A Geographic Mosaic of Speciation in Eurosta solidaginis

Abstract

Ecological speciation occurs when divergent selection in different habitats creates strong selection for reproductive isolation which counteracts the homogenizing effects of gene flow between populations using those different habitats. Multiple divergent selection pressures can affect isolation and the strength of these selection pressures can vary geographically. Sympatric pairs of populations using different habitats in separate geographic areas may experience varying degrees of diversifying selection. Differing degrees of reproduction isolation between the populations result in what may be called a “Geographic Mosaic of Speciation”. A Geographic Mosaic of Speciation provides an opportunity to study the process of speciation in various degrees of completion. Eurosta solidaginis (Diptera: Tephritidae) is a fly that induces galls on tall goldenrod (Solidago altissima; Compositae) in North America with populations in the forest and prairie biomes of Minnesota, which are under divergent ecologically-based selection pressures. Previous research indicates that the goldenrod, the gallfly, and the natural enemies of the gallfly in the prairie and the forest biomes are genetically differentiated. I measured characteristics of each member of this three-trophic-level interaction across the prairie-forest biome border in order to characterize divergent selection on members of this interaction. I found that differences in the host plants, flies, and their natural enemies were distributed in a geographic mosaic at the prairie-forest biome border. Some neighboring populations were highly differentiated in a range of characteristics while others showed less differentiation. I then tested two pairs of neighboring fly populations to measure their degree of reproductive isolation and their adaptation to host-plants. The goldenrods from these paired sites were morphologically differentiated from each other. Flies from three out of the four sites tested had higher survival on plants from their own site than plants from the neighboring site, which shows that differences in plant morphology are correlated with differential selection of host-plants on E. solidaginis flies. Reproductive isolating mechanisms of the flies across the boundary are also distributed in a geographic mosaic which supports our hypothesis that these flies show a Geographic Mosaic of Speciation.