How environmental factors affect gene expression

Environmental factors can cause changes in the frequency of a particular gene expression within a population or species of organism. For example, if a habitat is fairly light in color, then organisms that are light in color tend to blend in better. This allows them to hide from predators and/or be better camouflaged predators themselves.
If that environment were to drastically change, that species might struggle to survive. For example, if a fire blackened all of the trees in an area, a lightly colored organism would no longer blend well with the environment. Any random genetic changes that occurred that allowed for a darker coloring would likely make that darker individual more fit for that environment. That organism would likely live to pass on its unique adaptation, and after a while, the entire species would be darker than it was before. The gene expression that wasn't advantageous before would now be commonly displayed as a result of an environmental change. This is exactly what happened with the Peppered Moth in the latter half of the 19th century.
http://www.mothscount.org/text/63/peppered_moth_and_natural_selection.html