Heterochrony
Heterochrony refers to the biological processes that involve changes in the timing, rate, and duration of development and growth patterns from an ancestor to its descendants. This phenomenon can lead to significant morphological differences and is categorized into two main patterns: paedomorphosis and peramorphosis. Paedomorphosis results in descendants having features that are truncated or immature compared to their ancestors, with neoteny being a notable example—humans display neotenous traits when compared to ancestral primates. Conversely, peramorphosis describes a situation where descendant forms exceed those of their ancestors, leading to enhanced traits through mechanisms like acceleration, hypermorphosis, and predisplacement.
Key historical figures in the study of heterochrony include Karl Ernst von Baer, who emphasized the relationship between early and late developing features in organisms, and Ernst Haeckel, who notably proposed that ontogeny recapitulates phylogeny, a concept now largely viewed as outdated. The study of heterochrony has gained renewed interest, particularly in understanding how slight changes in developmental timing can result in the emergence of new structures and possibly new species. While research continues, the application of heterochrony remains in its early stages, and the full implications for evolutionary biology are still being explored.
Heterochrony
Heterochronic phenomena are processes by which changes in the timing, rate, and duration of an ancestral pattern of growth and development result in changes in descendants. There are approximately six types of heterochrony, though there can also be overlap between the various types and among species. One of the six types of heterochrony is neoteny, in which a descendant’s slower rate of development causes it to be an immature or truncated expression of its ancestor. Humans are an example of neoteny when compared with ancestral primates.
Paedomorphosis and Peramorphosis
The six types of heterochrony fall into two general patterns: paedomorphosis and peramorphosis. In these patterns, ancestral and descendant ontogenetic trajectories are compared. If the descendant morphology exceeds or surpasses the ancestral morphology, this is called peramorphosis. There are three types of peramorphosis: acceleration, hypermorphosis, and predisplacement. In acceleration, the beginning and end of development occur simultaneously in both the ancestral and descendant ontogenetic trajectories. The rate of development is faster in the descendant, however, so its morphology transcends that of the ancestor. In hypermorphosis, development begins at the same time and ensues at the same rate in both the ancestor and descendant; however, the descendant continues development for a longer interval—it stops growing later. In predisplacement, the descendant begins development earlier than the ancestor, and the rate of development and the time at which growth ceases remain the same. The result of both predisplacement and hypermorphosis is a longer interval of growth. In acceleration, however, the interval of growth is the same in both the ancestor and its descendant; only the rate of growth has changed.
If a descendant morphology is a truncated or an abbreviated version of ancestral morphology, this is called paedomorphosis. There are also three types of paedomorphosis: neoteny, progenesis, and postdisplacement. In progenesis, descendant development begins and proceeds at the ancestral rate but stops sooner. In postdisplacement, the ancestral rate of development and the time of cessation are the same in the descendant; however, development begins later. Thus, for post displacement and progenesis, the time interval over which the descendant morphology develops is truncated when compared to the ancestral ontogenetic trajectory. In contrast, neoteny is characterized by a slower descendant rate of development but an unchanged time interval for growth and development.
The Contributions of Von Baer and Haeckel
Two scientists of the nineteenth century were especially important contributors to the early ideas concerning development and its impact on evolutionary change. Karl Ernst von Baer suggested that the features that appear early in ontogeny are those that are shared by the most organisms, whereas the features that appear later in ontogeny are those that are shared by successively smaller groups of organisms. This maxim has been called von Baer’s law. Ernst Heinrich Haeckel held a different, more restricted concept of ontogeny. He popularized the phrase “ontogeny recapitulates phylogeny.” By this, he meant that the ontogenetic or developmental phases through which an organism passes can be interpreted as being equal to the sequence of events that occurred during the evolutionary history of that particular organism. Haeckel’s narrower concept of the relationship between ontogeny and phylogeny has been rejected by modern biologists in favor of von Baer’s law.
