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The impact on Darwinian fitness of variations in fecundability and in early fetal loss has been investigated by computer simulation. The magnitude of the impact has been expressed in terms of “mortality equivalents,” i.e., the amount of children and adult mortality which is necessary to achieve the same reduction in fitness as that produced by a given change in one of these components of fertility. It has been shown that substantial increases in mortality are required to produce decreases in fitness which can arise from relatively modest changes in fecundability or early fetal loss.