Most mutations are thought to be neutral or slightly deleterious.  The rate at which deleterious mutations occur and the distribution of their effects are important parameters in evolutionary genetic theory, and several studies have quantified the genomic rate of deleterious mutation by through laboratory assays of fitness in experimental lines where spontaneous mutations have been allowed to accumulate over many generations.  Although this information allows us to parameterize genetic models, it offers little insight in how spontaneous mutations may affect fitness that is dependent on ecological interactions.  
Results/Conclusions

Mutations were accumulated in 30 experimental lines of Daphnia pulex via single offspring descent for an average of 45 generations, at which point they were assayed for morphological characteristics that influence the vulnerability of Daphnia to predation and compared to 15 controls in which mutations were not allowed to accumulated.  The traits assayed in juvenile growth rate experiments included body length, eye size and tailspine length in neonates and 4-day old primiparous individuals.  Overall bodysize was reduced by ~14% in MA linces compared to controls, as was tailspine length by ~32%. Both of these changes increase the vulnerability of Daphnia to predators.  However, this was apparently not driven by a general, allometrically consistent decrease in size of all body parts.  Eye size increased slightly (<3%) in lines that accumulated mutations, meaning that relative eye size increased dramatically.  Because Daphnia eyes are dark in an otherwise transparent organism, increased eye size increases the vulnerability of Daphnia to size-selective visual predators.  This suggests that spontaneous deleterious mutations can act independently on different aspects of morphology.