Abstract
The cladoceran Daphnia is an ideal organism for the comparative study of life history strategies due to its extensive geographic range and the great deal known of its biology. Biotic selection pressures on components of life history strategies include predation, food and competition. Abiotic pressures include temperature, photoperiod and environmental chemistry. Stochastic life history theory, based on the differential survivorship of juveniles and adults, is also an important component in the evolution of Daphnia life histories. As an example of the interaction of these pressures, 2 Daphnia spp. and their life histories are compared. D. pulex has a very broad geographic range extending through most temperate areas of North America, and has a correspondingly broad array of life history characteristics but may be characterized as maturing rapidly and reproducing with many large broods of small neonates. D. middendorffiana, an arctic species, requires more time to mature and produces many fewer broods of large neonates.