THE GENETIC COMPONENT IN PLANT SIZE HIERARCHIES - NORMS OF REACTION TO DENSITY IN A POLYGONUM SPECIES

An important motivation for the study of variability in size and repro ductive output in plant populations is its potential relation to natur al selection. However, very few data are available to assess the genet ic component of fitness-related traits in competing plant populations, or the differential performance of plant genotypes at different densi ties. To address these issues we conducted an experiment using 25 geno types of a colonizing herbaceous annual, Polygonum pensylvanicum. Thes e genotypes were randomly sampled from a natural population and cloned by axillary meristem enhancement. Cloned plants were grown in a glass house at three densities spanning the range encountered in the natural population (from individually grown to 850 individuals/ml). The growt h and fate of a total of 1400 individuals were followed over the cours e of a 10-wk growing period. Variability in size and reproductive outp ut (as measured by the coefficient of variation of vegetative and repr oductive dry mass) increased with density. Early plant size measures w ere positively correlated with subsequent relative growth rates in den se populations, but not among individually grown plants. These observa tions indicate the likely importance of asymmetric or ''one-sided'' co mpetition in the dense populations. The proportion of variance in fina l size and reproduction explained by genotype was generally higher for individually grown plants than for plants grown under crowded conditi ons. We suggest that this may result from asymmetric competitive inter actions working to amplify early size differences resulting primarily from environmental and developmental ''noise.'' The same genotypes wer e not superior across all densities. Qualitative (''cross-over'') inte ractions for fitness-related characters were observed in comparing gen otype performance between the individually grown vs. the low and high density treatments. Genotypes with an early size advantage were predic tably favored in dense populations, but the genetic correlation betwee n early and final performance was weaker among individually grown plan ts. In sum, density increased relative variation in fitness correlates such as reproductive biomass, but decreased the heritability of these traits. The response of selection is the product of these two opposin g forces. Applying our results to some elementary quantitative genetic models suggests that the potential for natural selection would increa se with population density, while the potential for genetic drift woul d decrease. Such patterns may be of particular evolutionary importance in colonizing annuals, whose life histories imply an alternate exposu re of genotypes to high and low densities.