Evolution of Facultative Paedomorphosis in Newts and Salamanders (Caudata)
Evolution is the ultimate result of a number of influences which either promote or limit change. Ontogenesis, or the development of a new organism can arise from small changes in genes that induce phenotypic variation. A vertebrate usually experiences three different events during its life, birth, metamorphosis, and puberty. These three typically occur in order and any deviation in this cycle results in heterochrony. Heterochony involves the timing or the rate of these developmental changes which play a major role in both macro- and microevolution. Evidence from fossil and living records suggest that heterochronic processes were involved in the evolution of a large number of vertebrate and invertebrate species, including humans. The evolution history of amphibians presents a classic example of heterochrony in nature. Within the amphibian class the process of metamorphosis is often interrupted which gives rise to a distinct evolutionary process called paedomorphosis (Haken 1989). Organisms in this clade exhibit metamorphic and paedomorphic life-history strategies that help them survive in detrimental environments. Although there are centuries worth of research trying to elucidate the principles involved in amphibian evolution and incorporation of paedomorphosis in their development, a clear and concise theory is yet to be reached. However, is has become clear that the determinants for this phenomenon are multifactorial and include factors such as climate variation, resource partitioning, sex-specific fitness and fecundity and age at maturation. No one single factor is responsible for paedomorphosis, instead they form part of an additive effect which results in the prevalence and incidence of this polymorphism (Denoël et al. 2005). Metamorphosis is defined as the post-embryotic transformation from a larva into a juvenile. This process involves an extensive transformation of the organism, often a radical change in the body plan organization. In most cases, metamorphosis arises from climate and habitat changes. In order to produce larval organisms that are able to survive in these changing conditions paedomorphosis takes place. Paedomorphosis is an evolutionary process in which metamorphosis is interrupted and ancestral larval and juvenile traits are exhibit or retained at adult stage. Scientists consider paedomorphosis an alternative ontogenetic pathway that results in increased fitness for survival in unfavorable habitats. This process has played an essential role in the evolutionary history of amphibians, where disruption of metamorphosis occurred in nine of ten families in the salamander and newts species (Laudet 2011). Large morphological variation within the amphibian clade results from two processes: paedomorphosis (underdevelopment) and peramorphosis (overdevelopment) which achieve variation without large genetic changes. There are two processes involved in paedomorphosis, neoteny (deceleration) and progenesis (hypomorphosis). Both of these processes entail retardation in development, however they differ in that during neoteny reproduction occurs at the same age as it does in metamorphosed organisms, while during progenesis reproduction occurs at an earlier age. In newts and salamanders, neoteny is usually the causative agent for Paedomorphosis (Ryan and Semlitsch 1998). Paedomorphs can either be facultative or obligated, differing on whether or not they have completely lost the availability to undergo metamorphosis. When both paedomorphic and metamorphic phenotypes are observed within a single population the individuals are believed to be facultative paedomorphs. In this case, paedomorphosis is reversible and results from phenotypic plasticity and its only exhibit by a portion of the entire population. Conversely, if the paedomorphic phenotype is fixed within a single population this would be an example of obligated...
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