6.4 Human Responses to Environmental Stress

Created by: CK-12/Adapted by Christine Miller

Figure 6.4.1 Brace yourself! 

Oh, the Agony!

Wearing braces can be very uncomfortable, but it is usually worth it. Braces and other orthodontic treatments can re-align the teeth and jaws to improve bite and appearance. Braces can change the position of the teeth and the shape of the jaws because the human body is malleable. Many phenotypic traits — even those that have a strong genetic basis — can be molded by the environment. Changing the phenotype in response to the environment is just one of several ways we respond to environmental stress.

Types of Responses to Environmental Stress

There are four different types of responses that humans may make to cope with environmental stress:

  1. Adaptation
  2. Developmental adjustment
  3. Acclimatization
  4. Cultural responses

The first three types of responses are biological in nature, and the fourth type is cultural. Only adaptation involves genetic change and occurs at the level of the population or species. The other three responses do not require genetic change, and they occur at the individual level.

Adaptation

An adaptation is a genetically-based trait that has evolved because it helps living things survive and reproduce in a given environment. Adaptations generally evolve in a population over many generations in response to stresses that last for a long period of time. Adaptations come about through natural selection. Those individuals who inherit a trait that confers an advantage in coping with an environmental stress are likely to live longer and reproduce more. As a result, more of their genes pass on to the next generation. Over many generations, the genes and the trait they control become more frequent in the population.

A Classic Example: Hemoglobin S and Malaria

Probably the most frequently-cited example of a genetic adaptation to an environmental stress is sickle cell trait. As you read in the previous section, people with sickle cell trait have one abnormal allele (S) and one normal allele (A) for hemoglobin, the red blood cell protein that carries oxygen in the blood. Sickle cell trait is an adaptation to the environmental stress of malaria, because people with the trait have resistance to this parasitic disease. In areas where malaria is endemic (present year-round), the sickle cell trait and its allele have evolved to relatively high frequencies.  It is a classic example of natural selection favoring heterozygotes for a gene with two alleles. This type of selection keeps both alleles at relatively high frequencies in a population.

To Taste or Not to Taste

Another example of an adaptation in humans is the ability to taste bitter compounds. Plants produce a variety of toxic compounds in order to protect themselves from being eaten, and these toxic compounds often have a bitter taste. The ability to taste bitter compounds is thought to have evolved as an adaptation, because it prevented people from eating poisonous plants. Humans have many different genes that code for bitter taste receptors, allowing us to taste a wide variety of bitter compounds.

A harmless bitter compound called phenylthiocarbamide (PTC) is not found naturally in plants, but it is similar to toxic bitter compounds that are found in plants. Humans’ ability to taste this harmless substance has been tested in many different populations. In virtually every population studied, there are some people who can taste PTC (called tasters), and some people who cannot taste PTC, (called nontasters). The ratio of tasters to non-tasters varies among populations, but on average, 75 per cent of people can taste PTC and 25 per cent cannot.

Tongue
Figure 6.4.2 The tiny red dots on the surface of the tongue consist of clumps of taste buds that contain receptor proteins for certain chemicals. We can taste those chemicals that bind strongly with any of the receptors.

Like many scientific discoveries, human variation in PTC-taster status was discovered by chance. Around 1930, a chemist named Arthur Fox was working with powdered PTC in his lab. Some of the powder accidentally blew into the air. Another lab worker noticed that the powdered PTC tasted bitter, but Fox couldn’t detect any taste at all. Fox wondered how to explain this difference in PTC-tasting ability. Geneticists soon determined that PTC-taster status is controlled by a single gene with two common alleles, usually represented by the letters T and t. The T allele encodes a chemical receptor protein (found in taste buds on the tongue, as illustrated in Figure 6.4.2) that can strongly bind to PTC. The other allele, t, encodes a version of the receptor protein that cannot bind as strongly to PTC. The particular combination of these two alleles that a person inherits determines whether the person finds PTC to taste very bitter (TT), somewhat bitter (Tt), or not bitter at all (tt).

