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Showing papers in "Annual Review of Ecology, Evolution, and Systematics in 1997"


Journal ArticleDOI
TL;DR: Riparian zones possess an unusually diverse array of species and environmental processes as discussed by the authors, related to variable flood regimes, geographically unique channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor.
Abstract: Riparian zones possess an unusually diverse array of species and environmental processes. The ecological diversity is related to variable flood regimes, geographically unique channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor. The resulting dynamic environment supports a variety of life-history strategies, biogeochemical cycles and rates, and organisms adapted to disturbance regimes over broad spatial and temporal scales. Innovations in riparian zone management have been effective in ameliorating many ecological issues related to land use and environmental quality. Riparian zones play essential roles in water and landscape planning, in restoration of aquatic systems, and in catalyzing institutional and societal cooperation for these efforts.

2,502 citations


Journal ArticleDOI
TL;DR: Understanding of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment.
Abstract: Savannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (facilitation). The strength and sign of the interaction varies in both time and space, allowing a rich array of possible outcomes but no universal predictive model. Simple models of coexistence of trees and grasses, based on separation in rooting depth, are theoretically and experimentally inadequate. Explanation of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment. Prediction of the consequences of manipulating tree biomass through clearing further requires an understanding of how trees modify light, water, and nutrient environments of grasses. Understanding the nature of coexistence between trees and grass, which under other circumstances...

2,436 citations


Journal ArticleDOI
TL;DR: Overall studies of parasite communities suggest that the action of processes determining species richness of parasite assemblages becomes less detectable as focus shifts from parasite faunas to infracommunities.
Abstract: Parasite communities are arranged into hierarchical levels of organization, covering various spatial and temporal scales. These range from all parasites within an individual host to all parasites exploiting a host species across its geographic range. This arrangement provides an opportunity for the study of patterns and structuring processes operating at different scales. Across the parasite faunas of various host species, several species-area relationships have been published, emphasizing the key role of factors such as host size or host geographical range in determining parasite species richness. When corrections are made for unequal sampling effort or phylogenetic influences, however, the strength of these relationships is greatly reduced, casting a doubt over their validity. Component parasite communities, or the parasites found in a host population, are subsets of the parasite fauna of the host species. They often form saturated communities, such that their richness is not always a reflection of that of the entire parasite fauna. The species richness of component communities is instead influenced by the local availability of parasite species and their probability of colonization. At the lowest level, infracommunities in individual hosts are subsets of the species occurring in the component community. Generally, their structure does not differ from that expected from a random assembly of available species, although comparisons with precise null models are still few. Overall studies of parasite communities suggest that the action of processes determining species richness of parasite assemblages becomes less detectable as focus shifts from parasite faunas to infracommunities.

1,479 citations


Journal ArticleDOI
TL;DR: Experimental, theoretical, and empirical studies of homoploid hybrid speciation suggest that it is feasible, although evolutionary conditions are stringent, and hybridization may be important as a stimulus for the genetic or chromosomal reorganization envisioned in founder effect and saltational models of speciation.
Abstract: The origin of new homoploid species via hybridization is theoretically difficult because it requires the development of reproductive isolation in sympatry. Nonetheless, this mode is often and carelessly used by botanists to account for the formation of species that are morphologically intermediate with respect to related congeners. Here, I review experimental, theoretical, and empirical studies of homoploid hybrid speciation to evaluate the feasibility, tempo, and frequency of this mode. Theoretical models, simulation studies, and experimental syntheses of stabilized hybrid neospecies indicate that it is feasible, although evolutionary conditions are stringent. Hybrid speciation appears to be promoted by rapid chromosomal evolution and the availability of a suitable hybrid habitat. A selfing breeding system may enhance establishment of hybrid species, but this advantage appears to be counterbalanced by lower rates of natural hybridization among selfing taxa. Simulation studies and crossing experiments also suggest that hybrid speciation can be rapid—a prediction confirmed by the congruence observed between the genomes of early generation hybrids and ancient hybrid species. The frequency of this mode is less clear. Only eight natural examples in plants have been rigorously documented, suggesting that it may be rare. However, hybridization rates are highest in small or peripheral populations, and hybridization may be important as a stimulus for the genetic or chromosomal reorganization envisioned in founder effect and saltational models of speciation.

