Heterochronic Processes Research Articles

The role of time and timing in hominid dental evolution

AbstractExactly when during evolution hominids acquired their extended extra‐uterine growth period is a contentious issue. In order to shed light on the tempo and mode of ontogenetic changes during hominid evolution, research has focused on the pattern and, to a lesser extent, the rate of growth observed in the developing dentition of extant and extinct hominoid taxa. From these data, the absolute timing of events has often been inferred, either implicitly or explicitly. Differences in patterns of growth, especially of the eruption of teeth, are reasonably well documented among hominoids. However, data on the absolute timing of dental developmental events are much more scarce, rendering tentative all inferences about timing from patterns alone. Such inferences are even more tentative when they involve interpreting ontogenetic trajectories in extinct species such as Plio‐Pleistocene hominids, which almost certainly had unique patterns of maturation. In order to contribute to the debate about possible relations between pattern and timing in the developing dentition, we have collated information that specifically relates to the absolute timing of developmental events in extant and extinct hominoids and, hence, also to the rate at which processes occur. In doing so, we have attempted to identify both developmental constraints and possible heterochronic processes that may have led to the extended growth period characteristic of humans. There appears to be growing evidence that evolution toward an extended hominid ontogeny did not follow a path that can be described as a simple heterochronic event.

Dimorphism inKheraicerasSpath from the Callovian Chari Formation, Kutch, India

KheraicerasSpath, 1924, is known mainly from macroconchs. Matching of dimorphic pairs at species level is difficult due to the rarity of microconchs and consequent difficulty in establishing contemporaneity. The present study reveals dimorphism withinKheraiceras cosmopolitum(Parona and Bonarelli, 1895) from the lower part of the Callovian Chari Formation, Kutch, India, where microconch closely resembles macroconch except for the size, being about three times smaller and having an aperture characterized by terminal construction. The evolution ofKheralcerasfromBullatimorphitesBuckman is based on macroconchs only. The discovery of coeval microconch specimens and study of known microconchs within the lineage indicate “parallel” evolution in macro- and microconchs and improves the understanding of evolutionary trends that involve complex heterochronic processes.

HETEROCHRONY AND ALLOMETRY: LESSONS FROM THE WATER STRIDER GENUS LIMNOPORUS.

Heterochrony and allometry both deal with evolutionary modifications of ontogenies. Although data about both morphology and age are required to identify heterochronic processes, age data are not needed to study allometry. Using a simple graphical model, we show that allometric patterns cannot be used to infer the underlying heterochronic processes. We present a case study of the water strider genus Limnoporus Stål (Heteroptera: Gerridae) to illuminate the distinct roles that allometry and heterochrony play in integrated studies of the evolution of form. Multivariate analyses reveal several evolutionary modifications of growth trajectories (changes in direction, lateral transposition, and ontogenetic scaling), which are fairly consistent with the hypothesized phylogeny of the genus. Because there is no positive correlation between instar durations and size increments, size cannot be used as a proxy for age data in studies of heterochrony. In fact, a measure of overall size itself shows a remarkable variety of heterochronic changes among the six species. Mixtures of several heterochronic processes predominate over the more unitary reflections of "pure" processes. Heterochronic changes in different branches of the phylogeny, apparently independent of size scaling, suggest considerable potential for adaptive evolution. "Local" differentiation of ontogenetic traits within small clades may be at least as important as "global" evolutionary trends in large clades and will often be missed in "global" analyses.

A DYNAMICAL THEORY OF HETEROCHRONY: TIME-SEQUENCING CHANGES IN ECOLOGY, DEVELOPMENT AND EVOLUTION

This work provides a foundation for a quantitative dynamical theory of heterochronic processes in the evolution of colonial invertebrate animals including Bryozoans, Siphonophores and Ants. These processes are environmentally induced changes in the time-sequencing of growth and development which can produce alterations in the morphotypes or castes within an individual colony. Motivation comes from Křivan’s theory of environmentally induced constraints on population densities for ecological interactions, but the present theory is second order with allometric production variables xi and population densities for morphotypes, Ni. We are able to unite ecological theory and the allometric form of the Wilson Ergonomic Theory via projective differential geometry and Wagner spaces which provide a natural description of environmentally induced time-sequencing changes altering the allometric curve of a species. Such changes define a model of heterochronic processes important in paleontology.

COMPARATIVE ONTOGENY OF A WILD CUCURBIT AND ITS DERIVED CULTIVAR.

