MEASURES OF PHENOTYPIC SELECTION ARE BIASED BY PARTIAL INBREEDING

When populations are partially inbred due to the population structure or to a mixed mating system like partial self-fertilization, some indi viduals will be more inbred than others. This heterogeneity among indi viduals in the history of inbreeding can greatly complicate the interp retation of measures of quantitative genetic variability when the trai ts studied exhibit inbreeding depression. Partial inbreeding can also bias measures of phenotypic selection toward the detection of strong d irectional and stabilizing selection. In this paper, data are presente d from several inbreeding experiments conducted on two partially selfi ng, annual populations of the monkeyflower Mimulus guttatus that show that the means of many of the morphological and phenological traits me asured were affected by inbreeding. These findings imply that estimate s of heritabilities and additive genetic covariances would not reflect the potential for these populations to respond to selection. Phenotyp ic selection analyses conducted on naturally occurring plants, involvi ng linear regressions of relative seed production on the traits, revea led significant directional selection on many of the same quantitative traits measured in the inbreeding studies. However, when the same sel ection analyses were performed on plants with known histories of inbre eding, part of the statistical relationship between relative seed numb er and the traits was found to be due to the mating system: inbred ind ividuals had both lower seed production and different mean values for the traits than outcrossed individuals. It is also shown, with a hypot hetical example, that partial inbreeding can bias measures of stabiliz ing selection toward the detection of strong stabilizing selection. Pa rtial inbreeding therefore tends to make directional and stabilizing s election appear stronger than it is, and it may be that natural select ion in the wild is actually weaker than many studies of partially inbr ed species suggest.