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If phenotypic plasticity is a trait subject to selection and evolution, we need to quantify reaction norms and to identify the potential target(s) of selection. Previously we proposed the use of polynomial coefficients as reaction norm descriptors. However, the interpretation of these coefficients is not straightforward if the reaction norm is not linear. Therefore, we instead propose the use of characteristic values to describe reaction norms. We depict such characteristic values for linear, quadratic, and logistic reaction norms, discuss how they might evolve, and illustrate their estimation using data on abdominal pigmentation of Drosophila simulans., (© 1998 The Society for the Study of Evolution.)

In a heterogeneous world, the optimal strategy for an individual is to continually change its phenotype to match the optimal type. However, in the real world, organisms do not behave in this fashion. One potential reason why is that phenotypic plasticity is costly. We measured production and maintenance costs of plasticity in the freshwater crustacean Daphnia pulex (Cladocera: Crustacea) in response to the presence of chemical signals from a predator, the insect Chaoborus americanus. We looked at three changes in juvenile body size and shape: body length, body depth, and tailspine length. Fitness costs were measured as changes in adult growth and fecundity, and summarized as the intrinsic rate of increase (r) for individuals reared in the presence or absence of Chaoborus extract. The cost of plasticity was measured as a multiple regression of mean clone fitness against trait and trait plasticity. We found scant evidence for either production or maintenance costs of plasticity. We also failed to find direct costs of these juvenile structures, which is surprising, as others have found such costs. We attribute the lack of measurable direct or plasticity costs to a decrease in metabolic rates in the presence of the Chaoborus extract. This decrease in metabolic rate may have compensated for any cost increases. We call for more extensive measures of the costs of plasticity, especially under natural conditions, and the incorporation of costs into evolutionary models., (© 1998 The Society for the Study of Evolution.)