SHORT-TERM EVOLUTION IN THE SIZE AND SHAPE OF PEA APHIDS

Phenotypic evolution in contemporary populations can generally be witn essed only when novel selective forces produce rapid evolution. Exampl es of conditions that have led to rapid evolution include drastic envi ronmental change, invasion of a new predator, or a host-range expansio n. In cyclical parthenogens, however, yearly cycles of phenotypic evol ution may occur due to the loss of adaptation during recombination in the sexual phase (genetic slippage), permitting an opportunity to obse rve adaptive evolutionary change in contemporary populations that are not necessarily subject to new patterns of natural selection. In insec t herbivores, comparative studies suggest that morphological features that aid individuals in remaining on the plant or exploiting it as a f ood source are likely targets for selection. Here, we estimated the ge netic variability of morphological traits in a cyclical parthenogen, t he pea aphid (Acyrthosiphon pisum), to determine the potential for the ir evolution and we tested the hypothesis that size and/or shape evolv es by clonal selection during one season of parthenogenetic reproducti on. Genetic variation in a set of morphological traits was estimated u sing laboratory-reared descendents of clones collected from a single a lfalfa field in May 1988 and April 1989 (henceforth, the ''early'' col lections). In both years, there was significant clonal heritability ea rly in the season both for overall morphology and for several individu al aspects of size and shape. Because the course of short-term evoluti onary change in the multivariate phenotype is a function of patterns o f genetic covariance among characters, genetic correlations between si ze and 12 shape variables were also estimated for these early collecti ons. A comparison between the mean phenotype of each early collection and that of a corresponding ''late'' collection made from the same fie ld seven to eight clonal generations later in the same years revealed qualitatively similar changes in the average multivariate morphologica l phenotypes between the time periods in both years, although the diff erence was only significant for the 1989 samples. The pattern of genet ic correlations that we estimated early in the 1989 season between ove rall size and various shape variables suggests that the observed short -term evolutionary changes in shape could have been due to natural sel ection acting only to increase overall size. We tested this hypothesis by estimating selection on size using a separate data set in which bo th demographic and morphological variables were measured on individual s reared under field conditions. Highly significant regressions of ind ividual relative fitness on size were found for two major fitness comp onents. Thus, it is likely that the evolutionary change in morphology that we observed is attributable to natural selection, possibly acting primarily through body size. A shift back to smaller size between the late 1988 and early 1989 collections from the same field suggests tha t either a cost of recombination or opposing selective forces during o verwintering may produce persistent yearly cycles of morphological evo lution in this cyclically parthenogenetic species.