Meeting Abstract
Patterns of phenotypic evolution can abruptly shift as species move between adaptive zones. Salamanders have frequently transitioned across three distinct life cycle strategies that range from aquatic-to-terrestrial (biphasic), to fully aquatic (paedomorphosis), to fully terrestrial (direct development). These transitions are often associated with changes in body form such as loss of digits, limb reduction, or body elongation. However, the relationships among these traits remain unresolved. Here we use an evolutionary modeling approach to test whether life cycle transitions in salamanders have influenced rates, optima and integration of primary locomotory structures (limbs and trunk). We show that paedomorphic salamanders have elevated rates of limb evolution with optima shifted toward smaller size and fewer digits compared to all other salamanders. Rate of hindlimb digit evolution is shown to decrease in a gradient as life cycles become more terrestrial. Paedomorphic salamanders exhibit reduced correlations between limb lengths as well as between forelimb digit loss and vertebral number. Paedomorphs have a higher correlation between hindlimb digit loss and increases in vertebral number. Our results generally demonstrate constraint on limb evolution due to terrestrial locomotion that, when lifted, leads to higher rates of trait evolution and shifts in optima and integration. The basic tetrapod body form of most salamanders and the independent losses of terrestriality life stages provide an important framework for understanding the evolutionary and developmental relationships between the limb skeleton and vertebral column.
