Abstract
SummaryForty-one third chromosomes extracted from a natural population ofDrosophila melanogasterwere assessed for net fitness and for the quantitative characters viability, net fertility, female productivity, male weight, abdominal bristle number, and sternopleural bristle number. Net homozygous and heterozygous fitness of the third chromosomes was estimated by competition against a marked balancer third chromosome. Average fitness of the homozygous lines relative to wild-type heterozygotes was 0·13, indicating substantial inbreeding depression for net fitness. All significant correlations of quantitative characters with fitness and with each other were high and positive. Homozygous fitness is strongly correlated with net fertility, viability, and female productivity, moderately associated with male weight, and not significantly associated with bristle traits. The combination of metric traits which best predicts homozygous fitness is the simple multiple of viability and female productivity. Heterozygous fitness is not correlated with homozygous fitness; furthermore, the relative contribution of metric traits to fitness in a heterozygous population is likely to be different from that deduced from homozygous lines. These observations are consistent with a model of genetic variation for fitness in natural populations caused by segregation of rare deleterious recessive alleles.
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