INBREEDING DEPRESSION IN FOUR POPULATIONS OF COLLINSIA HETEROPHYLLA NUTT (SCROPHULARIACEAE).

The effects of one and two generations of inbreeding were studied in plants from four natural populations of the annual plant, Collinsia heterophylla, using inbred and outcrossed plants generated by hand pollinations to create expected inbreeding coefficients ranging from 0-0.75. The selfing rates of the populations were estimated using allozyme markers to range from 0.37-0.69. Inbreeding depression was mild, ranging from 5-40%, but significant effects were detected for characters measured at all stages of the life cycle. Fitness components declined significantly with the inbreeding coefficient, and regression of fitness characters on inbreeding coefficients gave no evidence of any strongly synergistic effects attributable to the different genetic factors that contribute to decline in fitness under inbreeding. The magnitude of inbreeding depression did not clearly decrease with the populations' levels of inbreeding. This is not surprising because the selfing rates are similar enough that it is unlikely that the populations have been characterized for long periods of time by these different inbreeding levels. © 1996 The Society for the Study of Evolution.