Premium
The Genetic Component in Plant Size Hierarchies: Norms of Reaction to Density in a Polygonum Species
Author(s) -
Thomas S. C.,
Bazzaz F. A.
Publication year - 1993
Publication title -
ecological monographs
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.254
H-Index - 156
eISSN - 1557-7015
pISSN - 0012-9615
DOI - 10.2307/2937099
Subject(s) - biology , herbaceous plant , ecology , population , competition (biology) , genetic variation , reproduction , perennial plant , agronomy , botany , demography , genetics , gene , sociology
An important motivation for the study of variability in size and reproductive output in plant populations is its potential relation to natural selection. However, very few data are available to assess the genetic component of fitness—related traits in competing plant populations, or the differential performance of plant genotypes at different densities. To address these issues we conducted an experiment using 25 genotypes of a colonizing herbaceous annual, Polygonum pensylvanicum. These genotypes were randomly sampled from a natural population and cloned by axillary meristem enhancement. Cloned plants were grown in a glasshouse at three densities spanning the range encountered in the natural population (from individually grown to 850 individuals/m 2 ). The growth and fate of a total of 1400 individuals were followed over the course of a 10—wk growing period. Variability in size and reproductive output (as measured by the coefficient of variation of vegetative and reproductive dry mass) increased with density. Early plant size measures were positively correlated with subsequent relative growth rates in dense populations, but not among individually grown plants. These observations indicate the likely importance of asymmetric or "one—sided" competition in the dense populations. The proportion of variance in final size and reproduction explained by genotype was generally higher for individually grown plants than for plants grown under crowded conditions. We suggest that this may result from asymmetric competitive interactions working to amplify early size differences resulting primarily from environmental and developmental "noise." The same genotypes were not superior across all densities. Qualitative ("cross—over") interactions for fitness—related characters were observed in comparing genotype performance between the individually grown vs. the low and high density treatments. Genotypes with an early size advantage were predictably favored in dense populations, but the genetic correlation between early and final performance was weaker among individually grown plants. 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 opposing forces. Applying our results to some elementary quantitative genetic models suggests that the potential for natural selection would increase with population density, while the potential for genetic drift would decrease. Such patterns may be of particular evolutionary importance in colonizing annuals, whose life histories imply an alternate exposure of genotypes to high and low densities.