Most previous studies of evolutionary modification of form in plants have focused primarily on individual organs or flowers. Few have investigated the role of evolutionary changes in timing or position at the level of whole plant ontogeny. This study compares ontogenies of the primary shoots of two subspecies of Cucurbita argyrosperma, one a cultivar and the other its wild progenitor. Differences in flowering times between these subspecies suggested that the cultivar may have evolved from the wild subspecies via heterochronic processes leading to paedomorphosis. Analyses showed that both subspecies are similar in vegetative architecture and rates of leaf production. Earlier flowering in the cultivar, both in terms of position and absolute time, appears to have arisen through progenesis. Initial observations of leaf blade morphology led to the hypothesis that paedomorphosis and gigantism also may have been involved in the evolution of leaf blade shape in the cultivar: all leaves of the cultivar are larger and visually similar in shape to early leaves of the wild subspecies. However, quantitative analysis revealed that leaves of the cultivar are neither geometrically, nor solely allometrically larger versions of early leaves of the progenitor. Leaf shape in the cultivar exhibits novel features as well as effects of allometry shared with the progenitor, hence a simple hypothesis of paedomorphic evolution of leaf shape is not supported.

A mathematical theory of evolution by heterochrony in colonial invertebrates

This work provides a foundation for a quantitative dynamical theory of heterochronic processes in The Evolution of Colonial Animals including Bryozoans, Siphonophores and Social Insects. These are environmentally induced changes in the time-sequencing of growth and development of a colonial individual (e.g. Ant colony, Portuguese Man-of-War, etc.) which can, via the Finsler differential geometric theory of Wagner transformations, produce alterations in social interactions. This mathematical theory predicts the usual Paleontological Allometric relationships between fossil morphospecies and their ancestors. Emphasis is on the extensive Bryozoan fossil record because of its great importance in establishing The Theory of Punctuated Evolution of N. Eldredge and S. J. Gould. But, facultative neoteny, as occurs in Salamanders is also described by this model, as is behavioral switching in Ants.

Variations morphologiques de l'oursin irrégulier disasteroidea Collyrites (Pygomalus) Analis (Agassiz): Implications taxonomiques, paléoécologiques et évolutives

The morphological and anatomical variations of the test of Collyrites (Pygomalus) analis ( Agassiz) are analyzed for 17 samples from the Upper Bathonian of the Paris Basin (Nievre, Saône-et-Loire and Sarthe), well located in the standard Jurassic ammonite zones (Retrocostatum and Discus). The biometrical description of the morphology is emphazised by the analysis of 8 variables and 8 indices worked with statistical methods. The large number of specimens and the range of size allow the study, of morphological modifications, during the ontogeny. In each population, the general shape presents a huge variability between the young and the adults. In the same area, the morphological differences between samples successive in time are not significant and compatible with a pera-hypermorphic cline controlled by anagenetic processes, where Collyrites (Pygomalus) analis ( Agassiz) represents one of the, evolutionary steps. Within such a time interval, the species is morphologicaly well delimited and some “taxa”, previously described as species or subspecies, are here considered as morphological variants due to heterochronyn ontogeny. The comparison between samples isochronous at the scale of ammonite zone, but coming from different areas of the Paris Basin, shows important morphological differences on the maximum size and the height of the test. Once more, these differences have no taxonomic value; they are interpreted as accomodations to some environmental factors and the more gibbous specimens are considered as ecophenotypes. In consequence, the observed morphological palsticity is interpreted as the result of the action of both heterochronic processes in the ontogeny and adaptative reply facing to environmental constraints.

Structural and molecular approaches to the phylogeny of Amphibia

Abstract The structuralistic and molecular approaches to the phylogeny of living amphibians are briefly reviewed with emphasis on recent paleontological findings supporting their possible origin from Paleozoic branchiosaurids; however the scarcity of early Mesozoic fossils improves the weight of neontological research on the problem of interOrder relationships of lissamphibians. Ontogenetic studies have provided many new facts of phyletic interest, even if the larvae of urodeles on the one hand, and of anurans on the other, seem to assume extremely different biological roles, and are then scarcely comparable with each other. Changes in temporal programming of morphogenetic processes severely modify adult mor‐phophysiology at a probably low genetic cost; neoteny and pedomorphosis have effectively characterized the natural history both of extinct and living amphibians, and seem to constitute powerful preadaptive mechanisms opening new econiches and paralleling the radiation of speciose genera or tribes of lissamphibians. Heterochronic processes find a counterpart at karyological level because both genome size and karyotype morphology are modified with the ontogenetic ≪patterning characterizing these taxa. The importance (if any) of repetitive DNA fractions (especially satellite sequences) in the production of microevolutionary changes is currently under examination, and in the future will probably modify the perhaps too mechanicistic vision linking protein changes to specific evolutionary trends, giving more weight to the role played by non‐transcriptive genomic components in the control of morphogenetic processes, and thus in producing adaptation and evolution.