1,206 citations


Journal ArticleDOI
TL;DR: Competitive ability in belowground competition is correlated with such attributes as root density, surface area, and plasticity either in root growth or in the properties of enzymes involved in nutrient uptake.
Abstract: Belowground competition occurs when plants decrease the growth, survival, or fecundity of neighbors by reducing available soil resources. Competition belowground can be stronger and involve many more neighbors than aboveground competition. Physiological ecologists and population or community ecologists have traditionally studied belowground competition from different perspectives. Physiologically based studies often measure resource uptake without determining the integrated consequences for plant performance, while population or community level studies examine plant performance but fail to identify the resource intermediary or mechanism. Belowground competitive ability is correlated with such attributes as root density, surface area, and plasticity either in root growth or in the properties of enzymes involved in nutrient uptake. Unlike competition for light, in which larger plants have a disproportionate advantage by shading smaller ones, competition for soil resources is apparently more symmetric. Below...

1,044 citations


Journal ArticleDOI
TL;DR: The maximum likelihood method is described and how likelihood ratio tests of a variety of biological hypotheses can be formulated and tested using computer simulation to generate the null distribution of the likelihood ratio test statistic is described.
Abstract: One of the strengths of the maximum likelihood method of phylogenetic estimation is the ease with which hypotheses can be formulated and tested. Maximum likelihood analysis of DNA and amino acid sequence data has been made practical with recent advances in models of DNA substitution, computer programs, and computational speed. Here, we describe the maximum likelihood method and the recent improvements in models of substitution. We also describe how likelihood ratio tests of a variety of biological hypotheses can be formulated and tested using computer simulation to generate the null distribution of the likelihood ratio test statistic.

967 citations


Journal ArticleDOI
TL;DR: These trends are derived from a sample of 40 independent clades of terrestrial animals, primarily vertebrates, and indicate that SSD increases with size where males are the larger sex, but decreases with sizeWhere females are larger, a trend formalized as “Rensch's rule.”
Abstract: Sexual size dimorphism (SSD) is common in both plants and animals, and current evidence suggests that it reflects the adaptation of males and females to their different reproductive roles. When species are compared within a clade, SSD is frequently found to vary with body size. This allometry is detected as β ≠ 1, where β is the slope of a model II regression of log(male size) on log(female size). Most frequently, β exceeds 1, indicating that SSD increases with size where males are the larger sex, but decreases with size where females are larger, a trend formalized as “Rensch's rule.” Exceptions are uncommon and associated with female-biased SSD. These trends are derived from a sample of 40 independent clades of terrestrial animals, primarily vertebrates. Their extension to plants and aquatic animals awaits quantitative assessments of allometry for SSD within these groups. Many functional hypotheses have been proposed to explain the evolution of allometry for SSD, most featuring sexual selection on males ...

898 citations


Journal ArticleDOI
TL;DR: This work has shown that replacement of vulnerable taxa by rapidly spreading taxa that thrive in human-altered environments will ultimately produce a spatially more homogenized biosphere with much lower net diversity.
Abstract: ▪ Abstract Extinction is rarely random across ecological and geological time scales. Traits that make some species more extinction-prone include individual traits, such as body size, and abundance. Substantial consistency appears across ecological and geological time scales in such traits. Evolutionary branching produces phylogenetic (as often measured by taxonomic) nesting of extinction-biasing traits at many scales. An example is the tendency, seen in both fossil and modern data, for higher taxa living in marine habitats to have generally lower species extinction rates. At lower taxononomic levels, recent bird and mammal extinctions are concentrated in certain genera and families. A fundamental result of such selectivity is that it can accelerate net loss of biodiversity compared to random loss of species among taxa. Replacement of vulnerable taxa by rapidly spreading taxa that thrive in human-altered environments will ultimately produce a spatially more homogenized biosphere with much lower net diversity.

