Changes in the normal types of environmental conditions to which species are exposed can lead to dramatic reductions in the fitness of organisms, and these conditions are often referred to as 'stressful'. They may be catastrophic such as droughts or mass flooding events, or subtle such as small increases in sea or air temperature. Both types of changes potentially have an enormous effect on population density and ultimately lead to changes in species distributions and community composition. However, while stressful conditions are often seen in a negative light because they decrease biodiversity, they may also have positive effects by being linked to rapid evolutionary responses; for instance, species radiations within the fossil record have often been linked to periods of environmental change, and geographic regions with a high level of animal and plant diversity are often exposed to marked fluctuations in environmental conditions that produce periods of local extinction and expansion.
One reason why stressful conditions might produce evolutionary changes is that they represent periods of intense selection. Numerous laboratory-based experiments in model organisms like Drosophila melanogaster, mice, and Escherichia coli bacteria have demonstrated that rapid evolution can occur following intense selection. There are also numerous examples of natural populations of organisms adapting under intense selection due to stressful conditions, such as the evolution of highly resistant pest populations following exposure to chemicals, and evolutionary changes in both morphological and physiological traits in birds exposed to food shortages resulting from climatic stressors. Apart from producing more intense selection, stressful conditions can also influence evolutionary rates in other ways explored further below.
The question of how stressful conditions influence the evolutionary potential of populations has become increasingly important because human activities are rapidly producing habitat fragmentation, thermal and precipitation shifts associated with global warming and increasing quantities of pollutants entering the environment. There are numerous examples of species adapting in the face of such adversity but similarly there are many examples highlighting that not all species successfully adapt. The low plant diversity typical of metal-contaminated soils and the loss of endemics in areas that are becoming warmer highlight the limitations of evolutionary responses. Under many circumstances strong directional selection will lead to adaptation but why do we see an absence of a selection response in other cases, and how might evolutionary changes be promoted in such situations?
In this article we consider conditions under which stress might increase or decrease the likelihood of adaptation. We focus on current human-induced disturbances such as habitat fragmentation, the direct and indirect effects of global warming, and the effects of insecticides and pollutants. Within this framework we discuss potential genetic and ecological limits that may prevent future adaptation and what this means for species conservation. We finish by highlighting current evidence for adaptation to human-induced environmental stressors and the potential to use genes as markers of environmental change. A theme emerging from work undertaken to date is that populations and species markedly differ in their propensities to evolve. This makes predictions difficult in specific instances, but generalizations may emerge about specific groups of organisms as more data are collected.
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