Paedomorphosis in Evolution and Development

Some evolutionary changes of species result in the retention of juvenile characters in their adult life. This is known as paedomorphosis or neoteny or progenesis. It plays an important role in the origin of many taxa from subspecies to phyla. Paedomorphosis is also known as pedomorphosis. They usually retain the organism of juvenile or larval traits for their later adult life. The paedomorphosis occurs in two aspects. If one could find sexual maturation relative to the rest of the development, it is known as progenesis. If one could study the retardation in body development with regular reproduction activity, it is known as neoteny. 

Amphibians are the known best example of paedomorphosis. They will arrest the development in larval. So, some aquatic species will continue their sexual maturity inside the organism, they are capable to reproduce. In some other species, few morphological features are retarded. But the number of morphological features retarded may vary from species to species. Neotenic body features are most common in human beings. It shows some morphological defects in adults and it may vary from one to another. 

In other species, all morphological development is retarded, but this organism remains juvenilized but sexually matured. Such a shift in reproductive capability would appear to have adaptive significance for the organisms. According to the evolutionary theory, the process of paedomorphosis arrests the larval stages in their developmental phases of existing organisms, which give rise to the wholly new organisms under certain circumstances. 

Heterochrony

Both progenesis and neoteny may result in paedomorphism. It is a type of heterochrony. This heterochrony is much important for evolutionary biology, domestication, and evolutionary development of species. Heterochrony usually occurs due to the changes in the size, characteristics, shape, and even presence of certain features and organs of the species. According to heterotopy, a small change in spatial positioning in the embryo can also create morphological innovations. Heterochrony is further divided into two types they are intraspecific heterochrony and interspecific heterochrony.  The intraspecific heterochrony shows the variation in a species. Likewise, the phylogenetic variations, which include a variation of descendant species with respect to an ancestral species will comes under interspecific heterochrony. 

History & Etymology About Neoteny

Paedomorphosis is the result of neoteny. This occurs due to the retention of juvenile traits into the adult form, which results in retention of somatic development or progenesis, it increases the development process and the juvenile form becomes a sexually mature adult. The word neoteny is derived from the German word Neotenie. The word neoteny and the concept of neoteny has been traced from the bible. The term neoteny was invented by Julius Kollmann in 1885. He described the maturation of axolotl’s, which remains in a tadpole condition like aquatic stage complete with gills. This remains different from other adult amphibians like frogs and toads. In 1926, Louis Bolk described neoteny as the major process in humanization. In 1977, Stephen Jay Gould noted that Bolk’s account constituted the justification of both racism and sexism. Later he acknowledged the Bolk’s as his core idea about humans are different from other primates to become a sexually mature and infantile stage of body development. 

Paedomorphosis Evolution 

Neoteny terminates the development of an organism into an adult. So, this is termed eternal childhood. During this form of heterochrony, some developmental stage of childhood is extended and certain development process will remains as normal in childhood. The neoteny has been implicating as a behavior changes and developmental cause of childhood. This further increases brain plasticity. 

Paedogenesis can be observed from Ambystoma mexicanum (Axoloti). Axolotls will reach their full sexual maturity, but they get resist in the development of fins and gills. It means, the species looks still in the juvenile stage. But they remain in the same aquatic environments. Initially, this condition was considered hypomorphosis, but at that stage, the species will stop growing hormonally and genetically. In paedogenesis, it traped the genetic growth. Paedogenesis is also known as progenesis.  

During the avian cranial evolution, paedomorphosis plays a predominant role. The beaks and skulls of living adult birds controlled the anatomy of the juvenile theropod dinosaurs from which they developed. Extant birds have large brains and eyes related to the rest of the skull. This condition can see in adult birds, which represents the young stage of a dinosaur.  Here, a juvenile avian ancestor may have large eyes, a short face, a thin palate, tall and thin postorbitals, restricted adductors, narrow jugal bone, and a short and bulbous braincase. If the organism is aging, they would change widely in their cranial morphology to develop overlapping bone and a robust skull. According to molecular experiments, birds retain with its juvenile morphology. This represents both members of the WNT signaling pathway and fibroblast growth factor 8(FGF8). These are facilitated to achieve paedomorphosis in birds. These signaling pathways are playing important roles in facial patterns in other vertebrates. The retention of the juvenile ancestral state also creates some anatomical changes like a highly kinetic skull with many small non-overlapping bones. This is believed to have facilitated the evolution of cranial kinesis in birds.  This plays an important role in their ecological success. 

Paedomorphic Vs Overwintering Larval

Both paedomorphosis larva and overwintering larval show a similar relationship in the adult state, while their larval stage is retarded. But they show some differences. Usually, newts and salamanders will reach sexual maturity without losing their gills, while their larval stage is retarded. But some populations will remain immature during the larvae overwinter in water.  The larva which is affected by the overwintering will attain a large body size than the Paedomorphosis larva.