Reduced inbreeding depression in peripheral relative to central populations of a monocarpic herb

Many temperate taxa were confined to warmer latitudes during the last glacial maximum. As their ranges expanded when climates warmed, genetic drift and inbreeding in relatively small peripheral populations are expected to have reduced genetic diversity and the segregating genetic load. Therefore, inbreeding depression in peripheral populations might be lower than in centrally located sites. We evaluated the consequences of inbreeding for fitness traits in six central and six northern peripheral populations of the herb Campanulastrum americanum. Inbreeding reduced performance for all traits. Inbreeding depression in peripheral populations was lower than in central populations. This difference increased across the life cycle from similar levels for germination, to central populations having three times the inbreeding depression for adult traits. Geographical patterns of inbreeding depression suggest that mating system variation and potential future mating system evolution in many temperate taxa might reflect, at least in part, nonequilibrium conditions associated with historic range changes.