885 citations


Journal ArticleDOI
TL;DR: The significance of introgressive hybridization in evolution and diversification of animals is reconsidered to determine if the generally negative assessment of these processes is warranted.
Abstract: Although hybridization and introgression have been considered important in generation of plant diversity, their role in evolutionary diversification of animals remains unclear In this review, we reconsider the significance of introgressive hybridization in evolution and diversification of animals to determine if the generally negative assessment of these processes is warranted Unlike the situation for plants, hybrid animal taxa appear to be relatively rare This could, however, be due to negative attitudes toward hybridization and difficulty in detecting such forms Hybridization has been responsible for instantaneous creation of several unique complexes of polyploid and unisexual animals Allopolyploidy has allowed for diversification, whereas unisexual taxa have acted as conduits of gene exchange among related sexual species Many instances of diploid, bisexual taxa of hybrid origin have been put forward, but few have been carefully tested Changing attitudes toward hybrids and technological advances should allow for careful consideration of hypothesized hybrid taxa and will undoubtedly increase the number of known animal hybrid taxa

634 citations


Journal ArticleDOI
TL;DR: Quantitative morphological studies reveal profound changes in evolutionary rates corresponding with the generation of morphological disparity at low taxonomic diversity during the early radiation of many clades.
Abstract: The diversity of organismic form has evolved nonuniformly during the history of life. Quantitative morphological studies reveal profound changes in evolutionary rates corresponding with the generation of morphological disparity at low taxonomic diversity during the early radiation of many clades. These studies have also given insight into the relative importance of genomic and ecological factors in macroevolution, the selectivity of extinction, and other issues. Important progress has been made in the development of morphological spaces that can accommodate highly disparate forms, although this area still needs more attention. Other future directions include the relationship between morphological and ecological diversification, geographic patterns in morphological diversity, and the role of morphological disparity as a causal factor in macroevolution.

509 citations


Journal ArticleDOI
TL;DR: It is found that the responses of ecosystems to short-term experimental treatments involve less change in species composition than is found in natural communities that have diverged in response to local conditions over longer periods.
Abstract: Trophic structure, the partitioning of biomass among trophic levels, is a major characteristic of ecosystems. Most studies of the forces that shape trophic structure emphasize either “bottom-up” or “top-down” regulation of populations and communities. Recent work has shown that these two forces are not mutually exclusive alternatives, but efforts to model their interaction still often yield unrealistic predictions. We focus on the problems involved with modeling situations in which community composition, including both the number of trophic levels and the species composition within a trophic level, can change. We review the development of these ideas, emphasizing in particular how compositional change can alter theoretical expectations about the regulation of trophic structure. A comparison of studies on the effects of predators and resource productivity in limnetic ecosystems reveals an intriguing disparity between the results of manipulative experiments and those of correlational studies. We suggest that this contrast is a result of the difference in the temporal scales operating in the two types of studies. Ecosystem-level variables may appear to approach an equilibrium in short-term press experiments; however, processes such as invasion and extinction of species will not have time to play out in most such experiments. We found that the responses of ecosystems to short-term experimental treatments involve less change in species composition than is found in natural communities that have diverged in response to local conditions over longer periods. We argue that the results of short-term experiments support the predictions of models in which

Journal ArticleDOI
TL;DR: Experiments show that the dominance theory, which posits that “speciation genes” act as partial recessives in hybrids, can explain Haldane's rule for hybrid inviability and that faster evolution of hybrid male steriles plays an important role.
Abstract: ▪ Abstract Haldane's rule—the preferential sterility or inviability of hybrids of the heterogametic (XY) sex—characterizes speciation in all known animals. Over the past decade, an enormous amount of experimental and theoretical work has been devoted to explaining this pattern. This work has falsified several once-popular theories and, more important, has produced a strong consensus on the likely causes of Haldane's rule. Experiments show that the dominance theory, which posits that “speciation genes” act as partial recessives in hybrids, can explain Haldane's rule for hybrid inviability. Dominance likely also contributes to Haldane's rule for sterility. Recent experiments further show that faster evolution of hybrid male steriles plays an important role. Faster evolution of X-linked loci may also contribute, though the evidence here is weaker. Evolutionary geneticists now largely agree that the simultaneous action of these forces explains Haldane's rule.

