Ecological differentiation of natural populations of Drosophila melanogaster, Drosophila simulans, and another drosophilid, Zaprionus tuberculatus, in ''Evolution Canyon,'' Mount Carmel, Israel, is well established. The fitness complex of D. melanogaster includes oviposition temperature preferences, tolerance to high temperature, drought stress and starvation, and different longevity patterns. This remarkable differentiation has evolved despite small interslope distances (only 100 -400 m), within easy dispersal distance. The differences between populations are those expected from genetic adaptation to local microclimates. How such differentiation could evolve and be maintained despite the likelihood of genetic exchange between populations is a challenging question. We hypothesized that interslope microclimatic differences caused strong differential selection for stress tolerance, accompanied by behavioral differentiation (habitat choice and reduced migration rate), reinforced by sexual isolation. Here we report highly significant mate choice by flies from different slopes of the canyon, with preference for sexual partners originating from the same slope. No preferences were found when the sexual partners belonged to different isofemale lines from the same slope.T he origin of reproductive isolation is the central event in the evolution of biological species (1, 2). Sexual isolation may evolve either in allopatry or sympatry, with and without geographical separation, respectively. It is widely accepted that postmating (postzygotic) isolation results from and completes the evolutionary divergence, whereas premating (prezygotic) isolation is associated with its initial stages (refs. 3-5 and references therein). The selectionist paradigm explains both allopatric and sympatric premating isolation as a result of divergent selection. However, the proposed versions of this selection-induced process vary significantly, especially with respect to sympatry. The main problem of sympatric speciation can be formulated as follows (3,(6)(7)(8)(9)(10). Is sexual isolation an incidental by-product of genetic divergence? Or, alternatively, is it selected directly because of its contribution to higher fitness?No clear answers to these questions are yet available. We are still far from understanding how isolation evolves, mainly because of scarcity of evidence, especially in the sympatric scenario. Understanding how selection works, quantifying its effects, and characterizing its interaction with other factors contributing to the differentiation of natural populations (migration, habitat choice, sexual behavior, etc.) are among the major issues of evolutionary biology. A wide range of genetic organization patterns, from major genes and compact blocks of epistatic (coadapted) genes to multiple loci dispersed over many chromosomes, was revealed in laboratory studies (refs. 11-13; for review, see also ref. 14). Much less is known about the population-genetic basis of adaptation and speciation in nature, despite an abundance of field ...