 

If the ability to taste bitter compounds is advantageous, why does every human population studied contain a significant percentage of people who are nontasters? Why has the nontasting allele been preserved in human populations at all? Some scientists hypothesize that the nontaster allele actually confers the ability to taste some other, yet-to-be identified, bitter compound in plants. People who inherit both alleles would presumably be able to taste a wider range of bitter compounds, so they would have the greatest ability to avoid plant toxins. In other words, the heterozygote genotype for the taster gene would be the most fit and favored by natural selection.

Most people no longer have to worry whether the plants they eat contain toxins. The produce you grow in your garden or buy at the supermarket consists of known varieties that are safe to eat. However, natural selection may still be at work in human populations for the PTC-taster gene, because PTC tasters may be more sensitive than nontasters to bitter compounds in tobacco and vegetables in the cabbage family (that is, cruciferous vegetables, such as the broccoli, cauliflower, and cabbage pictured in Figure 6.4.3).

  • People who find PTC to taste very bitter are less likely to smoke tobacco, presumably because tobacco smoke has a stronger bitter taste to these individuals. In this case, selection would favor taster genotypes, because tasters would be more likely to avoid smoking and its serious health risks.
  • Strong tasters find cruciferous vegetables to taste bitter. As a result, they may avoid eating these vegetables (and perhaps other foods, as well), presumably resulting in a diet that is less varied and nutritious. In this scenario, natural selection might work against taster genotypes.

Figure 6.4.3 Cruciferous vegetables.

Developmental Adjustment

It takes a relatively long time for genetic change in response to environmental stress to produce a population with adaptations. Fortunately, we can adjust to some environmental stresses more quickly by changing in nongenetic ways. One type of nongenetic response to stress is developmental adjustment. This refers to phenotypic change that occurs during development in infancy or childhood, and that may persist into adulthood. This type of change may be irreversible by adulthood.

Phenotypic Plasticity

Developmental adjustment is possible because humans have a high degree of phenotypic plasticity, which is the ability to alter the phenotype in response to changes in the environment. Phenotypic plasticity allows us to respond to changes that occur within our lifetime, and it is particularly important for species (like our own) that have a long generation time. With long generations, evolution of genetic adaptations may occur too slowly to keep up with changing environmental stresses.

Developmental Adjustment and Cultural Practices

Developmental adjustment may be the result of naturally occurring environmental stresses or cultural practices, including medical or dental treatments. Like our example at the beginning of this section, using braces to change the shape of the jaw and the position of the teeth is an example of a dental practice that brings about a developmental adjustment. Another example of developmental adjustment is the use of a back brace to treat scoliosis (see images in Figure 6.4.4). Scoliosis is an abnormal curvature from side to side in the spine. If the problem is not too severe, a brace, if worn correctly, should prevent the curvature from worsening as a child grows, although it cannot straighten a curve that is already present. Surgery may be required to do that.

Figure 6.4.4 Scoliosis can be prevented from worsening by shaping the phenotype with a back brace.

Developmental Adjustment and Nutritional Stress

An important example of developmental adjustment that results from a naturally occurring environmental stress is the cessation of physical growth that occurs in children who are under nutritional stress. Children who lack adequate food to fuel both growth and basic metabolic processes are likely to slow down in their growth rate — or even to stop growing entirely. Shunting all available calories and nutrients into essential life functions may keep the child alive at the expense of increasing body size.

Table 6.4.1 shows the effects of inadequate diet on children’s’ growth in several countries worldwide. For each country, the table gives the prevalence of stunting in children under the age of five. Children are considered stunted if their height is at least two standard deviations below the median height for their age in an international reference population.

