Natural Selection on Chromosome inversions in Drosophila pseudoobscura

    The Standard (ST) arrangement of Chromosome III is related to the Arrowhead (AR) pattern by a paracentric inversion involving about a third of the chromosome. Other chromosome polymorphisms such as Chiracahua (CH) involve multiple inversions, and inversions-within-inversions.

    Th Dobzhansky and co-workers demonstrated that particular inversion types replaced others on reproducible geographic, altitudinal, and seasonal gradients. The graph below shows that ST is most common at sea level, and is replaced by AR at high altitudes. The patterns are repeated throughout the range of the species. Seasonal patterns repeat annually. Temperature adaptation was demonstrable in the laboratory.