INBREEDING AND ITS FITNESS EFFECTS IN AN INSULAR POPULATION OF SONG SPARROWS (MELOSPIZA-MELODIA)

Inbreeding depression is thought to be a major factor affecting the ev olution of mating systems and dispersal. While there is ample evidence for inbreeding depression in captivity, it has rarely been documented in natural populations. In this study, I examine data from a long-ter m demographic study of an insular population of song sparrows (Melospi za melodia) and present evidence for inbreeding depression. Forty-four percent of all matings on Mandarte Island, British Columbia, were amo ng known relatives. Offspring of a full-sib mating (f = 0.25) experien ced a reduction in annual survival rate of 17.5% on average. Over thei r lifetime, females with f = 0.25 produced 48% fewer young that reache d independence from parental care. In contrast, male lifetime reproduc tive success was not affected by inbreeding. Reduced female lifetime r eproductive success was mostly due to reduced hatching rates of the eg gs of inbred females. Relatedness among the parents did not affect the ir reproductive success. Using data on survival from egg stage to bree ding age, I estimated the average song sparrow egg on Mandarte Island to carry a minimum of 5.38 lethal equivalents (the number of deleterio us genes whose cumulative effect is equivalent to one lethal); 2.88 of these lethal equivalents were expressed from egg stage to independenc e of parental care. This estimate is higher than most estimates report ed for laboratory populations and lower than those reported for zoo po pulations. Hence, the costs of inbreeding in this population were subs tantial and slightly above those expected from laboratory studies. Var iability in estimates of lethal equivalents among years showed that co sts of inbreeding were not constant across years.