We present a quantitative method for describing how heterochronic changes in ontogeny relate to phyletic trends. This is a step towards creating a unified view of developmental biology and evolutionary ecology in the study of morphological evolution. Using this representation, we obtain a greatly simplified and logical scheme of classification. We believe that this scheme will be particularly useful in studying the data of paleontology and comparative morphology and in the analysis of processes leading to adaptive radiation. We illustrate this scheme by examples drawn from the literature and our own work.

Many scientists argue that we are either entering or in the midst of the sixth great mass extinction. Intense human pressure, both direct and indirect, is having profound effects on natural environments. The amphibians-frogs, salamanders, and caecilians-may be the only major group currently at risk globally. A detailed worldwide assessment and subsequent updates show that onethird or more of the 6,300 species are threatened with extinction. This trend is likely to accelerate because most amphibians occur in the tropics and have small geographic ranges that make them susceptible to extinction. The increasing pressure from habitat destruction and climate change is likely to have major impacts on narrowly adapted and distributed species. We show that salamanders on tropical mountains are particularly at risk. A new and significant threat to amphibians is a virulent, emerging infectious disease, chytridiomycosis, which appears to be globally distributed, and its effects may be exacerbated by global warming. This disease, which is caused by a fungal pathogen and implicated in serious declines and extinctions of >200 species of amphibians, poses the greatest threat to biodiversity of any known disease. Our data for frogs in the Sierra Nevada of California show that the fungus is having a devastating impact on native species, already weakened by the effects of pollution and introduced predators. A general message from amphibians is that we may have little time to stave off a potential mass extinction.chytridiomycosis ͉ climate change ͉ population declines ͉ Batrachochytrium dendrobatidis ͉ emerging disease

Extinctions are normal biological phenomena Both mass extinctions in geological time and local extinctions in ecological time are well documented, but rates of extinction have increased in recent years--especially in vet. tebrateg including amphibians--as illustrated by recent re. ports of their population declines and range reductions We suggest that long-term population data are necessary for rigorously evaluating the significance of the amphibian decliruet Due to the physiological constraint~ relatively low mobility, and site fidelity of amphibian& we suggest that many amphibian populations may be unable to recolonize areas after local extinction