Paedomorphosis in Amphibians
Evolution of Facultative Paedomorphosis in Newts and Salamanders (Caudata)
INTRODUCTION
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
INTRODUCTION
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
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