Table 6.4.1

Percentage of Stunting in Young Children in Selected Countries (2011-2015)

Percentage of Stunting in Young Children in Selected Countries (2011-2015)
Country Per cent of Children Under Age 5 with Stunting
United States 2.1
Turkey 9.5
Mexico 13.6
Thailand 16.3
Iraq 22.6
Philippines 33.6
Pakistan 45.0
Papua New Guinea 49.5

After a growth slow-down occurs and if adequate food becomes available, a child may be able to make up the loss of growth. If food is plentiful, the child may grow more rapidly than normal until the original, genetically-determined growth trajectory is reached. If the inadequate diet persists, however, the failure of growth may become chronic, and the child may never reach his or her full potential adult size.

Phenotypic plasticity of body size in response to dietary change has been observed in successive generations within populations. For example, children in Japan were taller, on average, in each successive generation after the end of World War II. Boys aged 14-15 years old in 1986 were an average of about 18 cm (7 in.) taller than boys of the same age in 1959, a generation earlier. This is a highly significant difference, and it occurred too quickly to be accounted for by genetic change. Instead, the increase in height is a developmental adjustment, thought to be largely attributable to changes in the Japanese diet since World War II. During this period, there was an increase in the amount of animal protein and fat, as well as in the total calories consumed.

Acclimatization

Other responses to environmental stress are reversible and not permanent, whether they occur in childhood or adulthood. The development of reversible changes to environmental stress is called acclimatization. Acclimatization generally develops over a relatively short period of time. It may take just a few days or weeks to attain a maximum response to a stress. When the stress is no longer present, the acclimatized state declines, and the body returns to its normal baseline state. Generally, the shorter the time for acclimatization to occur, the more quickly the condition is reversed when the environmental stress is removed.

Acclimatization to UV Light

A common example of acclimatization is tanning of the skin (see Figure 6.4.5). This occurs in many people in response to exposure to ultraviolet radiation from the sun. Special pigment cells in the skin, called melanocytes, produce more of the brown pigment melanin when exposed to sunlight. The melanin collects near the surface of the skin where it absorbs UV radiation so it cannot penetrate and potentially damage deeper skin structures. Tanning is a reversible change in the phenotype that helps the body deal temporarily with the environmental stress of high levels of UV radiation. When the skin is no longer exposed to the sun’s rays, the tan fades, generally over a period of a few weeks or months.

Figure 6.4.5 Tanning of the skin occurs in many people in response to exposure to ultraviolet radiation from the sun.

Acclimatization to Heat

Another common example of acclimatization occurs in response to heat. Changes that occur with heat acclimatization include increased sweat output and earlier onset of sweat production, which helps the body stay cool because evaporation of sweat takes heat from the body’s surface in a process called evaporative cooling. It generally takes a couple of weeks for maximum heat acclimatization to come about by gradually working out harder and longer at high air temperatures. The changes that occur with acclimatization just as quickly subside when the body is no longer exposed to excessive heat.

Acclimatization to High Altitude

Base camp in Nepal
Figure 6.4.6 Mountaineers must spend 4-5 days acclimatizing to high altitude before attempting to climb to the summit of Mount Everest.

Short term acclimatization to high altitude occurs as a response to low levels of oxygen in the blood.  This reduced level of oxygen is detected by carotid bodies, which will trigger in increase in breathing and heart rate.  Over a period of weeks the body will compensate by increasing red blood cell production, thereby improving the oxygen-carrying capacity of the blood.  This is why mountaineers wishing to climb to the peak of Mount Everest must complete the full climb in portions; it is recommended that climbers spend 2-3 days acclimatizing for every 600 metres of elevation increase.  In addition, the higher to altitude, the longer it make take to acclimatize; climbers are advised to spend 4-5 days acclimatizing at base camp (whether the base camp in Nepal or China) before completing the final leg of the climb to the peak.  The concentration of red blood cells gradually decreases to normal levels once a climber returns to their normal elevation.

