THE SYMMETRY OF CORRELATED SELECTION RESPONSES IN ADAPTIVE EVOLUTION - AN EXPERIMENTAL-STUDY USING DROSOPHILA

The relationship between the processes of density-dependent and age-sp ecific selection has been investigated by examining a common phenotype , urea resistance, which has apparently evolved in response to each of these selection mechanisms. Twenty populations that have experienced differing levels of age-specific selection show differences in egg-to- adult viability in environments with high levels of urea. Among this g roup of populations, it appears that resistance to urea is correlated with longevity, but not development time. Ten populations kept at extr eme larval densities for many generations also show responses to urea: those kept at high larval densities appear to be most resistant to ur ea. However, these populations show no differences in adult longevity. An additional five populations were selected directly for urea resist ance by adding this compound to the larval food environment. Again, th ere was a strong response to this artificial selection, with urea resi stance increasing dramatically, but these populations showed no respon se in adult longevity or resistance to crowding when compared to five control populations. There is clearly no simple relationship between l ongevity and larval urea resistance. It may be that age-specific and d ensity-dependent selection induce similar changes in this phenotype, b ut do so through different genetic and physiological pathways. We sugg est that these data are not consistent with the view of constant and s ymmetric genetic variance-covariance matrices. These data support a mo re prominent role for observations of evolutionary trajectories rather than static measurements of genetic components of variance.