The evolution of gastropod shell form: a developmental model illustrating the roles of heterochrony and non-heterochronic changes

The shape of an isometric gastropod shell can be described completely by specifying the pattern of shell secretion around the aperture (relative to aperture size) and the growth rate of the aperture itself. These descriptors provide a “natural” morphometric in that they correspond to the specific biological processes involved in constructing the shell.Describing shell form in this way allows us to specify what developmental changes must occur during the transition of one shell form to another. In particular, we can distinguish between transitions that can occur through purely heterochronic processes (changes in growth rate) and those that require a change in the specific pattern in which cells of the mantle lay down shell. We can also investigate just what changes occur during the ontogeny of non-isometric shells.Any change in either the pattern of shell secretion or the growth rate of the animal leads to changes in a number of classical morphometric measures, such as apex angle and whorl expansion rate. Those transformations resulting from changes in growth rate, however, are much more predictable than those resulting from changes in the pattern of shell production. A slight increase in the growth rate of the animal, for instance, produces a correspondingly slight increase in the apex angle and the rate of whorl expansion. By contrast, the consequences of a slight change in the pattern of shell production are highly sensitive to just how that change was achieved.Data from 8 genera of marine snails show that the variance within each genus, relative to the variance among all genera, is smaller for measures of aperture shape (which can only be altered through a change in the pattern of secretion of shell material) than for characters that can change through heterochronic transformations (such as apex angle). Furthermore, the shell forms of a number of non isometric shells can be described by a constant pattern of shell production and a variable growth rate.Heterochronic changes thus appear to be the preferred mechanism for changing phenotype in gastropod shells. Those characters that can only be altered by changing the pattern of shell production around the mantle, such as aperture shape, appear to be more conservative than those that can be changed through purely heterochronic transitions. This is consistent with the idea that mutations which alter many characters in a highly correlated manner have a higher probability of being favored by selection than those with relatively unpredictable consequences.

Ontogenetic and evolutionary patterns of septal neck transformation in the Ammonoidea

Four types of septal necks are present in the Ammonoidea: (1) retrochoanitic (entirely projected adapically), (2) modified retrochoanitic (projected adorally or both adorally and adapically on the dorsal side and adapically on the ventral side), (3) amphichoanitic (projected both adorally and adapically), and (4) prochoanitic (entirely projected adorally). Each septal neck is continuous with the rest of the septum and consists primarily of a nacreous layer. A spherulitic-prismatic deposit called the auxiliary deposit commonly appears on the adoral side of the septal neck and covers the inner surface of the nacreous layer. Another spherulitic-prismatic deposit (cuff) may also occur on the adapical side of prochoanitic necks. In all of the Paleozoic suborders except Goniatitina (Prolecanitina, Bactritina, Anarcestina, Agoniatitina, Clymeniina, Gonioclymeniina and Tornoceratina), type 1 necks are present throughout ontogeny (1→1). In the Goniatitina as well as in the Ceratitina, type 1 necks either persist throughout ontogeny (1→1) or develop into type 2 necks (1→2). In the Phylloceratina, type 1 necks are present in early to middle ontogeny, later developing into type 3 necks (1→3). In the Lytoceratina, type 1 necks are only present in very early ontogeny and are immediately replaced by type 4 necks (1→4). In the Ancyloceratina and Ammonitina (with few exceptions), only type 4 necks occur (4→4). Type 1 necks evidently represent the primitive condition. Variation among suborders in the ontogenetic timing of the transformation from the primitive condition to one of the three more derived conditions may be the result of heterochronic processes such as acceleration or predisplacement. However, the pattern of septal neck transformation within suborders is more or less stable and was probably established at the time each suborder originated.

