In 19th century Great Britain, Peppered Moth populations were affected by industrialization. Lighter moth populations used to predominate over darker ones, because the darker ones stood out against light-colored trees, and were therefore more easily spotted and eaten by predator birds. However, during the 19th century, factory soot started covering trees and buildings, turning them darker; this caused the darker moths to suddenly be better camouflaged against predators. So over several years, the dark population of moths gradually increased above the lighter population. This example illustrates that natural selection acts most directly on: