Variable selection on Eurosta's gall size, III: Can an evolutionary response to selection be detected?

Abstract

AbstractStandard quantitative genetic theory predicts that when a trait is exposed to selection, the between‐generation change in the phenotypic mean, Δz̄i, will be equal to the product of the trait's heritability and the selection differential, h2S. By extension, this theory implies that if a number of replicate populations are exposed to varying intensities of selection, the between‐generation changes in means should covary with the selection differential applied. This relationship offers an opportunity for a statistical test to detect evolutionary change when selection is measured in replicate populations. If an evolutionary response to phenotypic selection occurs, the regression of over Si, where i indicates population, will have a positive slope. This statistical test was applied to data on the insect Eurosta solidaginis (Diptera: Tephritidae). The larvae of this fly induce galls on the stems of the host plant, Solidago altissima (Asteraceae). Previous work has shown that gall size is a heritable trait of the insect. Further, size‐dependent attack on Eurosta larvae by parasitoids selects for larger gall size (Weis and Abrahamson, 1986). Long‐term data on phenotypic selection in 16 populations across 5 generations were analyzed for selection response. Apparent upward evolutionary responses were seen in 2 of the 4 between‐generation transitions. However, no response was seen when the analysis was applied to the cumulative change in gall size. Examination of the data suggested that some of the change in mean gall size was a developmental response to spatial and temporal variation in the environment. Non‐linear developmental effects of environment, when combined with non‐linear fitness functions, can induce a spurious selection response; these non‐linear relationships can account for the apparent evolutionary change gall size found in the by‐generation analysis. Thus, there is no reliable evidence for evolutionary change in Eurosta's gall size over the generations studied. Stasis of gall size in the face of ongoing selection may be due to counterbalancing selection on the gallmaker imposed by host plant resistance.