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 disjunct, 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.