EVOLUTIONARY EFFECTS OF SELECTION ON AGE AT REPRODUCTION IN LARVAL AND ADULT DROSOPHILA-MELANOGASTER

Two sets of three replicate lines of Drosophila melanogaster were arti ficially selected by reproduction at either a 'young' or an 'old' age. The pure lines, the hybrids between the lines within a selection regi men and the base stock from which the lines were derived were compared for longevity, early and late fertility, development time, larval via bility and adult thorax length. Comparison of hybrid with pure lines s howed some evidence for inbreeding depression in the lines from both s election regimes. Comparison of hybrid lines with the base stock did n ot provide evidence for any trade-off in either males or females betwe en early fertility on the one hand and late life fertility and longevi ty on the other. Nor was there any clear evidence of a trade-off betwe en pre-adult and adult fitness components. There was evidence of inadv ertent selection for rapid development in both selection regimens, esp ecially in the females of the 'young' lines, and this complicated the interpretation of the responses and correlated responses to selection. An improvement in adult performance in the 'old' line males relative to the base stock appeared to be attributable to reversal of mutation accumulation. Comparison of the hybrid 'young' and 'old' lines with th e base stock did not support the idea that the superior longevity and late life fertility of the 'old' lines relative to the 'young' lines c ould be accounted for by the effects of mutation accumulation in the ' young' lines. The results point to the need to compare selected lines with their base stock when deducing responses and correlated responses to selection and to avoid unintentional selection. In this type of ex periment, larval density should be standardized during selection, and adults should not be under pressure for rapid maturation.