EVOLUTIONARY RESPONSES OF DROSOPHILA-MELANOGASTER LIFE-HISTORY TO DIFFERENCES IN LARVAL DENSITY

The study examined the effects of evolution at two different larval de nsities on pre-adult and adult fitness traits. Five replicate selectio n lines each were cultured at either 50 or 150 larvae per vial, avoidi ng selection on development time, age at breeding or for adaptation to adult density, one or more of which factors has been a confounding va riable in previous studies. Low density selection lines evolved extend ed development times at both growth densities. The extended developmen t times were associated with greater adult body size at the lower grow th density only, and particularly in females. The lines did not differ significantly in larval competitive ability at either growth density. At neither growth density did the early adult fertility of females or the lifespan of either sex differ between the lines from the two sele ction regimes, but at the lower growth density the late fertility of l ow density line females was significantly enhanced. The results sugges t that larval density does have important effects on the expression an d resolution of life history trade-offs in Drosophila melanogaster, bu t that these may be somewhat different from those reported in previous studies.