The relationship between fitness and response to selection in Drosophila melanogaster.

The relationship between the selection response exhibited by a morphological trait and the correlated response observed on fitness values has been investigated to improve our understanding of micro-and macro-evolutionary processes. The research was designed to test the validity of the "homeostatic" and "metric deviation" models (Robertson 1956). The results show that: 1) correlated changes in fertility values are larger when selection for minus-variant wing length is applied to males than when it is performed on females, 2) within each of the selected lines, M, F and MF, the reproductive fitness of the populations obtained from vg/vg x +/vg crosses differs from that of the populations derived from +/vg X vg/vg crosses, 3) reproductive fitness values of populations derived from vg/vg X +/vg crosses are fairly constant in selected lines obtained by means of different selective procedures, while mean wing length changes; the corresponding populations from +/vg X vg/vg crosses show, on the contrary, a proportionality between fitness and selected trait values. The lack of consistency of these results with the currently accepted hypotheses on the relationship between selection response and correlated changes in fitness leads to the conclusion that the two models considered do not satisfactorily interpret our results; these are better explained by a developmental relationship between fitness and selected trait.