Chapter 5
Evolution Review: Human Variation Today
Synopsis: The biocultural approach of physical anthropology emphasizes that human evolution
and variation are shaped by both biology and culture, that is, by both genetic factors and
environmental factors. Physical anthropologists apply this concept in various ways. For instance,
the relationship between the unique life history stages of humans and our sociocultural behaviors
is an area of interest. Additionally, human populations across the globe vary in the ways that
homeostasis (physiological equilibrium) is maintained, and such adaptations are a product of
evolutionary processes operating over a wide range of environmental settings. The broad range
of human adaptability—to life in hot and cold climates, among extremes of latitude and altitude,
and with variable access to key nutrients—is a major contributor to the success of our species:
humans survive and reproduce across a diverse array of environments worldwide.
1. Define Bergmann’s rule and Allen’s rule in relation to climate adaptation. What is one
example of an environmental factor that can lead to the development of body-shape
characteristics that deviate from those predicted by these two rules?
Bergmann’s and Allen’s rules are general principles that explain the relationship between body
shape and adaptations to climate. Bergmann’s rule states that in mammals, including humans,
body size increases as populations move farther away from the equator. That is, a larger, stockier
build is adaptive in colder climates, while a smaller, more slender physique is adaptive in
warmer climates. Allen’s rule is similar to Bergmann’s, but with specific reference to limb size.
Limbs are shorter and stockier in colder, high-latitude environments and longer and more slender
in warmer, low-latitude environments. In combination, these two general rules relate body
proportions to thermoregulatory adaptations (i.e., heat dissipation and retention). Some examples
of factors that can lead to deviations from body proportions expected under Bergmann’s and
Allen’s rules include malnutrition and chronic disease stress during critical periods of growth
and development, as well as an earlier age at menarche in females.
2. What is clinal variation? Identify two physiological effects of solar radiation that likely acted
as selective pressures in shaping the pattern of clinal variation observed for human skin color.
For many biological traits, humans exhibit a pattern of clinal variation, or geographically
continuous variation in which some phenotypic characteristic gradually changes from one
population to the next. A prime example of clinal variation in humans is the relationship between
skin color and latitude. Variation in skin color is influenced by the competing needs of the body
to absorb UV radiation for synthesizing vitamin D (a steroid hormone produced in the skin that
regulates calcium absorption and mineralization of the skeleton) and to protect the body from
overexposure to UV radiation that can lead to deficiencies in folate (a vitamin essential for DNA
synthesis and repair, especially during cell division). At low latitudes, where exposure to UV
radiation is high, darker skin pigmentation is adaptive, as it allows for increased protection
against folate deficiencies. At high latitudes, with less exposure to UV radiation, lighter skin
pigmentation is adaptive, as it allows for sufficient vitamin D synthesis.
3. In early life, human females are more highly buffered against environmental stresses than are
human males. In addition, human females enter puberty, adolescence, and reproductive
senescence earlier than human males do. Why are such features advantageous from an
evolutionary standpoint? (Hint: Focus on implications for fitness, or reproductive success.)
The idea that females are more highly buffered against environmental insults compared to males
can be explained by the greater investment of females in reproduction compared to males.
Buffering against environmental stressors during critical periods of female growth and
development increases the chances that these females will be able to support the heightened
demands of pregnancy and lactation during their reproductive years, when their energetic
expenditure in producing off spring is much greater than that of their male counterparts. Entering
into the life history stages of puberty and adolescence at an earlier age increases female
reproductive fitness by maximizing the length of the overall reproductive period prior to
menopause. Undergoing reproductive senescence at an earlier age can also provide fitnessrelated benefits, as postmenopausal women often play important roles in caring for their
grandchildren (e.g., providing food, protection, and key social information), thereby increasing
their own fitness as well (i.e., the grandmother hypothesis).
4. In what ways have each of the four types of adaptation discussed in this chapter contributed to
the ability of human populations to inhabit areas spanning the entire surface of the globe?
Consider various factors including climate and nutrition. (Hint: Think about an extreme
geographical setting. How might the ways that a resident maintains homeostasis differ from the
ways a visitor maintains homeostasis?)
Humans display a very high degree of adaptive flexibility, and different populations exhibit
different types of genetic adaptations (e.g., to climate) and developmental adaptations (e.g., to
altitude), which allow for longterm survival across a wide array of environmental settings.
Physiological adaptations allow individuals experiencing new or extreme environmental
stressors to maintain internal homeostasis over the short term, and cultural adaptations can
further buffer individuals from environmental stressors, allowing for long- term survival in
extreme environmental settings, to which an individual may not be genetically or
developmentally adapted.
As an example, consider differences in the ways that a resident of a high-altitude community
might maintain homeostasis compared to a short-term visitor. The resident will likely exhibit
genetic adaptations (e.g., alleles for high oxygen saturation in hemoglobin) and developmental
adaptations (e.g., barrel chests and increased lung volume), allowing for increased acquisition
and more efficient use of the limited oxygen supply over the long term. A short-term visitor to a
high-altitude setting will gradually become acclimatized, through the production of more red
blood cells and vasodilation—that is, an expansion in the diameter of arteries and veins allowing
for increased blood flow and access to oxygen-saturated hemoglobin. The visitor may also
require cultural adaptations to maintain homeostasis (and possibly to survive), such as the use of
oxygen tanks or medical supplies should they develop severe hypoxia and mountain sickness
(nausea, loss of appetite, fatigue, and shortness of breath).
5. Many scientists characterize the conditions of modern populations worldwide (but especially
Western societies) as conducive to an obesity pandemic. Discuss the ways in which adaptive
features from our evolutionary past may have become modern maladaptations. (Hint: Think
about the differences in nutritional resources and activity patterns between ourselves and our
hominin ancestors.)
Much of human evolutionary history is characterized by a hunting and gathering subsistence
pattern, in which our ancestors were limited only by the availability of food resources across the
landscape and the technologies available for acquiring them. Under such circumstances,
behavioral and physiological traits that allowed for maximizing the acquisition and storage of
nutritional energy conferred an adaptive advantage. In times of plenty, our evolutionary
ancestors acquired and consumed a great amount of caloric energy, with excess calories stored in
the adipose (fat) tissue. In times of food shortage, their bodies drew upon these stored calories in
order to meet energy requirements and ensure survival. However, in our modern environment
(and especially among Western societies), characterized by “supersized” food portions, highcalorie and high-fat foods, a relatively lower threat of food shortage, and a generally sedentary
lifestyle compared to that of our ancestors, these once-adaptive characteristics are contributing to
an increased prevalence of extra weight and obesity in the present.