All animals and plants respond to changes in the environment during their life cycle. This flexibility is known as phenotypic plasticity and allows organisms to cope with variable environments. A common source of environmental variation is predation risk, which describes the likelihood of being attacked and killed by a predator. Some species can respond to the level of predation risk by producing morphological defences against predation. A classic example is the production of pedestals and head spikes in the water flea, Daphnia pulex, which defend against predation from Chaoborus midge larvae. Previous studies of these defences have focussed on changes in pedestal size and the number of spikes along a gradient of predation risk. Although these studies have provided a model for continuous plasticity, they do not capture the whole-organism shape response to predation risk. In contrast, studies in fish and amphibians focus on shape as a complex, multi-faceted trait made up of different variables. In this study, we analyse how multiple aspects of shape change in D. pulex along a gradient of predation risk from C. flavicans. These changes are dominated by the inducible morphological defence, but there are also changes in the size and shape of the head and the body. We detected change in specific modules of the body plan and a level of integration among modules. These results are indicative of a complex, multi-faceted response to predation and provide insight into how predation risk drives variation in shape and size at the level of the whole organism.

Populations experiencing varying levels of ionising radiation provide an excellent opportunity to study the fundamental drivers of evolution. Radiation can cause mutations, and thus supply genetic variation; it can also select against individuals that are unable to cope with the physiological stresses associated with radiation exposure. Since the nuclear power plant explosion in 1986, the Chernobyl area has experienced a spatially heterogeneous exposure to varying levels of ionising radiation. We sampled Daphnia pulex (a freshwater crustacean) from lakes across the Chernobyl area, genotyped them at ten microsatellite loci, and also calculated the current radiation dose rates. We then investigated whether the pattern of genetic diversity was shaped primarily by radiation-mediated supply of variation consistent with increased supply of de novo mutations, or by radiation-mediated selection and loss of variation at higher dose rates. We found that measures of genetic diversity, including expected heterozygosity and mean allelic richness (an unbiased indicator of diversity) were significantly higher in lakes that experienced higher radiation dose rates; this is consistent with mutation outweighing selection as the key evolutionary force in populations experiencing high radiation dose rates. We also found significant but weak population structure, and clear evidence for isolation by distance between populations. This evidence suggests that gene flow between nearby populations is eroding population structure, and that mutational input in high radiation lakes could, ultimately, supply genetic variation to lower radiation sites.