Microclinal variation for ovariole number and body size in Drosophila melanogaster in 'Evolution Canyon'.

Sites that display strong environmental contrasts in close proximity, such as 'Evolution Canyon' on Mt. Carmel, Israel, are natural theatres for investigating adaptive evolution in action. We reared Drosophila melanogaster from collection sites along altitudinal transects on the north- and south-facing canyon slopes in each of three temperature environments, and assessed genetic variation in ovariole number and body size between and within collection sites, and temperature plasticity. Both traits exhibited significant genetic variation within collection sites and phenotypic plasticity in response to temperature, but not genetic variation for plasticity. Between-site genetic variation in ovariole number was negatively correlated with altitude on both slopes of the canyon, and collections from the north- and south-facing slopes were genetically differentiated for male, but not female, body size. Genetic variation between sites within easy dispersal range is consistent with the action of strong natural selection, although neither the selective agent(s) nor the direct targets of selection are known.