An Introduction to the Invertebrates, Second Edition - tiera.ru

WHY HAS STUDY OF THE FRUIT FLY DROSOPHILA BEEN SO IMPORTANT?211mapping was checked and extended by experiments in which partsof the chromosomes were cut out, turned round or rearranged.T. H. Morgan, A. H. Sturtevant, H. J. Muller and C. B. Bridges in 1915published The Mechanism of Mendelian Heredity. The merging of cytologyand genetics into a single science was the first major contributionof the Drosophila work of the Morgan school.15.7.2 The evolutionary role of mutationsFurther work by Morgan’s group, and by others such as TheodosiusDobzhansky and (with a gift of cultures from Columbia)S. S. Chetverikov in Russia in the 1920s, established the connectionbetween Mendelism and Darwinism. At that time genetics andevolution were viewed very separately, with much controversy abouttheir relationship. H. De Vries, working on evening primroses(Oenothera biennis), observed sudden large heritable changes thathe termed ‘mutations’: these were generally deleterious and couldnot be seen as the raw material of evolution. Morgan redefinedmutations as small changes in genes, initially recessive butretained in the genotype, and showed how such mutations couldbe the source of variation acted on by natural selection. For studyof mutations Drosophila was a rewarding subject, having thecomplex insect structure with many characters under direct geneticcontrol. A large ‘library’ of mutations was soon acquired, rangingfrom large-scale malformations to patterns of bristles on thecuticle. Mutations were shown to occur in wild populations also, tobe imitated by heat shock at critical points in development(R. Goldschmidt) and to be caused by ionising radiation (Muller).A balanced polymorphism in the population was described byM. Teissier; the statistical approach to populations by R. A. Fisherwas especially important. Genes were revealed as large andintricate entities which interacted with each other and with theenvironment, and genetics became part of the ‘evolutionary synthesis’of the 1930s and 1940s.15.7.3 SpeciationSpeciation in Drosophila is marked, most spectacularly in Hawaii,where 450 species have been described. New Hawaiian islandsare continually being formed and are biologically ‘empty’ at first.Small insects are weak fliers and colonists are few: these speciatereadily as each island presents a great diversity of habitats, sexualselection is strong and lava flows may further divide populations.Many closely related species of Drosophila are therefore available forcomparison; often there is remarkably little genetic differencebetween them.