Journal ArticleDOI
TL;DR: No alternative strategy now available is clearly superior to the standard approach to the estimation of gene flow based on Wright's model and allozyme markers, but this may soon change as methods are developed that are clearly superior.
Abstract: ▪ Abstract The estimation of gene flow from the distribution of genetic markers in populations requires an indirect approach. Gene flow parameters are defined by demographic models, and population genetic models provide the link between these parameters and the distributions of genetic markers. Following the introduction of allozyme methods in the 1960s, a standard approach to the estimation of gene flow was developed. Wright's island model of population structure was used to relate the distribution of allozyme alleles in populations to Nem, the product of the effective population size and the rate of migration. Alternative strategies for the estimation of gene flow have been developed using different genetic markers, different models of demography and population genetics, and different methods of parameter estimation. No alternative strategy now available is clearly superior to the standard approach based on Wright's model and allozyme markers. However, this may soon change as methods are developed that ...

Journal ArticleDOI
TL;DR: Gene number is an important component of assessing conservation value, and Phylogenetic measures are better indicators of conservation worth than species richness, and measures using branch-lengths are better than procedures relying solely on topology.
Abstract: ▪ Abstract A variety of phylogenetic measures have been proposed to quantify distinctiveness, often held to mark species of high conservation worth. However, distinctiveness of species and their numbers have different implications for conservation policy, depending on whether moral, esthetic, or utilitarian reasons are accepted as justifying conservation. The utilitarian position values species according to increasing numbers, and as they are more, as opposed to less, distinctive. The view is taken that conservation should seek to maximize the preserved information of the planet's biota, best expressed in terms of genetic information held in genes and not in portions of the genome of uncertain or no function. Gene number is thus an important component of assessing conservation value. Phylogenetic measures are better indicators of conservation worth than species richness, and measures using branch-lengths are better than procedures relying solely on topology. Distance measures estimating the differences be...

Journal ArticleDOI
TL;DR: This work discusses three levels of flow effects as a means of examining the potential importance of flow in the settlement process, and explores the ecological consequences of settlement in different flow-regimes in light of the direct effects of flo...
Abstract: A feature common to many benthic marine plants and animals is the release of propagules that serve as the organism's only mechanism of dispersal. Successful dispersal depends to a large extent on the process of settlement—the transient phase between the pelagic life of the propagule and the benthic existence of the adult. The flow of water may affect settlement on three levels: 1. Flow can act by exerting hydrodynamic forces on settling propagules. These forces may affect the propagule's encounter with the substratum, its behavior following encounter, or both. 2. Flow may provide a settlement cue that induces active behavior of motile propagules. 3. Flow may act to mediate various settlement cues (e.g. sediment load and the concentration of attractants). We discuss these three levels of flow effects as a means of examining the potential importance of flow in the settlement process, and then we explore the ecological consequences of settlement in different flow-regimes in light of the direct effects of flo...

Journal ArticleDOI
TL;DR: The evolution of eusociality in Isoptera likely evolved in response to a variety of contributing elements and the selective pressures that they generated.
Abstract: Eusociality in Isoptera (termites) converges along many lines with colony organization and highly social behavior in the phylogenetically distinct insect order Hymenoptera (ants, bees, wasps). Unlike the haplodiploid Hymenoptera, however, both sexes of Isoptera are diploid. Termite families thus lack asymmetric degrees of genetic relatedness generated by meiosis and fertilization, so explanations for eusocial evolution based on such asymmetries are not applicable to Isoptera. The evolution of eusociality in termites likely occurred in small families in which most helpers retained developmental flexibility and reproductive options. A suite of ecological and life-history traits of termites and their ancestors may have predisposed them toward eusocial evolution. These characteristics include familial associations in cloistered, food-rich habitats; slow development; overlap of generations; monogamy; iteroparity; high-risk dispersal for individuals; opportunities for nest inheritance by offspring remaining in their natal nest; and advantages of group defense. Such life-history components are particularly persuasive as fostering social evolution because many are present in a broad group of eusocial taxa, including Hymenoptera, beetles, aphids, thrips, naked mole rats, and shrimp. The evolution of eusociality in Isoptera likely evolved in response to a variety of contributing elements and the selective pressures that they generated.

