JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Plants can employ two general strategies to defend themselves against herbivory: they can either reduce the amount of damage they experience (resistance), or they can tolerate herbivore damage. Theoretical considerations suggest that, in many cases, tolerance and resistance are redundant strategies, and may therefore be mutually exclusive adaptations. In this investigation of natural populations of the annual plant Arabidopsis thaliana we examine whether the pattern of selection acting on resistance and tolerance favors the evolution of one defense strategy, or the other, but not both. We found that the joint pattern of selection acting on tolerance and two resistance traits, trichome density and total glucosinolate concentration, indicated that there were not alternate peaks in the fitness landscape favoring either resistance or tolerance. Rather, selection favored the retention of both tolerance and resistance. One reason for the absence of mutually exclusive alternative resistance/tolerance strategies is the absence of a negative genetic correlation between resistance and tolerance. An unexpected result is the detection of disruptive selection acting on tolerance, which seems to result from a nonlinear relationship between tolerance and its costs.

Measuring natural selection has been a fundamental goal of evolutionary biology for more than a century, and techniques developed in the last 20 yr have provided relatively simple means for biologists to do so. Many of these techniques, however, share a common limitation: when applied to phenotypic data, environmentally induced covariances between traits and fitness can lead to biased estimates of selection and misleading predictions about evolutionary change. Utilizing estimates of breeding values instead of phenotypic data with these methods can eliminate environmentally induced bias, although this approach is more difficult to implement. Despite this potential limitation to phenotypic methods and the availability of a potential solution, little empirical evidence exists on the extent of environmentally induced bias in phenotypic estimates of selection. In this article, we present a method for detecting bias in phenotypic estimates of selection and demonstrate its use with three independent data sets. Nearly 25% of the phenotypic selection gradients estimated from our data are biased by environmental covariances. We find that bias caused by environmental covariances appears mainly to affect quantitative estimates of the strength of selection based on phenotypic data and that the magnitude of these biases is large. As our estimates of selection are based on data from spatially replicated field experiments, we suggest that our findings on the prevalence of bias caused by environmental covariances are likely to be conservative.

Plants can employ two general strategies to defend themselves against herbivory: they can either reduce the amount of damage they experience (resistance), or they can tolerate herbivore damage. Theoretical considerations suggest that, in many cases, tolerance and resistance are redundant strategies, and may therefore be mutually exclusive adaptations. In this investigation of natural populations of the annual plant Arabidopsis thaliana we examine whether the pattern of selection acting on resistance and tolerance favors the evolution of one defense strategy, or the other, but not both. We found that the joint pattern of selection acting on tolerance and two resistance traits, trichome density and total glucosinolate concentration, indicated that there were not alternate peaks in the fitness landscape favoring either resistance or tolerance. Rather, selection favored the retention of both tolerance and resistance. One reason for the absence of mutually exclusive alternative resistance/tolerance strategies is the absence of a negative genetic correlation between resistance and tolerance. An unexpected result is the detection of disruptive selection acting on tolerance, which seems to result from a nonlinear relationship between tolerance and its costs.

SUMMARYThe alternating least squares PARAFAC algorithm is a useful tool for resolving trilinear three-way data arrays. Occasionally, however, it becomes bogged down for many iterations in the vicinity of a poor quality resolution before moving on to a much superior optimum fit. We investigate this behavior in a simulation study and suggest ways of overcoming the obstacles it presents.