Environmental Stress as an Evolutionary Force

Stressful environmental conditions can be defined as those that lead to a sharp reduction in fitness in populations. That is, when changed environmental con­ ditions cause a drastic reduction in reproductive output, and when persistence of the conditions leads to permanent damage, these conditions constitute an environmental stress. Physical stresses that are encountered rarely in populations-such as periods of drought or extreme cold­ or that are encountered by a minority of a species-such as in populations that are located at distribution borders or are exposed to local chemical stresses arising from human activities-can, through their direct or indirect effects, lead to marked reductions in the size of populations (Glynn 1988, Hoffmann and Parsons 1991) and to repeated cycles of colonization and extinction (Andrewartha and Birch 1954).