The reason for this is presumably extensive gene flow, a consequence of the strong dispersal capabilities of the adult moths. In addition, there is no evidence of the species becoming insecticide resistant. This is likely a consequence of the weak selection pressure resulting from the extremely limited use in space and time of insecticides against the spruce budworm in most of its range. This variation in insecticide tolerance has been shown to be heritable and is therefore subject to natural selection if selection pressures become large enough. Because of its much greater spatial extent, which easily exceeds the scales of genetic mixing through moth dispersal, and because of its continuity, forecasts suggest that climate change has the potential to be a considerably stronger selective force than have historical insecticide applications.
More common genotypes are also subject to natural selection.
For instance, some species with large ranges may be composites of regionally adapted populations so common genotypes in the warmer areas may be able to successfully establish in the cooler areas as the climate warms. Although the spruce budworm, for example, has little genetic variation which has a geographic basis, a genetically-based cline in fecundity and egg weight has been discovered. In the north, a large egg is thought necessary to give young spruce budworms enough energy to survive the long winters. Hence, incidents of devastating mortality or explosive outbreaks need not characterize the impact of natural selection in a changed environment. It may be subtle, non-lethal effects cascading through food webs and ecosystems that cause many of the shifts in the abundance and distribution of biological populations that ultimately become environmental issues.