Clinal variation, intraspecific heterochrony, and microevolution in the Permian bryozoan Tabulipora carbonaria

Clinal geographic variation across an onshore-to-offshore environmental gradient occurs in the Lower Permian trepostome bryozoan Tabulipora carbonaria collected from three widespread calcareous shales of the Wreford Megacyclothem of Kansas. Separate multivariate statistical tests dclineate significant differences between populations within each shale unit, and between onshore, intermediate, and offshore populations pooled across all three shales, suggesting that even weakly-developed gradients may initiate different intraspecific morphological responses. Growth trajectories for populations along the cline also differ significantly, indicating astogenetic (developmental) heterochrony. Populations from onshore habitats are generally paedomorphic relative to those in more offshore settings, and exhibit pre- and postdisplacement and hypermorphosis in zooecial and acanthostyle characteristics. These heterochronic processes may have increased colonial reproductive potential and the efficiency of water flow for feeding and waste disposal in colonies from onshore habitats. Variation from tightly constrained development (astogenetic plasticity) decreased monotonically in an onshore-to-offshore direction; canalized growth may characterize colonies from more stable offshore habitats, whereas greater flexibility during the growth of colonies from unstable onshore biotopes may have increased their rate of survival. Populations of colonies from stratigraphically successive calcareous shales of the Wreford display patterns of growth that are nearly identical to those found in an offshore-to-onshore direction along the cline. Both clinal and temporal patterns probably resulted from selection for more paedomorphic morphologies in onshore, perhaps unstable, habitats and represent microevolution in T. carbonaria. These local adjustments to environmental conditions may produce variation that affects the rate of macroevolutionary change. □Bryozoa, clines, heterochrony, microevolution, Permian, variability.

The Trimmatom nanus Species Complex (Actinopterygii, Gobiidae): Phylogeny and Progenetic Heterochrony

-A cladistic analysis of 42 two-state characters of the adults of five species in the monophyletic Trimmatom nanus species group resulted in a fully resolved cladogram of 48 steps with a consistency index of 88% (excluding autapomorphies of the group and of terminal taxa). The initial analysis was based on 52 osteological and external two-state characters (although in both cases some characters were analyzed as ordered transformation series). The monophyly of three of the species-T. sagma, T. offucius, and T. nanus-was especially well corroborated, with 19 synapomorphies. Within this subgroup, eight synapomorphies unite T. offucius and T. nanus as sister species. These two species mature at smaller sizes and do not reach the lengths attained by the other three species in the species group. Reexamination of the eight apomorphic character states in similar-sized specimens of their sister species, T. sagma, led to the interpretation of three of the eight character states as being consistent with progenesis. Because these character states are the result of progenetic processes, they are subsumed as a single apomorphy of truncated development. The potential to reassign the position of the progenetic (at this level of analysis) character states on the tree is explored. Some character states involving lack of ossification that cannot be positively identified as synapomorphies of T. offucius and T. nanus by comparison with juveniles of T. sagma and T. zapotes may be interpreted as the result of heterochronic shifts earlier in development. Cladograms therefore can be used on real data sets to identify progenetic characteristics and their level of universality. [Progenesis; phylogeny; heterochrony; truncated development; Trimmatom.] The publication of Gould's (1977) book on ontogeny and phylogeny was followed by an increased interest in heterochronic phenomena (e.g., McNamara [1988] and references cited therein). Numerous workers have investigated the consequences of truncated development at small size, or progenesis (e.g., Alberch et al., 1979; Alberch and Alberch, 1981). Often the detection of heterochronic processes has been probed with a two-taxon statement involving the inferred ancestor or closest relative, which is used to polarize the character states (e.g., Schweitzer et al., 1986; Anstey, 1987: his Tables 2-4). In the case study by Alberch and Alberch (1981), a derived salamander was compared morphologically and ontogenetically with two generalized members of the same genus. In these examples, an implicit (if imprecise) phylogeny is assumed. In many cases, the interpretation given will, in fact, be correct, but for the wrong reasons. In essence, the problem associated with these approaches is the lack of explicit descriptions of the procedure for detecting heterochronic results in nature (Fink, 1982: 254). There is no mechanism to distinguish the distribution of plesiomorphy and apomorphy on the phylogenetic tree because there is no tree to begin with, and therefore assessments of relative plesiomorphy or apomorphy cannot be made. As Fink and others have shown in theoretical models, such distinction is essential to protocols for research on heterochrony (Fink, 1982: his Figs. 1, 2, 1988; Kluge and Strauss,