Journal ArticleDOI
TL;DR: From a cluster analysis of insect mouthparts, this work recognizes 34 fundamental mouthpart classes among extant insects and their geochronol...
Abstract: ▪ Abstract One of the most intensively examined and abundantly documented structures in the animal world is insect mouthparts. Major structural types of extant insect mouthparts are extensive, consisting of diverse variations in element structure within each of the five mouthpart regions—labrum, hypopharynx, mandibles, maxillae, and labium. Numerous instances of multielement fusion both within and among mouthpart regions result in feeding organs capable of ingesting in diverse ways foods that are solid, particulate, and liquid in form. Mouthpart types have a retrievable and interpretable fossil history in well-preserved insect deposits. In addition, the trace-fossil record of insect-mediated plant damage, gut contents, coprolites, and insect-relevant floral features provides complementary data documenting the evolution of feeding strategies during the past 400 million years. From a cluster analysis of insect mouthparts, I recognize 34 fundamental mouthpart classes among extant insects and their geochronol...

Journal ArticleDOI
TL;DR: A major transition from field-based observations and conjecture to carefully controlled laboratory selection experiments has permitted crucial tests of theories of density-dependent natural selection and a deeper understanding of the mechanisms of adaptation to different levels of population crowding.
Abstract: The development of theory on density-dependent natural selection has seen a transition from very general, logistic growth-based models to theories that incorporate details of specific life histories. This transition has been justified by the need to make predictions that can then be tested experimentally with specific model systems like bacteria or Drosophila. The most general models predict that natural selection should increase density-dependent rates of population growth. When trade-offs exist, those genotypes favored in low-density environments will show reduced per capita growth rates under crowded conditions and vice versa for evolution in crowded environments. This central prediction has been verified twice in carefully controlled experiments with Drosophila. Empirical research in this field has also witnessed a major transition from field-based observations and conjecture to carefully controlled laboratory selection experiments. This change in approach has permitted crucial tests of theories of density-dependent natural selection and a deeper understanding of the mechanisms of adaptation to different levels of population crowding. Experimental research with Drosophila has identified several phenotypes important to adaptation, especially at high larval densities. This same research revealed that an important trade-off occurs between competitive ability and energetic efficiency.

Journal ArticleDOI
TL;DR: A more comprehensive theory of alternatives is suggested that will increase the ability to make predictions regarding the existence and expression of alternative reproductive behaviors in both sexes and interactions between the sexes.
Abstract: The study of alternative reproductive behaviors in fishes has contributed to our general understanding of reproductive strategies and mating systems. Despite extensive research on the mechanisms and patterns of alternatives, two important factors have not been addressed, and both may strongly influence the evolution of alternative reproductive behaviors. First, alternative female reproductive behaviors exist and should be considered in theoretical and empirical work. Second, interactions between the sexes will influence the evolution of alternative reproductive behaviors. In this review, we explore these two points and suggest the development of a more comprehensive theory of alternatives that will increase our ability to make predictions regarding the existence and expression of alternative reproductive behaviors in both sexes.

Journal ArticleDOI
TL;DR: Extensive recently published data suggest that the differences in variation for X-linked and autosomal genes for Drosophila, mice, and humans are consistent with the differences predicted theoretically based on the relative effective population sizes.
Abstract: The evolutionary genetics of haplodiploids and X-linked genes share many features and are different from diploid (autosomal) genes in many respects. For example, the conditions for a stable polymorphism, the amount of genetic load, and the effective population size are all expected to be quite different between haplodiploids or X-linked genes and diploids. From experimental data, the genetic load for X-linked genes is much less than autosomal genes and appears less for haplodiploids than for diploids. The observed amount of molecular variation for haplodiploids is much less than that for diploids, even more so than predicted from the differences in effective population size. Extensive recently published data suggest that the differences in variation for X-linked and autosomal genes for Drosophila, mice, and humans are consistent with the differences predicted theoretically based on the relative effective population sizes.

