HETEROCHRONIES: PERAMORPHOSIS AND PEDOMORPHOSIS (WITH EXAMPLES) - BIOLOGY - 2025

The heterochronies are a set of morphological changes - crucial in macroevolution - changes that occur or arrangements on the speed and timing of development. They are classified into two large families: pedomorphosis and peramorphosis.

The first, pedomorphosis, refers to the retention of the youthful appearance by the adult, if we compare them with the ancestor species. In contrast, in peramorphosis (also known as recapitulation) the adults present exaggerated characteristics in the descendant species.

Source: I, Drow male

Each of these families of heterochronies has in turn three mechanisms that explain the existence of the aforementioned patterns. For pedomorphosis they are progenesis, neoteny and post-displacement, while the mechanisms of peramorphosis are hypermorphosis, acceleration and pre-displacement.

Currently, understanding the relationship between development patterns and evolution is one of the most ambitious goals of biologists and that is why the “evo-devo” discipline was born. Heterochronies are a key concept in this branch.

What are heterochronies?

Traditionally, one usually speaks of two levels of changes in evolutionary biology, microevolution and macroevolution. The first is widely studied and seeks to understand the changes that occur in allele frequencies in members of a population.

In contrast, the according to exchange rate, macroevolution, implies the accumulation of changes at the microevolutionary level that lead to diversification. The famous paleontologist and evolutionary biologist SJ Gould points out two main ways in which macroevolutionary change can occur: innovation and heterochronies.

Heterochronies are all that set of variations that occur during the ontogenetic development of an individual, in terms of the time of appearance of a character or in the rate of its formation. This ontogenetic change has phylogenetic consequences.

In the light of evolutionary biology, heterochronies serve to explain a wide range of phenomena and function as a concept that unifies a model to explain diversity with phenomena related to development.

Today the concept has gained much popularity and researchers apply it at different levels - it no longer exclusively understands morphology - including cellular and molecular levels.

At what level do heterochronies occur?

The comparison established in the heterochronies is made based on the descendants compared with their ancestors. In other words, the descendants of a group are compared to the outer group. This phenomenon can occur at different levels - call it a population or a species.

For example, we are aware that in our populations not all developmental phenomena occur at the same time in all individuals: the age of changing teeth is not homogeneous in the population, and neither is the age of the first menstruation in girls.

A key factor is the time frame used in the study. It is recommended that it be a temporarily limited study of a closely related group.

Conversely, comparisons at higher levels (phyla, for example) using approximate sampling of time periods, will be emphasized and will reveal scored patterns of differences that cannot be used to infer processes.

How are they studied?

The simplest and quickest way to pinpoint the potential evolutionary events that can be explained by heterochronies is by observing and analyzing the fossil record. The idea in this procedure is to be able to recognize the changes that have occurred in terms of size and age.

From the point of view of paleontologists, heterochronies are key processes to understand the evolution of a certain group and to be able to trace the phylogenetic relationships between them.

Ontogenetic processes that affect growth rate

Pedormofosis

Pedomorphosis occurs when adult forms exhibit typical features or characteristics of juveniles.

There are three events that can lead to pedomorphosis. The first is progenesis, where the time for the formation of the trait is shortened, generally caused by advancement in sexual maturity.

Neotheony, on the other hand, reduces the rate of change in ontogenetic development. Therefore, the youthful features are maintained in the adult. Finally, post-displacement involves the development of a late-onset trait.

Peramorphosis

Peramorphosis is an exaggeration or an extension of a certain morphology of the adult individual, when compared to its ancestor.

As in pedomorphosis, peramorphosis can be explained by three events. Hypermorphosis encompasses a delay in the age of maturity, so the body grows until maturity arrives. This process represents an extension of the ontogenetic process.

Acceleration refers to the increase in exchange rates. In contrast to the previous case, in acceleration, the age of sexual maturity is the same for ancestors and descendants. Finally, pre-displacement refers to the earliest onset of the appearance of a feature.

In vertebrates, peramorphosis appears to be more of a theoretical model than an event that occurs in reality. There are scarce data and in very specific cases of the process.

Examples

Heterochronies in the development of

Heterochronies can also be studied at the molecular level and there are different methodologies to carry out these investigations.

For example, Kim et al. (2000) sought to understand the heterochronies in early development of different Drosophila species - known as fruit flies.

The results suggest that in the three evaluated species (D. melanogaster, D. simulans, and D. pseudoobscura) there is a temporal displacement of the ontogenetic trajectory in the early stages of development. D. simulans showed earlier expression patterns, followed by D. melanogaster and ending with and D. pseudoobscura.

The time scales in which gene expression varied between species was less than half an hour. The authors speculate that there are epigenetic interactions between the expression of the genes studied and the synchronization of the cell cycle that lead to existing morphological differences between species.

Salamanders

Salamanders are the classic example of neoteny, specifically the species Ambystoma mexicanum. The adult forms of this species exhibit their characteristic gills, typical of the juvenile stages.

Humans

It is speculated that the morphology of humans is the product of a neoteny event. If we compare the structures of our skull, for example, we will find more similarities with a juvenile form of our simian ancestor than with the adult variations.

References

1. Goswami, A., Foley, L., & Weisbecker, V. (2013). Patterns and implications of extensive heterochrony in carnivoran cranial suture closure. Journal of evolutionary biology, 26 (6), 1294-1306.
2. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw – Hill.
3. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
4. Kim, J., Kerr, JQ, & Min, GS (2000). Molecular heterochrony in the early development of Drosophila. Proceedings of the National Academy of Sciences, 97 (1), 212-216.
5. Smith, KK (2003). Time's arrow: heterochrony and the evolution of development. International Journal of Developmental Biology, 47 (7-8), 613-621.