Phylogenetic patterns of trait and trait plasticity evolution: Insights from amphibian embryos

Abstract

Environmental variation favors the evolution of phenotypic plasticity. For many species, we understand the costs and benefits of different phenotypes, but we lack a broad understanding of how plastic traits evolve across large clades. Using identical experiments conducted across North America, we examined prey responses to predator cues. We quantified five life-history traits and the magnitude of their plasticity for 23 amphibian species/populations (spanning three families and five genera) when exposed to no cues, crushed-egg cues, and predatory crayfish cues. Embryonic responses varied considerably among species and phylogenetic signal was common among the traits, whereas phylogenetic signal was rare for trait plasticities. Among trait-evolution models, the Ornstein-Uhlenbeck (OU) model provided the best fit or was essentially tied with Brownian motion. Using the best fitting model, evolutionary rates for plasticities were higher than traits for three life-history traits and lower for two. These data suggest that the evolution of life-history traits in amphibian embryos is more constrained by a species' position in the phylogeny than is the evolution of life history plasticities. The fact that an OU model of trait evolution was often a good fit to patterns of trait variation may indicate adaptive optima for traits and their plasticities.

Keywords: Anaxyrus; Hyla; Lithobates; Pseudacris; Rana; phylogenetic inertia.

Figure 1.

Time to hatching, mass at hatching, and Gosner stage at hatching for 20 species of amphibian embryos when exposed to cues from either no predator, crushed conspecific eggs, or crayfish consuming conspecific eggs. To test for population effects, three species were selected from two locations. Abbreviations refer to the states in which the animals were collected. Asterisks indicate species in which crushed-egg cues differed from the control; crosses indicate species in which crayfish cues differed from the control (p < 0.05). Data are means ± 1 SE.

Figure 2.

Growth rate (i.e., mass ÷ time to hatch) and developmental rate (i.e., Gosner stage ÷ time to hatch) for 20 species of amphibian embryos when exposed to cues from either no predator, crushed conspecific eggs, or crayfish consuming conspecific eggs. To test for population effects, three species were selected from two locations. Abbreviations refer to the states in which the animals were collected. Asterisks indicate species in which crushed-egg cues differed from the control; crosses indicate species in which crayfish cues differed from the control (p < 0.05). Data are means ± 1 SE.