The Meaning of Developmental Time: A Metric for Comparative Embryology

The lack of a suitable metric for developmental time has prevented a comparison of developmental rates and tempos between species, thus precluding analyses of heterochronic processes in evolution that do not depend on the use of morphology or size as an index of age. At least two problems exist in using clock time itself as the measure of developmental time: temperature effects within a species, and size effects across species. Consideration of two important distinctions that can be made in the notion of time-that between time as sequence and time as duration and that between extrinsic and intrinsic time-suggests that a metric, to be useful, must satisfy seven criteria: (1) it should be independent of morphology; (2) it should be independent of size; (3) it should depend on only one a priori homologous event; (4) it should be unaffected by changes in temperature for any given species; (5) closely related organisms should undergo homologous events at similar developmental ages as measured by the metric; (6) it should increase monotonically with clock time; and (7) it should be defined in a physically measurable way. I propose that physiological time, here defined as the integral of mass-specific metabolic rate over clock time, may be such a metric; this definition is based on an extension of previous concepts of physiological time to reflect ontogenetic changes in mass and metabolic rate. The proposed metric inherently satisfies criteria 1, 3, 6, and 7; satisfaction of the others must be assessed empirically. In spite of problems with strict comparability, literature data from 22 vertebrate species strongly suggest that criterion 2 is also satisfied, although they cast doubt on the satisfaction of criterion 5. No available data bear directly on criterion 4, although circumstantial evidence suggests that it may also be satisfied. A more rigorous assessment of the hypothesis awaits additional data, especially those allowing a correlation between physiological time and morphological development. However, even if this particular proposal for a metric proves unworkable-which is likely, given its simple formulation and broad scope-the criteria suggested will serve to judge any future proposal of a metric for developmental time. For the present, one must at least be aware of the hidden assumptions that have entered into much previous work on heterochrony, particularly that of an equivalence between size and age. Without a metric for developmental time, the extent and meaning of evolutionary changes in developmental timing simply cannot be assessed.

Heterochrony in human evolution: The case for neoteny reconsidered

This paper reviews the evidence for the claim that neoteny, or morphological juvenilization resulting from dissociation and retardation of ancestral rates of shape change, has played a key role in human evolution. Accepted categories and processes of heterochrony are reviewed, and the available data on human growth, variation, and evolution are analyzed in light of expected results. Relatively weak concordance with predictions is found. Mistakes of fact and interpretation fall into several categories, including (1) confusion of neoteny with paedomorphosis resulting from other heterochronic processes; (2) conflation of growth prolongation in time with morphological shape retardation; (3) failure to move beyond superficial shape similarities to underlying homologous growth processes and patterns; (4) failure to identify a paedomorphic basis for key anatomical novelties in human evolution; and (5) establishing an essentially untestable framework for analysis of the hypothesis. Key areas that might contribute new data to this debate are discussed, particularly the genetic and epigenetic control of the covariation of morphology and development during ontogeny and evolution. The primary reasons that arguments in favor of neoteny in human evolution have persisted probably relate to anthropocentric factors and the search for a single basis for the important morphological and behavioral transformations characterizing our lineage.

Hétérochronies, variabilité morphologiqueet tendances évolutives chez les brachiopodes jurassiques

The morphological variability of species of Mesozoic brachiopods, especially rhynchonellids, arise main ly from their ontogenetic development. It appears on the biconvex shell through various ontogenetic trajectories resulting from heterochronic processes. at the scale of evolutionary trends, the same processes explain a large part of the morphological changes, which use both paedo- and peramorphose. To be detected and understood, their mechanisms have to be compared with the ontogenetic grounds of the population's variability. In spite of their relatively poor branching power of evolution, theses trends are helpfull to specify evolutionary processes. Their links with environmental data will certainly be of great interest for a weighted analysis of adaptation.

