Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1987. This study describes an investigation of lines of Drosophila melanogaster (the fruit fly), divergently selected intermittently over 600 generations for geotaxis (orientation and movement with respect to gravity), that involves genetic analyses and changes in selection pressure. The extreme geotactic expressions observed in the negatively geotactic (high) and positively geotactic (low) lines were considered to be examples of instinctive behavior because (1) they seemed to be associated with increased fitness and (2) they were "population-typical." Chromosomal assays, performed early in the history of these lines (1959-60) and again more recently (1984), showed gradual changes in the effects of individual chromosomes of both lines, and small but significant increases in interactions among chromosomes of the low line. Breeding analyses of synthesized "chromosome lines" and of the selected lines suggested that there were two or three major segregating loci associated with geotactic differences. When selection pressure was reversed in the low line and then relaxed on several occasions, the median geotactic score returned towards the original expression, this suggesting that increased fitness was associated with extreme expressions of geotaxis (the high line showed similar changes but of much smaller magnitude). This example of "genetic homeostasis" was discussed in terms of coadaptation. These empirical observations have allowed a greater understanding of genetic changes associated with long-term selection, the genetics associated with instinctive behavior, and the evolution of coadaptation. U of I Only Restricted to the U of I community idenfinitely during batch ingest of legacy ETDs