| Abstract: | I studied the causes of variation and selection on clutch size in a population of Darwin's Medium Ground Finches (Geospiza fortis) on Isla Daphne Major, using data collected over a nine-year period (1976–1984). Quantitative-genetic analyses were carried out using the first clutch laid by a female in a given year. I used both unadjusted clutch-size values and values adjusted for between-year differences in mean clutch size for repeatability and regression analyses. Repeatability of clutch size was small (≤8%) and nonsignificant in all cases. Sib-sib analyses and parent-offspring regressions gave no evidence of a significant additive genetic component to clutch-size variation. Slopes of mother-daughter regressions were actually negative, suggesting possible maternal effects of mother's clutch size on daughter's clutch size. There was a small positive relationship between female age and clutch size but no effect of male or female body size or of large-scale differences in habitat quality on clutch size. Selection on clutch size was generally directional and positive: in almost all years in which successful breeding occurred, large clutches tended to fledge more chicks and produce more young surviving to the following year, possibly because there was no trade-off between clutch size and the weights of individual chicks at fledging. Thus, sustained directional selection for large clutch size may have reduced additive genetic variation in clutch size to low levels in this population. The size of a female's clutch may be primarily determined by unidentified proximate environmental factors which vary from year to year, rather than by any long-term optimization of clutch size with respect to adult survival. |
