Substantial genetic differentiation, as great as among species, exists between populations of Drosophila melanogaster inhabiting opposite slopes of a small canyon. Previous work has shown that prezygotic sexual isolation and numerous differences in stressrelated phenotypes have evolved between D. melanogaster populations in ''Evolution Canyon,'' Israel, in which slopes 100 -400 m apart differ dramatically in aridity, solar radiation, and associated vegetation. Because the canyon's width is well within flies' dispersal capabilities, we examined genetic changes associated with local adaptation and incipient speciation in the absence of geographical isolation. Here we report remarkable genetic differentiation of microsatellites and divergence in the regulatory region of hsp70Ba which encodes the major inducible heat shock protein of Drosophila, in the two populations. Additionally, an analysis of microsatellites suggests a limited exchange of migrants and lack of recent population bottlenecks. We hypothesize that adaptation to the contrasting microclimates overwhelms gene flow and is responsible for the genetic and phenotypic divergence between the populations.thermotolerance ͉ hsp70 ͉ P element ͉ genetic distance ͉ premating isolation A recurrent issue in evolutionary biology is the amount of genetic isolation required for incipient species to diverge from a common ancestor, with geographical isolation and its impact on gene flow a prominent source of genetic isolation (1-3). Indeed, Mayr (4, 5) has stressed repeatedly the role of geographical separation of populations in the origin of species. Although several evolutionary mechanisms may give rise to new species in contiguous populations, in the absence of these mechanisms gene flow would seem to exert a potent homogenizing force on incipient isolates (6, 7). Nonetheless, profound adaptive radiation of species can occur without large scale geographical isolation, as in the Ͼ450 species of haplochromine cichlids from Lake Malawi (8-11). Critics reasonably may dispute that such cases are evidence for speciation without geographic separation, because in Lake Malawi (Ϸ500-km long) and other such instances, potential spatial and ecological separation are large relative to the dispersal abilities of the speciating populations.Accordingly, we have investigated ''Evolution Canyon,'' Israel, where Drosophila melanogaster occurs on north-and southfacing slopes with greatly differing climatic regimes and in the intervening region; the slopes are 400 m apart at the top and 100 m apart at the bottom (12-14). Although adult Drosophila can traverse several kilometers in a single day (15), populations on each slope have diverged in body size, heat and desiccation tolerance, oviposition thermal preference, fluctuating asymmetry, rates of mutation and recombination, and mate preference (16-18). We thus ask: Have the populations likewise diverged genetically according to microsatellite markers and a candidate gene strongly linked to resistance to environmental stress? Is this diver...

We report multiple cases in which disruption of hsp70 regulatory regions by transposable element (TE) insertions underlies natural variation in expression of the stress-inducible molecular chaperone Hsp70 in Drosophila melanogaster. Three D. melanogaster populations from different continents are polymorphic for jockey or P element insertions in the promoter of the hsp70Ba gene. All three TE insertions are within the same 87-bp region of hsp70Ba promoter, and we suggest that the distinctive promoter architecture of hsp genes may make them vulnerable to TE insertions. Each of the TE insertions reduces Hsp70 levels, and RNase protection assays demonstrate that such insertions can reduce transcription of the hsp70Ba gene. In addition, the TEs alter two measures of organismal fitness, inducible thermotolerance and female reproductive success. Thus, transposition can create quantitative genetic variation in gene expression within populations, on which natural selection can act.

Naturally occurring heat shock (HS) during pupation induces abnormal wing development in Drosophila; we examined factors affecting the severity of this induction. The proportion of HS-surviving adults with abnormal wings varied with HS duration and intensity, and with the pupal age or stage at HS administration. Pretreatment (PT), mild hyperthermia delivered before HS, usually protected development against HS. Gradual heating resembling natural thermal regimes also protected wing development against thermal disruption. Because of the roles of the wings in flight and courtship and in view of natural thermal regimes that Drosophila experience, both HS-induction of wing abnormalities and its abatement by PT may have marked effects on Drosophila fitness in nature. Because PT is associated with expression of heat-inducible molecular chaperones such as Hsp70 in Drosophila, we compared thermal disruption of wing development among hsp70 mutants as well as among strains naturally varying in Hsp70 levels. Contrary to expectations, lines or strains with increased Hsp70 levels were no more resistant to HS-disruption of wing development than counterparts with lower Hsp70 levels. In fact, wing development was more resistant to HS in hsp70 deletion strains than control strains. We suggest that, while high Hsp70 levels may aid cells in surviving hyperthermia, high levels may also overly stimulate or inhibit numerous signalling pathways involved in cell proliferation, maturation and programmed death, resulting in developmental failure.