. Science 280:1455–57.
Dickinson, M. H. 1999. Haltere-mediated equilibrium reflexes of the fruit fly, Drosophila melanogaster. Phil. Trans. R. Soc. London B 354:903–16.
Dickinson, M. H., C. Farley, R. J. Full, M. Koehl, R. Kram, and S. Lehman. 2000. Toward an integrative view of how animals move. Science 288:100–106.
Erwin, D. H. 1999. The origin of body plans. Amer. Zool. 39:617–29.
Fayyazuddin, A., and M. H. Dickinson. 1996. Haltere afferents provide direct, electrotonic input to a steering motor neuron in the blowfly, Calliphora. J. Neurosci. 16:5225–32.
Fite, K. V., A. R. Blaustein, L. Bengston, and J. E. Hewett. 1998. Evidence of retinal light damage in Rana cascadae: A declining amphibian species. Copeia 1998:906–14.
François, V., M. Solloway, M. W. O’Neill, J. Emery, and E. Bier. 1994. Dorsal-ventral patterning of the Drosophila embryo depends on a putative negative growth factor encoded by the short gastrulation gene. Genes & Dev. 8:2602–16.
Gardiner, D. M., and D. M. Hoppe. 1999. Environmentally induced limb malformations in mink frogs (Rana septentrionalis). J. Exptl. Zool. 284:207–16.
Gerhart, J., and M. Kirschner. 1997. Cells, embryos, and evolution. Malden, Mass.: Blackwell Science.
Goodfield, J. 1968. Theories and hypotheses in biology. Boston Studies Phil. Sci. 5:421–49.
Hall, B. K. 1999. Evolutionary developmental biology. 2d ed. Dordrecht: Kluwer Acad. Publ.
Harland, R., and J. Gerhart. 1997. Formation and function of Spemann’s organizer. Ann. Rev. Cell Dev. Biol. 13:611–67.
Harvey, P. H., and M. P. Pagel. 1991. The comparative method in evolutionary biology. Oxford: Oxford Univ. Press.
Hayes, J. B., A. R. Blaustein, J. M. Kiesecker, P. D. Hoffman, I. Pandelova, D. Coyle, and T. Richardson. 1996. Developmental response of amphibians to solar and artificial UVB sources: A comparative study. Photochem. Photobiol. 64:449–56.
Holley, S. A., P. D. Jackson, Y. Sasai, B. Lu, E. M. DeRobertis, F. M. Hoffman, and E. L. Ferguson. 1995. A conserved system for dorsal-ventral patterning in insects and vertebrates involving sog and chordin. Nature 376:249–53.
Johnson, P. T. J., K. B. Lunde, E. G. Ritchie, and A. E. Launer. 1999. The effect of trematode infection on amphibian limb development and survivorship. Science 284:802–4.
Kaufman, D. M. 1995. Diversity of new world mammals: Universality of the latitudinal gradients of species and Bauplans. J. Mammal. 76:322–34.
Kiesecker, J. M., and A. R. Blaustein. 1995. Synergism between UVB radiation and a pathogen magnifies amphibian embryo mortality in nature. Proc. Nat. Acad. Sci. 92:11049–52.
———. 1999. Pathogen reverses competition between larval amphibians. Ecology 80:2442–49.
Kubow, T. M., and R. J. Full. 1999. The role of the mechanical system in control: A hypothesis of self-stabilization in hexapedal runners. Phil. Trans. R. Soc. London B 354:849–61.
McMahon, J. A., S. Takada, L. B. Zimmerman, C.-M. Fan, R. M. Harland, and A. P. McMahon. 1998. Noggin-mediated antagonism of BMP signalling is required for growth and patterning of the neural tube and somite. Genes & Dev. 12:1438–52.
Mariani, F. V., and R. M. Harland. 1998. XBF-2 is a transcriptional repressor that converts ectoderm into neural tissue. Development 125:5019–31.
Martinez, M. M., R. J. Full, and M. A. R. Koehl. 1998. Underwater punting by an intertidal crab: A novel gait revealed by the kinematics of pedestrian locomotion in air versus water. J. Expt. Biol. 201:2609–23.
Mayr, E. 1982. The growth of biological thought: Diversity, evolution, and inheritance. Cambridge, Mass.: Harvard Univ. Press, Belknap Press.
———. 1988. Toward a new philosophy of biology: Observations of an evolutionist. Cambridge, Mass.: Harvard Univ. Press, Belknap Press.
———. 1991. One long argument: Charles Darwin and the genesis of modern evolutionary thought. Cambridge, Mass.: Harvard Univ. Press.
Muybridge, E. 1957. Animals in motion. New York: Dover Publ. Reprint of E. Muybridge. 1887. Animal locomotion. Philadelphia: Univ. of Pennsylvania Press.
Nordenskiöld, E. 1928. The history of biology: A survey. New York: Tudor Publ.
Northcutt, R. G. 1993. A reassessment of Goodrich’s model of cranial nerve phylogeny. Acta Anat. 148:71–80.
———. 1995. The forebrain of gnathostomes: In search of a morphotype. Brain Beh. Evol. 46:275–319.
———. 1996. The origin of craniates: Neural crest, neurogenic placodes, and homeobox genes. Israel J. Zool. 42:273–313.
Olsson, L., and B. K. Hall. 1999. Introduction to the symposium: Developmental and evolutionary perspectives on major transformations in body organization. Amer. Zool. 39:612–16.
Romer, A. S. 1960. The vertebrate body. Philadelphia: Saunders.
Sanderson, M. H., and L. Huffard, eds. 1996. Homoplasy: The recurrence of similarity in evolution. San Diego: Academic Press.
Sessions, S. K., R. A. Franssen, and V. L. Horner. 1999. Morphological clues from multilegged frogs: Are retinoids to blame? Science 284:800–802.
Sessions, S. K., and S. B. Ruth. 1990. Explanation for naturally occurring supernumerary limbs in amphibians. J. Expt. Zool. 254:38–47.
Spemann, O., and H. Mangold. 1924. Über Induktion von Embryonalanlagen durch Implantation artfremder Organisatoren. Arch. Mikr. Anat. u. Entw. Mech. 100:599–638.
Symstad, A. J., D. Tilman, J. Willson, and J. M. H. Knops. 1998. Species loss and ecosystem functioning: Effects of species identity and community composition. Oikos 81:389–97.
Tilman, D. 1999. The ecological consequences of changes in biodiversity: A search for general principles. Ecol. 80:1455–74.
Zimmerman, L. B, J. M. Jesus-Escobar, and R. M. Harland. 1996. The Spemann organizer signal noggin binds and inactivates bone morphogenetic protein 4. Cell 86:599–606.