Journal ArticleDOI
TL;DR: The data from several species of ants, bees, and wasps indicate that workers can obtain information on the genetic heterogeneity of their colonies and use that information to manipulate reproductive decisions.
Abstract: ▪ Abstract The life of social insects centers around sedentary colonies that can include individuals belonging to different patrilines or matrilines, with a turnover of reproductives. The colony is a scene for both cooperation and conflicts, and the conceptual framework for the evolution of social life and colony organization is provided by the kin selection theory. Variable molecular markers make it possible to dissect kinship within colonies, identifying patrilines and matrilines and estimating genetic relatednesses. Such markers have been used to test hypotheses on social conflicts between queens and workers (split sex ratio hypothesis), among workers (worker policing hypothesis), and among reproductive females (skew hypothesis). The data from several species of ants, bees, and wasps indicate that workers can obtain information on the genetic heterogeneity of their colonies and use that information to manipulate reproductive decisions. The social structure of colonies and the mode of colony founding af...

Journal ArticleDOI
TL;DR: It has been demonstrated that, in concert with an increase in average age, the environmental and geographic ranges ofOrdovician genera both increased significantly through the period, indicating a role for intrinsic factors in producing Ordovician biotic patterns.
Abstract: Although the history of life has been characterized by intermittent episodes of radiation that can be recognized in global compilations of biodiversity, it does not necessarily follow that these episodes are caused by processes that occurred uniformly around the world. Major diversity increases could be generated by the cumulative effects of different mechanisms operating simultaneously at several geographic or environmental scales. The purpose of this review is to describe ongoing research on the manifestations, at several scales, of the Ordovician Radiation, which was among the most extensive intervals of diversification in the history of life. Through much of the period, diversity was concentrated most heavily near regions of active mountain building and volcanism; differences in diversity patterns from continent to continent, and among regions within continents, reflect this overprint. While this suggests a linkage of the Radiation and tectonic activity, this is by no means the only mediating agent. Outcrop-based research in North America has demonstrated that tectonic activity was detrimental to some biotic elements, in contrast to its effects on other organisms. Moreover, in the Great Basin of North America where the local stratigraphic record is of particularly high quality, biotic transitions characteristic of the period occurred far more rapidly than observed in global compilations of diversity, suggesting that the global rate of transition may represent the aggregate sum of transitions that occurred abruptly, but at different times, around the world. Finally, it has been demonstrated that, in concert with an increase in average age, the environmental and geographic ranges of Ordovician genera both increased significantly through the period, indicating a role for intrinsic factors in producing Ordovician biotic patterns.

Journal ArticleDOI
TL;DR: The current data on the phylogeny and reproduction of unicellular eukaryotes are insufficient to determine whether it evolved several times or just once, and theoretically, gradual origin of amphimixis from apomixis, with each step favored by natural selection, is feasible.
Abstract: ▪ Abstract The life cycles of cellular species are reviewed from the genetic perspective. Almost all life cycles include stages during which only one genome is transmitted from a parent to its offspring. This, together with interorganismal gene exchange, which occurs regularly in at least some prokaryotes and in the majority of eukaryotes, allows selection to evaluate different alleles more or less independently. Regular genetic changes due to intraorganismal ploidy cycles or recombination may also be important in life cycles of many unicellular forms. Eukaryotic amphimixis is generally similar in all taxa, but the current data on the phylogeny and reproduction of unicellular eukaryotes are insufficient to determine whether it evolved several times or just once. Theoretically, gradual origin of amphimixis from apomixis, with each step favored by natural selection, is feasible. However, we still do not know how this process occurred nor what selection caused it. For reasons not entirely clear, some propert...

Journal ArticleDOI
TL;DR: This approach demonstrates that larval morphology has been evolving independently of adult morphology, that larv morphology displays more homoplasy than adult morphology; and that early developmental patterns are remarkably flexible.
Abstract: New robust phylogenies for echinoderms, based on congruent patterns derived from multiple data sets, provide a sound foundation for plotting the evolution of life-history strategies and comparing rates and patterns of larval and adult morphological change. This approach demonstrates that larval morphology has been evolving independently of adult morphology, that larval morphology displays more homoplasy than adult morphology, and that early developmental patterns are remarkably flexible. Larval morphology on its own can mislead phylogenetic analysis, not because of lateral gene transfer among distantly related taxa, but because of massive convergence in the form of nonfeeding larvae brought about by the loss of complex structures and the strong functional constraints on feeding larvae. The degree to which larval tissue is resorbed at metamorphosis is believed to be important in determining adult body plan. Although the correspondence is not precise, it does provide a model for understanding skeletal homol...