Enigmatic flies: Is Drosophila in the "Evolution Canyon" a model for incipient sympathic speciation?

Extensive studies of biota at "Evolution Canyon" (EC) on Mount Carmel (Nahal Oren Canyon, Israel) include Drosophila as one of the main model organisms. The microtopography of the EC permits interslope mixing by easy migration of the flies. Nevertheless, we found strong interslope differentiation in Drosophila populations derived from the opposite slopes involving habitat choice, as well as various aspects of induced changes in viability and longevity caused by short-term and lifetime high-temperature treatments. The most exciting findings were related to sexual behavior: interslope differences in mating propensity, sexual discrimination, reproductive activity, peculiarities in courtship song patterns, and significant positive assortative mating. Some of the foregoing effects were also confirmed in D. simulans. The evidence suggests that these populations are examples of ongoing divergence taking place regardless of gene flow. However, tests for interslope genetic differentiation in Drosophila, carried out in a number of laboratories, gave somewhat conflicting results. A possible explanation could be that adaptive differentiation can withstand destructive effects of interslope migration, but it should not necessarily be accompanied by differentiation for selectively neutral markers, unless the latter will be in linkage disequilibrium with selected loci. The last condition can also be maintained despite migration, but only under tight linkage and strong selection. For some Drosophila genes, linkage disequilibrium is known to decay within just a few kilobases. Thus, differentiation for adaptive trait complexes and relevant candidate genes seems to be much better evidence for interslope divergent selection than that displayed by genetic distances estimated using molecular markers.