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.