DISRUPTIVE SELECTION ON HABITAT PREFERENCE AND THE EVOLUTION OF REPRODUCTIVE ISOLATION: AN EXPLORATORY EXPERIMENT

A. pair of replicate experiments was carried out to examine the role of habitat specialization in the process of sympatric speciation. The rationale of the experimental design was that disruptive selection for habitat preference can facilitate the process of sympatric speciation by reducing an antagonistic interaction between the processes of recombination and selection. Replicate populations of Drosophila melanogaster were subjected to disruptive selection for habitat preference and simultaneous selection for positive assortative mating, for a period of 46 generations. The experimental protocol simulated the introduction of a frugivorous fly population into a new environment that contained a mosaic of eight equally abundant habitats. Only two habitats were suitable to the flies, and these required opposite patterns of geotaxis, phototaxis, chemotaxis, and developmental time to be successfully located. At the beginning of each generation, flies were placed as pupae into the middle of a maze containing eight discrete habitats. To produce disruptive selection on habitat preference, only those flies that chose one or the other of two selected habitats were permitted to contribute gametes to the next generation. Flies from each of the selected habitats were cultured separately to simulate independent carrying capacities within each habitat. Because flies rarely mated until they assorted themselves into the available habitats, genes affecting habitat preference pleiotropically produced assortative mating between flies with similar habitat preference. A genetic marker was used to continuously monitor the level of potential gene flow between the flies derived from the two selected habitats. During the course of the experiment, there was a gradual increase in the philopatry of the flies, indicating the development of habitat specialization. The evolution of habitat specialization resulted in substantial reproductive isolation between the subpopulations utilizing each habitat resource.