5.2: Acclimation and Adaptation

Species respond to environmental stressors through acclimation and adaptation. The distinction between the two is important; though they are linked, they operate at very different scales and with different short-term and long-term ramifications. Acclimation occurs when an individual organism experiences a change in its phenotype in response to an environmental stressor. This change is reversible and occurs very rapidly, sometimes in as little as a few minutes. Adaptation occurs when a population experiences a change in its genotype in response to an environmental stressor. This change occurs much more slowly, usually across many generations, and is a result of natural selection.

For example, imagine you set out to hike to the summit of Mt. Whitney in California’s Sequoia National Park. The peak is approximately 14,505 feet (~4420 meters) in elevation; high enough that the air is much thinner at the top (Fig 5.2.1) and therefore your lungs will struggle to obtain the oxygen necessary for you to breathe. As you hike higher and higher in elevation, your body will acclimate by producing more hemoglobin. Hemoglobin is a protein in your blood that carries oxygen from your lungs to your tissues and an increase in hemoglobin in your blood helps your body maintain oxygen in your bloodstream. In this example, the concentration of hemoglobin in your blood is the phenotype that changes in response to the environmental stressor of reduced oxygen availability. When you hike down to lower elevations, the hemoglobin concentrations in your blood will return to normal.

Figure \(\PageIndex{1}\): Diagram of the effects of altitude on oxygen availability. Considering sea level as 'baseline' (100%), reductions in O₂ levels are shown for the boundaries between low altitude, high altitude, very high altitude, extreme altitude, and the death zone (above which, human life cannot be sustained for long periods). For reference, the altitude of the tallest three mountains in the world (Everest, Aconcagua, and Denali) are provided, as well as the tallest mountain in California (Whitney). Image by L Gerhart Barley with biorender.com

Now consider the long-term impacts on a population of humans that live at very high elevations for many generations. The Tibetan Plateau, for example, has an average elevation of approximately 14,800 feet (~4500 meters) and has been inhabited by humans for possibly as long as 30,000 years. Genetic comparisons performed by Yi et al (2010) between Tibetan Plateau populations and their close relatives, the Han Chinese revealed differences between these groups in the frequency of different alleles for a gene called EPAS1. At this gene, both populations contained a particular allele that increases the production of cells called erythrocytes (which contain hemoglobin) in the blood; however, this allele was 78% more common in the high-elevation Tibetan population. In this case, differences in the frequency of EPAS1 alleles between these two populations represent an adaptation to long-term exposure to the environmental stressor of reduced oxygen availability. We will discuss alleles and their frequencies in different populations of organisms more in the inheritance sections.

In the following sections, we will discuss acclimational and adaptational responses to abiotic (temperature) and biotic (predator/prey interactions) stressors.