Daphnia Paleogenetics and Environmental Change: Deconstructing the Evolution of Plasticity

It is only recently that ecologists, including limnologists, have begun to appreciate the potential for organisms to evolve rapidly in response to environmental change. The resulting changes in the mean characteristics of the evolving populations have the potential to alter population, community and ecosystem responses to environmental changes - while those changes are still taking place. Viable diapausing eggs of zooplankton buried annually in lake sediments represent an historical record of the genotypes present at a series of time points in the past, typically dating back several decades. Because these eggs can be extracted from sediment layers of known age and hatched, the phenotypes of past populations can be reconstructed and evolutionary responses to recent environmental change revealed. This is especially valuable for investigating the nature of evolution of phenotypically plastic traits for which trait expression in different environmental conditions must be measured using living animals. When animals, hatched from sediments deposited at different times during a period of rapid change in the lake environment, are reared under a range of laboratory conditions, the nature of plasticity evolution can be discovered. Here, we use the results of two previously published studies of Daphnia evolutionary response to anthropogenic environmental change to deconstruct the patterns and rates of genetic changes in phenotypic plasticity. We find for the populations studied that (1) in three of four instances, plasticity evolved primarily by changes in the trait expressed in the environmental condition for which selection acted most strongly, rather than purely as a change in slope, and (2) the rates of evolution of plasticity were comparable to those for constitutive traits reported in the literature suggesting that plasticity does not necessarily retard the rate of evolution. Because Daphnia is a critical species in many limnetic ecosystems, and because in both of our study systems the characteristics of the Daphnia populations that evolved are known to influence community and ecosystem processes, this rapid adaptive evolution may be a critical component of population, community and ecosystem response to the accelerating pace of environmental change.