The evolution of reproductive modes and life cycles in amphibians

Amphibians have undergone important evolutionary transitions in reproductive modes and life-cycles. We compare large-scale macroevolutionary patterns in these transitions across the three major amphibian clades: frogs, salamanders, and caecilians. We analyse matching reproductive and phylogenetic data for 4025 species. We find that having aquatic larvae is ancestral for all three groups and is retained by many extant species (33–44%). The most frequent transitions in each group are to relatively uncommon states: live-bearing in caecilians, paedomorphosis in salamanders, and semi-terrestriality in frogs. All three groups show transitions to more terrestrial reproductive modes, but only in caecilians have these evolved sequentially from most-to-least aquatic. Diversification rates are largely independent of reproductive modes. However, in salamanders direct development accelerates diversification whereas paedomorphosis decreases it. Overall, we find a widespread retention of ancestral modes, decoupling of trait transition rates from patterns of species richness, and the general independence of reproductive modes and diversification.