Phenotypic selection in an artificial population of Impatiens pallida: the importance of the invisible fraction

Multiple-regression techniques for measuring phenotypic selection have been used in a large number of recent field studies. One benefit of this technique is its ability to discern the direct action of selection on traits by removing effects of correlated traits. However, covariation among traits expressed at different stages in an organism's life history is often poorly estimated because individuals that die before reaching adulthood cannot be measured as adults. Accurate estimates of trait covariances are necessary for the correct interpretation of the direct action of selection on a trait. If phenotypic characters expressed at different life-history stages are of interest, and mortality occurs between stages, the components of the selection model will be biased by not including those individuals that died (the "invisible fraction"). A laboratory experiment was designed using the common understory annual jewelweed (Impatiens pallida) to investigate the importance of this "invisible fraction" in the quantification of the strength of natural selection. Nine seedlings were planted into each of 40 plastic tubs. After 1 mo, half of these tubs were thinned by removing the four smallest plants. In the other half, the four largest plants were removed. Conditional directional selection gradients were significantly different for several traits between these two experimental populations, and differences in phenotypic variance-covariance matrices appear to have affected these estimates. These results suggest that methods such as those proposed by Lynch and Arnold should be applied to selection analyses in which within-generation mortality occurs and estimates of lifetime selection are desired.