Chapter 7 Human Variation

Overview

The human species is highly variable in every biological trait. Humans vary in their physiology, body proportions, skin color, and body chemicals. Many of these features influence susceptibility to disease and other forces of natural selection. Continued selection over time has produced adaptations of local populations to the environments in which they live. Much of human biological variation is geographic; that is, there are differences between population groups from different geographical areas. For example, northern European peoples differ in certain ways from those from eastern Africa, and those from Japan differ in some ways from those from the mountains of Peru. Between these populations, however, lie many other populations that fill in all degrees of variation between the populations we have named, and there is also a lot of variation within each of these groups.

Central to the study of human variation is the concept of a biological population, as defined in Chapter 5, p. 000 and as explained again below. Both physical features and genotypes vary from one person to another within populations, but there is also a good deal of variation between human populations from different geographic areas as the result of evolutionary processes. How do populations come to differ from one another? How do alleles spread through populations? How do environmental factors such as infectious diseases influence the spread? Why are certain features more common in Arctic populations and other features more common in tropical populations? Why have we traditionally thought of some of these variations as defining ‘races’? These are some of the questions that are explored in this chapter.

Chapter Outline

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Review Questions

THE PURPOSE of these review quizzes is to guide students in where their knowledge and understanding is strong, where it is weak, and where time should best be spent in studying.

CHAPTER 7:

  1. How do the several historical concepts of ‘race’ differ from one another?  What evidence sheds light on the shortcomings of each concept?  (If you cannot explain these things, or if you are not sure, then you need to reread Section 7.1.)
  2. How does the study of population genetics help us explain human variation?  What are the major findings regarding variation within populations, variation between populations, and the possibility of finding abrupt boundaries that will clearly separate groups of populations from one another?  What is genetic drift, and how is it best explained?     (If you cannot explain this, or if you are not sure, then you need to reread Section 7.2.)
  3. Why is malaria an important agent of selection?  How do the several forms of hemoglobin influence the shape of red blood cells?  Explain at least three genetically controlled traits that can influence one’s resistance to malaria or other major diseases.  (If you cannot explain these things, or if you are not sure, then you need to reread Section 7.3.)
  4. How do Bergmann’s and Allen’s rules influence selection for different human sizes and shapes across the globe?  How is sunlight an agent of selection, and what are its consequences over long spans of time?  (If you cannot explain these things, or if you are not sure, then you need to reread Section 7.4.)

Open Response Study Questions

These questions are designed to assess your understanding of the topics explored in this chapter. You can use these questions in three ways:

Before you start …

Read through the questions before you read the chapter to help prime you to read the text more carefully and strategically. Remember that you are just starting out on your learning journey, so don’t feel disheartened if you don’t know how to answer them yet!

Whilst you read …

As you work through the chapter, have another go at answering the questions to see how you are progressing. You can also answer the questions with the textbook open in front of you, in order to create model answers that can be used to refer back to later.

At the end …

Answer the questions once you have finished reading to see what you have learned. Check your responses against your model answers and use these to identify any gaps in your understanding.

DEFINITIONS:

In your own words, define ANY TWO of the following terms:

population                   Allen’s rule      balanced polymorphism            Hardy-Weinberg principle

codominant alleles      Gloger’s rule   cline    continuous variation  

balanced polymorphism

ESSAYS:

Answer any two of the following questions.  Make sure to answer all parts of any question you choose.

1. Select TWO of the following conditions:  sickle-cell anemia, thalassemia, G6PD deficiency.  For each of the two conditions you have chosen, describe and explain  (A) symptoms and consequences of the condition;  (B) geographic variation in the frequency of the condition;  (C) how natural selection acts upon the condition in different places; and (D) the reasons why the condition persists in human populations.

2. Explain the how natural selection has made human skin color darker, on average, in some places and lighter, on average, in other places by the operation of major opposing tendencies. In your answer, explain the reasons for the advantage of lighter pigmentation in some places and darker pigmentation in other places.

3. Explain the several concepts of race that have been used in the past, and how these concepts differ from one another.

4. Explain how the Dunkers of Pennsylvania and nearby states illustrate how genetic drift works.

5. Explain the several ways in which the concept of race has been used historically, as well as the ways in which biologists understand the concept today.

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Chapter 8
Nutrition, Circulation, and Health