Spatial Scale of Variation in Fitness of Erigeron annuus

Abstract

Environmental heterogeneity can maintain genetic variation when it disrupts the genetic correlation of fitness across environments such that no single genotype is superior in all sites. Whether spatial variation in selection leads to a stable polymorphism depends in part on the scale of environmental heterogeneity relative to dispersal and density dependence. I measured the lifetime survival and reproduction of six genotypes of Erigeron annuus (Asteraceae) along two 30-m transects in an early-successional field. Significant genotype-environment interactions for fitness occurred at scales of 10-20 cm and 1-3 m, which shows that selection was significantly heterogeneous at those scales. The spatial pattern of fitness variation was shown by the decline of the cross-environment genetic correlation of fitness as a function of distance. Correlations of fitness among plots separated by 10 cm and 75 cm were significantly larger than expected for randomly chosen plots. Beyond 1 m there was little similarity in the fitnesses of genotypes, and by 3 m the cross-environment correlation of fitness was marginally less than expected (P = .07). Overall, the genetic correlations were greater than zero, primarily because one genotype had uniformly low fitness. Two independent transects showed very similar spatial patterns of correlation with distance. These results show selection is fine grained and varies over spatial scales (20 cm and 3 m) that are similar to the scale of competition and dispersal, respectively.