The role of heterochrony in the evolution of spatangoid echinoids

Despite many imperfections the fossil record still providesthe only direct evidence of patterns of morphological change through time. By analysing the ontogenetic development of successive species of a lineage in a stratigraphic succession, it is possible to evaluate the nature of any heterochronic change and assess which particular paedomorphic or peramorphic processes might have been involved in morphological evolution. Spantangoids are useful in such studies because morphological characters which are used to differentiate living species are generally all preserved in fossil species. Furthermore, the autochthonous nature of many spatangoid assemblages and the intimate relationship between test morphology and enclosing sediment type allow the impact of environmental factors on the direction of morphological evolution to be assessed. Four lineages of Cainozoic spatangoids from Australia are discussed. The Paraster-Schizaster lineage shows peramorphic evolution, by a combination of hypermorphosis and acceleration of specific characters. Fossil species in a Hemiaster lineage provide an example of a lineage in which morphological evolution progressed by dissociated heterochronic events: some characters followed peramorphoclines, others paedomorphoclines. The Protenaster lineage is characterised by its great morphological conservation in most characters, except for the pore pairs of the phyllode: these form a peramorphocline produced by acceleration. The three species of the Lovenia lineage also show dissociated heterochronic events. The pronounced intra-specific variability in species of this lineage also occured by the action of heterochronic processes. In all these heterochronic lineages, and in the classic Micraster lineage in the European Cretaceous, which itself was probably driven by heterochrony, selection pressure favoured those species which were better adapted to burrowing in progressively finer-grained sediments.

Patterns of heterochrony in the fossil record

Recent analyses of the fossil record have revealed that heterochrony has played a significant role in the evolution of most marine invertebrate groups. Recognition of several different heterochronic processes has allowed their influence under different ecological regimes to be assessed. Furthermore, recent research has demonstrated significant differences in frequencies of these processes between groups and at their different stages of phylogenetic development. The fossil record shows that heterochron y has been an important component in the generation of evolutionary trends. Heterochrony is an important factor in macroevolution because it can result in relatively abrupt morphological change with only minimum alteration of the genome.

The biological and chronological maturation of early hominids

Recent studies on the rate and pattern of dental development indicate that the growth and maturation of early hominids were more similar to the extant apes than to modern humans. This contrasts with the previously held opinion derived from combined dental development, pattern and attrition studies claiming that early hominids were more hominine in their development (Mann, 1975). This paper explores the origin of this difference of opinion and reviews immature hominid dentitions with the benefit of improved radiographs and new data on the pattern and rate of pongid dental development. Paranthropus and Australopithecus specimens are shown to possess an ape-like development pattern but incisor development is specialized in the former and superficially human-like in pattern. The present and recent studies on dental development rate and pattern justify the position that early hominids were more ape-like in their growth and development. Therefore, ages at death calculated from pongid dental development schedules are provided for most immature early hominids. More detailed studies of early hominid developmental biology are now possible. It is suggested that divergent heterochronic processes characterize changes in brain/body proportions during hominid evolution. Relative rates of bone remodeling processes can now be identified on early hominid skeletons. The paleodemographic analysis of early hominids is little changed by the developmental model one chooses.

Heterochrony and Phylogenetic Trends

A model is proposed, based on examples that have been interpreted as phylogenetic trends, to explain how directional morphological evolution at the species level can arise by heterochrony. The examples illustrated are of Tertiary to Recent rhynchonellide brachiopods, Cambrian olenellid trilobites, living spatangoid echinoids, Tertiary to Recent schizasterid echinoids, Cenomanian ammonites and Silurian monograptids. Morphological discontinuities between species along morphological gradients (which can be recognised both spatially and/or temporally), and temporal morphological stasis within species, are both consistent with the punctuated equilibria model of macroevolution. It is argued that morphological discontinuities have arisen by selection of morphological novelties produced by heterochronic processes. These novelties are preadaptations which allow ecological and, consequently, genetic isolation from ancestral species. Establishment of a heterochronic morphological gradient is only possible given a suitable environmental gradient. The terms “paedomorphocline” and “peramorphocline” are proposed for these heterochronic morphological gradients. Paedomorphoclines and peramorphoclines each comprise a number of species occupying a series of adaptive peaks, which have evolved sequentially through time by selection along an environmental gradient.

The conceptual relationship between ontogeny and phylogeny

Studies of ontogenetic processes are fundamentally dependent on hypotheses of phylogeny. The model of Alberch et al. (1979) is reformulated in terms of phylogenetics and used to describe how heterochronic ontogenetic processes can be detected in nature. Heterochronic processes producing paedomorphosis can result in morphologies which resemble primitive (retained ancestral) traits; the conditions under which paedomorphic and primitive features can and cannot be distinguished are described. The utility of ontogeny for determination of evolutionary character transformations and character polarity and for detection of convergence and parallelism are considered. The ontogenetic criterion for assessing polarity is independent of hypotheses of phylogeny and may be as effective as outgroup comparison. Ontogenetic analysis may aid in the detection of convergence but not in the detection of parallelism.