Download Chapter 5 Evolution Matters: Human Variation Today

Chapter 5
Evolution Matters: 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 genetic factors and
environmental factors. Physical anthropologists employ such an understanding in a variety of
ways. For instance, human life history stages are unique in comparison with those of other
primate species, and the relationship between our unique life history and sociocultural behaviors
is an area of interest. Additionally, human populations across the globe vary in the ways that
homeostasis 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 differential
access to key nutrients—is a major contributor to the success of our species as a survivor and
reproducer 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 factors 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. Among humans, females are more highly buffered against environmental insults than
males in early life, and females enter into the life history stages of puberty, adolescence,
and reproductive senescence earlier than males. Discuss why such features are
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 offspring 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 fitness-related 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 long-term 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, high-calorie 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 overweight and obesity in the present.