Thesis #34766

The transition from aquatic to terrestrial reproduction in early tetrapods is viewed as a major adaptive change in the evolutionary history of vertebrates and critical for the colonization of land. Many extant amphibian species show partly or fully terrestrial modes of reproduction. This evolutionary shift from plesiomorphic, aquatic breeding to derived, terrestrial breeding has occurred multiple times independently. Because of this, there has been a historic interest in using extant amphibians as a model system for comparative studies on ‘terrestrialization’ of vertebrates, especially of early amniotes. Previous studies have highlighted a correlation between the spatial occurrence of terrestrial breeding amphibians with specific climatic and environmental conditions such as low latitudes and high levels of humidity and precipitation. Furthermore, it has been shown that highly derived terrestrial breeding strategies such as direct development (without an aquatic larval stage all together) does not always require ‘semi-terrestrial’ evolutionary intermediates.
African amphibians are often excluded from large-scale, comparative studies despite the high occurrence of terrestrially breeding species on this continent, including two out of three known genera of viviparous Anura. This thesis is dedicated to further our understanding of the evolution of terrestrial life history strategies in amphibians as a response to specific habitats and environmental pressures and the evolutionary history of derived terrestrial reproductive modes such as viviparity, focusing specifically on African systems. To achieve this, two strategies were employed. The first was to focus on a specific region, the Eastern Arc Mountains and adjacent lowlands of East Africa, and to investigate the distribution of terrestrial and aquatic breeding amphibians in relation to habitat types. The second strategy was to focus on a specific taxonomic group, the Bufonidae, and to investigate in more detail how specific life history traits are phylogenetically and spatially distributed. In particular how these traits have changed over time, whether lineages with different reproductive modes have diversified at different rates and to what extent the environment may have played a role in the evolution of viviparity.
Comparative phylogenetic analyses reveal that terrestrial breeding evolved frequently in forested and/or in topographically complex habitats, but also that these habitats are not exclusive to terrestrial breeders. Steep gradients appear to have a stronger effect than forest, but forest is nonetheless important. Viviparity, has evolved twice independently from different evolutionary precursors in African Bufonidae. Furthermore, this thesis shows that diversification rates have remained constant across lineages of Bufonidae with different reproductive modes and terrestrial modes of reproduction do not appear to have increased diversification rates compared to the plesiomorphic aquatic breeding strategy, though potentially it allowed the penetration into new habitats. The constant rate of diversification, without signs of temporal or clade specific bursts lends an explanations to why at least in bufonids, species richness is lower in Africa than in other tropical regions.