CONSTRAINTS ON THE EVOLUTION OF ATTRACTIVE TRAITS - SELECTION IN MALEAND FEMALE ZEBRA FINCHES

We examined natural and sexual selection in a laboratory population of zebra finches (Taeniopygia guttata), with a principal focus on bill c olor. Previous studies indicated that females prefer males with redder bills, whereas males prefer females with less red bills, For male red bill color, both the selection differential and selection gradient co efficients were significantly positive with fitness (the number of off spring surviving to independence), which was primarily due to differen ces in the time to the first nest and in reproductive rate but not to differences in survival. In contrast, for female red bill color, there was significant negative selection, which was due to differences in m ost stages of reproductive and survival selection but not to differenc es in the time to the first nest. There were no significant correlatio ns between male or female bill color and offspring weight or postfledg ing survival. All male plumage color traits (breast band, cheek patch, flank patch) showed nonsignificant selection for increase in size due to both the time to the first nest and for reproductive rate but show ed selection for decrease in size due to survival. Tarsus length was t he only male trait to show significant selection for decrease in size, The opposing selection on red bill color between the sexes indicates that the positive genetic correlation between male and female red bill color may constrain the evolutionary rate of red bill sexual dichroma tism.