Disruptive
selection on Drosophila scutellar
bristle number
Starting with
a typical normal distribution of bristle number in Generation
1 (top), Thoday & Gibson (1961) selected flies
with low numbers and high numbers of bristles, discarded the
others, and allowed the low-line and high-lines
to continue to reproduce in the same environment. By Generation
12 (bottom), the cage population had a bi-modal
distribution of bristle numbers, and individual flies
from the two groups showed some evidence of reduced
reproductive compatibility. Selection apparently produced
two distinctive "populations" in the same cage. (The
experiment has been difficult to reproduce).