Natural Selection on Chromosome inversions in Drosophila pseudoobscura

    Banding patterns are recognizable in preparations made from the giant salivary gland chromosomes of flies (Diptera), such as the many species of common fruit fly, Drosophila. In D. pseudoobscura of southwestern North America, banding patterns in Chromosome III vary by physical rearrangements (inversions) of portions of the chromosomes. The Arrowhead (AR) pattern differs from the Standard (ST) arrangement 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. The different patterns can be aligned by comparing banding patterns, and these relationships were the first detailed genetic evidence of natural selection in natural populations.

    Th Dobzhansky and co-workers demonstrated that particular inversion types replaced others on reproducible geographic and altitudinal 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.