Von Baer and Haeckel laid the foundation for ideas of heterochrony, but it was not until the later twentieth century that the subject of heterochrony was once again an area of active research. An important book by Stephen Jay Gould entitled Ontogeny and Phylogeny (1977) was instrumental in reopening the discussion initiated a century earlier. One of the most important things accomplished in Gould’s book was the distinction that was made between different types of paedomorphosis. Frequently, in earlier works dealing with species with a juvenile appearance, no distinction was made among neoteny, post displacement, and progenesis: All three were discussed as neoteny. This confusion prevented generalization.
Organisms for which neoteny is used to explain their origins include some salamanders of the family Ambystomatidae and the primates of the family Hominidae (the family that includes humans). In the latter case, Homo sapiens is considered a juvenilized anthropoid; the reduction in the amount of hair and the longer period of infancy can be used as evidence for this supposition.
Studying Heterochronic Change
Two kinds of information are necessary for documenting heterochronic processes of change. Because frequent reference must be made to ancestral and descendant ontogenetic trajectories, it is crucial to have available, for the organisms under study, an estimate of their phylogenetic, or evolutionary, history. This is not an easily obtained body of evidence, and much scientific debate has occurred over the procedures that should be followed in seeking to unravel phylogenetic history. Some researchers even doubted that it would ever be possible to obtain evidence sufficient for the task. Nevertheless, some small consensus has emerged. This is especially gratifying for students of heterochrony, who depend so much upon the phylogenies that anchor their conclusions.
It is also necessary to document ontogeny, development, and/or growth in either a quantitative or qualitative sense. The quantitative measurement of growth is relatively straightforward. Measurements are taken from specimens of known ages. It is most desirable to take measurements from the same individual specimens throughout their ontogenies. This is not always possible, nor is it always possible to obtain accurate ages for the available specimens. Another problem is that the ages of the specimens of different species might not be directly comparable. These are among the factors that complicate the otherwise simple process of measuring the sizes and shapes of specimens over time—that is, measuring in various places along an ontogenetic trajectory. Assuming that appropriate measurements have been gathered, various statistical procedures can be applied to the accumulated data. These procedures include some rather sophisticated multivariate statistics.
The qualitative documentation of an onto genetic trajectory suffers from many of the same sources of complication. In this approach, onto genies are conceptualized as a series of discrete stages, phases, events, or appearances. These sequences are determined for the organisms under examination, and the stages that bear close resemblance are sometimes considered identical. The problem with this view is that it conceives of ontogeny as being composed of static sequences; this is a largely Haeckelian view. In fact, ontogenies are best viewed as dynamic, which makes it conceptually difficult to compare isolated parts of an indivisible ontogeny.
An unanswered question of evolutionary biology concerns the processes by which new morphologies and new organisms originate. Although many promising inroads have been made, none has been more hopeful than the idea of heterochrony. The conjecture is that small shifts in the timing, rate, and duration of ontogeny contribute to the appearance of new structures or perhaps even new organisms. This approach has so far proved both effective and promising, but it has not yet been applied to a sufficient range of organisms. Thus, it is not yet possible to tell whether heterochrony will prove to be a universally applicable approach to the study of the origin of new structures and species.
Principal Terms
Acceleration: a faster rate of growth during ontogeny that causes a particular characteristic to appear earlier in a descendant ontogeny than it did in the ancestral ontogeny
Heterochrony: any phenomenon in which there is a difference between the ancestral and descendant rate or timing of development
Hypermorphosis: a phenomenon in which the rate and initiation of growth in the descendant are the same as in the ancestor, but the cessation of development takes place later
Ontogeny: the life history of an individual, including both its embryonic and postnatal development
Phylogeny: the history of a lineage of organisms, often illustrated by analogy to the branches of a tree
Postdisplacement: a form of paedomorphosis in which the initiation of growth in a descendant occurs later than in the ancestor, ensues at the ancestral rate, and ceases at the ancestral point
Predisplacement: a form of peramorphosis in which the initiation of growth in a descendant occurs earlier than in the ancestor, ensues at the ancestral rate, and ceases at the ancestral point
Bibliography
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