Non-adaptive Plasticity Research Articles

Temperature, Growth Rate, and Body Size in Ectotherms: Fitting Pieces of a Life-History Puzzle

The majority of ectotherms grow slower but mature at a larger body size in colder environments. This phenomenon has puzzled biologists because classic theories of life-history evolution predict smaller sizes at maturity in environments that retard growth. During the last decade, intensive theoretical and empirical research has generated some plausible explanations based on nonadaptive or adaptive plasticity. Nonadaptive plasticity of body size is hypothesized to result from thermal constraints on cellular growth that cause smaller cells at higher temperatures, but the generality of this theory is poorly supported. Adaptive plasticity is hypothesized to result from greater benefits or lesser costs of delayed maturation in colder environments. These theories seem to apply well to some species but not others. Thus, no single theory has been able to explain the generality of temperature-size relationships in ectotherms. We recommend a multivariate theory that focuses on the coevolution of thermal reaction norms for growth rate and size at maturity. Such a theory should incorporate functional constraints on thermal reaction norms, as well as the natural covariation between temperature and other environmental variables.

PLASTICITY AND CANALIZATION OF INSECT REPRODUCTION: TESTING ALTERNATIVE MODELS OF LIFE HISTORY TRANSITIONS

Life histories may show phases of both plasticity and canalization in response to feeding rate. Models for life history canalization and plasticity postulate a threshold for initiation of canalized developmental events. Some models postulate adaptive plasticity, whereas others postulate nonadaptive plasticity that results from environmental modulation of fixed development. These models have been tested by changing feeding rate at various times and determining when timing of life history events becomes unresponsive to those changes. This approach has been criticized because putative thresholds are usually not known. We use an alternative experimental design to test models of reproductive plasticity and canalization, and to estimate thresholds, in the grasshopper Romalea microptera. We develop mathematical models for published verbal models that predict how life history timing changes with feeding rate. Alternative models predict distinct relationships of time to oviposition vs. mean food intake that we test via experimental manipulation of food intake and nonlinear regressions. Regressions yield estimates of both the threshold and the duration of post-threshold development. A model postulating a fixed threshold and canalized post-threshold development provides the best, most parsimonious fit to data for this grasshopper. Thus, the simplest model, postulating no adaptive variation in development, is supported, a result that is consistent with previous experiments on this system using changing feeding rates. We use the estimate of the threshold (in units of food eaten) and measurements of hemolymph protein content to estimate the threshold in units of physiologically relevant storage. These results elucidate the structure of reproductive plasticity in this system and how this alternative experimental approach can provide testable predictions of developmental thresholds for further experiments on life history plasticity and canalization.

Towards a more synthetic view of evolution

The 21st century will be the century of biology, and evolutionary biology will be the linchpin. Yes, this is a bold statement, but it is certainly in keeping with Dobzhansky's famous dictum about evolution. Dobzhansky was one of the major architects of the Modern Synthesis. This book by Schlichting and Pigliucci (SP the Final Synthesis is just gathering steam. Note that these dates are just rough guides and not meant to imply that significant work did not occur outside those periods. What is critical is that within these periods the relevant disciplines all become focussed on the same research program. We are now in the midst of this final joining of biological disciplines. This joining, if history is a guide, will take the next couple of decades. This joining will be driven from the evolutionary side. It is the evolutionary side within which the disciplines are linked by a synthetic theory. The molecular side is primarily linked by a shared set of techniques and a viewpoint that centers all questions ultimately on DNA sequences. Because the evolution side is much more theory driven, that is where synthetic concepts tend to arise. This book by S&P is an excellent example of such theorydriven synthesis. Also, if truth be told, as a whole evolutionary biologists tend to have a larger perspective than those in many other biological disciplines. Looking back on the Modern Synthesis is useful for the task ahead (Mayr and Provine, 1980). In particular, if we learn anything from that episode in our history, we