An experimental investigation of how intraspecific competition and phenotypic plasticity can promote the evolution of novel, complex phenotypes

Intraspecific competition has long been considered a key driver of evolutionary diversification, but whether it can also promote evolutionary innovation is less clear. Here we examined the interplay between competition and phenotypic plasticity in fuelling the origins of a novel, complex phenotype – a distinctive carnivore morph found in spadefoot toad tadpoles (genus Spea) that specializes on fairy shrimp. We specifically sought to explore the possible origins of this phenotype by providing shrimp to Scaphiopus holbrookii tadpoles (the sister genus to Spea that does not produce carnivores) while subjecting them to competition for their standard diet of detritus. Previous research had shown that this species will eat shrimp when detritus is limited, and that these shrimp-fed individuals produce features that are redolent of a rudimentary Spea carnivore. In this study, we found that: (1) behavioural and morphological plasticity enabled some individuals to expand their diet to include shrimp; (2) there was heritable variation in this plasticity; and (3) individuals received a growth and development benefit by eating shrimp. Thus, novel resource use can arise via plasticity as an adaptive response to intraspecific competition. More generally, our results show how competition and plasticity may interact to pave the way for the evolution of complex, novel phenotypes, such as the distinctive carnivore morph in present-day Spea.

Ontogenetic and interspecific variation in timing of movement and responses to climatic factors during migrations by pond-breeding amphibians

Pond-breeding amphibians from temperate regions undertake overland migrations to reproduce in aquatic habitats. In turn, their offspring metamorphose and emigrate to upland, terrestrial habitats. We examined the diel patterns and daily variability of migrations of adult and juvenile amphibians in response to climatic cues. Of the eight species ( Ambystoma talpoideum (Holbrook, 1838), Ambystoma tigrinum (Green, 1825), Bufo terrestris (Bonnaterre, 1789), Hyla gratiosa LeConte, 1856, Pseudacris crucifer (Wied-Neuwid, 1838), Pseudacris ornata (Holbrook, 1836), Rana sphenocephala Cope, 1886, and Scaphiopus holbrookii (Harlan, 1935)) that we observed, all migrated almost exclusively at night except for the recently metamorphosed B. terrestris, which frequently migrated diurnally (>50% of captures). Additionally, we correlated daily captures of adult and juvenile A. talpoideum, A. tigrinum, B. terrestris, and R. sphenocephala to maximum and minimum daily temperatures, number of previous days without rain, total rainfall during the previous 24 h, and interactions of these variables. Rain was often the most important predictor of amphibian movements. However, species differed in their response to climatic factors, with some species and age classes being more dependent on rain for migrations than others. Rapid changes in regional weather patterns may affect species’ migrations differently, possibly altering arrival times of reproductive adults or affecting the likelihood of successful migrations.