Cultural Responses

More than any other species, humans respond to environmental stresses with learned behaviors and technology. These cultural responses allow us to change our environments to control stresses, rather than changing our bodies genetically or physiologically to cope with the stresses. Even archaic humans responded to some environmental stresses in this way. For example, Neanderthals used shelters, fires, and animal hides as clothing to stay warm in the cold climate in Europe during the last ice age. Today, we use more sophisticated technologies to stay warm in cold climates while retaining our essentially tropical-animal anatomy and physiology. We also use technology (such as furnaces and air conditioners) to avoid temperature stress and stay comfortable in hot or cold climates.

6.4 Summary

  • Humans may respond to environmental stress in four different ways: adaptation, developmental adjustment, acclimatization, and cultural responses.
  • An adaptation is a genetically based trait that has evolved because it helps living things survive and reproduce in a given environment. Adaptations evolve by natural selection in populations over a relatively long period to time. Examples of adaptations include sickle cell trait as an adaptation to the stress of endemic malaria and the ability to taste bitter compounds as an adaptation to the stress of bitter-tasting toxins in plants.
  • A developmental adjustment is a non-genetic response to stress that occurs during infancy or childhood, and that may persist into adulthood. This type of change may be irreversible. Developmental adjustment is possible because humans have a high degree of phenotypic plasticity. It may be the result of environmental stresses (such as inadequate food), which may stunt growth, or cultural practices (such as orthodontic treatments), which re-align the teeth and jaws.
  • Acclimatization is the development of reversible changes to environmental stress that develop over a relatively short period of time. The changes revert to the normal baseline state after the stress is removed. Examples of acclimatization include tanning of the skin and physiological changes (such as increased sweating) that occur with heat acclimatization.
  • More than any other species, humans respond to environmental stress with learned behaviors and technology, which are cultural responses. These responses allow us to change our environment to control stress, rather than changing our bodies genetically or physiologically to cope with stress. Examples include using shelter, fire, and clothing to cope with a cold climate.

6.4 Review Questions

  1. List four different types of responses that humans may make to cope with environmental stress.
  2. Define adaptation.
  4. Explain how natural selection may have resulted in most human populations having people who can and people who cannot taste PTC.
  5. What is a developmental adjustment?
  6. Define phenotypic plasticity.
  7. Explain why phenotypic plasticity may be particularly important in a species with a long generation time.
  8. Why may stunting of growth occur in children who have an inadequate diet? Why is stunting preferable to the alternative?
  9. What is acclimatization?
  10. How does acclimatization to heat come about, and what are two physiological changes that occur in heat acclimatization?
  11. Give an example of a cultural response to heat stress.
  12. Which is more likely to be reversible — a change due to acclimatization, or a change due to developmental adjustment? Explain your answer.

6.4 Explore More

Could we survive prolonged space travel? – Lisa Nip, TED-Ed, 2016.

How this disease changes the shape of your cells – Amber M. Yates, TED-Ed, 2019.

 

Attributions

Figure 6.4.1

Free_Awesome_Girl_With_Braces_Close_Up by D. Sharon Pruitt from Hill Air Force Base, Utah, USA on Wikimedia Commons is used under a  CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.

Figure 6.4.2

Tongue by Mahdiabbasinv on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 6.4.3

Figure 6.4.4

Scoliosis_patient_in_cheneau_brace_correcting_from_56_to_27_deg by Weiss H.R. from Scoliosis Journal/BioMed Central Ltd. on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 6.4.5

Figure 6.4.6

Nepalese base camp by Mark Horrell on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

References

TED-Ed. (2016, October 4). Could we survive prolonged space travel? – Lisa Nip. YouTube. https://www.youtube.com/watch?v=upp9-w6GPhU&feature=youtu.be

TED-Ed. (2019, May 6). How this disease changes the shape of your cells – Amber M. Yates. YouTube. https://www.youtube.com/watch?v=hRnrIpUMyZQ&feature=youtu.be

Weiss, H. (2007). Is there a body of evidence for the treatment of patients with Adolescent Idiopathic Scoliosis (AIS)? [Figure 2 – digital photograph],  Scoliosis, 2(19). https://doi.org/10.1186/1748-7161-2-19

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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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