VARIABLE SELECTION ON THE TIMING OF GERMINATION IN COLLINSIA VERNA (SCROPHULARIACEAE)

Natural selection on the timing of seed germination was investigated in a natural population of the winter annual Collinsia verna (Scrophulariaceae) for two years. The goal was to quantify 1) the importance of the timing of seed germination to life history evolution in this population and 2) variation in selection in time and space. During fall germination, seedlings were assigned to cohorts on the basis of their dates of germination. Growth, survivorship, and reproduction were censused throughout both years. Selection on the timing of germination was quantified using linear and quadratic regressions during three ecologically important periods in the life cycle, using the techniques of Lande and Arnold (1983) and Arnold and Wade (1984a, 1984b). Comparisons were made between years and on two spatial scales within years. Overall, selection favored early-germinating plants in the first year. The primary determinant of the relationship of the timing of germination to fitness was fecundity selection, rather than viability selection on seedlings. Fecundity selection was respondible for from 54% to 80% of the change in the mean time of germination. Significant disruptive selection characterized the second field season, again mediated mainly through fecundity selection. There was also temporal and spatial heterogeneity in selection on this character. Transects and quadrats differed significantly in the direction and magnitude of natural selection. In addition, the direction of selection changed between episodes for the transects. The results illustrate the importance of the timing of germination to life-history evolution in this annual plant and the complex action of natural selection on this character.