Variation in Bromus sterilis on farmland: evidence for the origin of field infestations.
Results from a common garden experiment and from a survey of isozymic variation conducted in southern England during 1987 and 1988, using population samples of Bromus sterilis collected from five pairs of field-boundary and mid-field habitats, provided no evidence of habitat-correlated differentiation of the populations in response to the differences in selective pressure between cropped and boundary habitats. Pairs of populations from the same field could not be differentiated either by quantitive characteristics or by allele frequencies, though populations from different fields could be distinguished. Results from a more intensive survey of isozymic variation, in a population occuring along a hedge-field interface, provided further evidence of the close genetic relationship between hedge and field populations. A strong association in allele frequencies was detected between samples from adjacent sections of hedge and field but not between samples from more distant sections. It was suggested that boundary populations act as a major seed source for continual re-infestation of the field, and that the unstable nature of ephemeral field infestations prevents ecotypic differentiation.