CLINAL VARIATION ASSOCIATED WITH EDAPHIC ECOTONES IN HYBRID POPULATIONS OF GAILLARDIA PULCHELLA

The variety pulchella of the outcrossing annual plant species Gaillardia pulchella consists of two edaphic races in central Texas which are divergent for one morphological and four electrophoretic characters. Reduced pollen stainability in F₁ hybrids suggests the races are also divergent in chromosome structure. The recent proliferation of this species on roadsides and in pastures has led to hybridization between these races. An analysis of character variation in three hybrid populations revealed significant clinal variation associated with edaphic ecotones, and the width of these clines was found to vary among characters in a consistent pattern. It is argued that this pattern is the result of different characters experiencing different effective selection regimes, with narrower clines reflecting greater differentials in effective selection. Several mechanisms are discussed by which selection may impede the transgression of alleles across the ecotones in these populations. The results of this study are compared to the results of parallel studies on the autogamous annual species Avena barbata in California, and it is suggested that the difference between these two species in the width of clines separating edaphic ecotypes may be accounted for by their different breeding systems.