Selection for oviposition site preference in Drosophila melanogaster.

It has long been a point of contention among biologists whether speciation can occur sympatrically as well as allopatrically. These experiments were set up in order to test the reactions of populations of D. melanogaster subjected to divergent selection for oviposition site under allopatric and sympatric selection regimes. Flies were presented with a choice of either a Potato based medium or a Sugar/killed yeast medium on which they could lay their eggs. The selection intensity was 50% under both regimes. Divergence occurred as quickly and to an approximately equal magnitude in all the Sympatric and Allopatric populations. This occurred despite there being up to 12.5% gene flow between the subpopulations of the Sympatric cages. Habituation was shown to have a significant effect on a females choice of oviposition site and a males courtship success. Flies selected on the Potato medium developed into adults quicker on the Potato medium than did flies selected on the Sugar medium. A small amount of reproductive isolation was observed in some of the Allopatric and Sympatric cages after 18 months of selection