Ecological Genetics in Drosophila athabasca: Its Effect on Local Abundance

In the vicinity of Deer Isle, Maine, two members of the Drosophila affinis subgroup Drosophila athabasca and D. affinis show demographic evidence of competing with each other. When both species are plentiful in collections, their densities are negatively correlated. They separate ecologically on the basis of island area, D. athabasca being the more commonly collected species on the mainland and large islands while D. affinis is often the more common on smaller islands. The environment on islands from 10 to 30 hectares in area does not appear to be intrinsically worse for D. athabasca than that on the larger islands or the mainland. Island populations of D. athabasca have slower development and lower fertility than mainland populations. These may be important components of fitness in competitive situations. As measured by the within- and between-family components of variance for a morphological trait, wing length, island populations of D. athabasca show lower levels of genetic variability than do nearby mainland populations. Therefore they may be more inbred, and this may cause low fitness on islands. Because this area is near the northern limit of D. affinis's range, it is postulated that periodic population crashes and attendant inbreeding preadapt this species to island life, where inbreeding is a natural consequence of small population size. In general, population structure may often have significant effects on the ecology of species.