Chapter 9 Populations and Reproduction

Overview

Imagine a world where people must share a room with 4 to 12 others. A room of one’s own is a rare luxury. In fact, people who have any housing at all consider themselves fortunate, because so many people have none. Drinking water is in short supply each summer, and overworked sewer systems are breaking down all the time; many millions have no sewer system at all. Jobs are scarce, and well-paying jobs are almost unheard of. Beggars crowd every street, and each garbage can is searched through time after time by starving people looking for something to sustain them.

Some parts of the world already experience these conditions. Some experts predict that a future like this may be in store for all of us unless something is done soon, and on a massive scale, to control population growth.

The Earth has recently undergone the most rapid population increase in all of human history. From 2.5 billion people in 1950, the total world population more than doubled to 6 billion in 1999 (Figure 9.1). As of 2020, the United Nations estimates that the world’s population increases each year by about 86 million people, a rate which would double the world’s population again in about 64 years.  The U.N. also estimates that the growth rate of the world’s population has begun to slow down a bit, and they predict that the total world population will stabilize at about 11 billion people (plus or minus 1.5 billion) by the year 2100.

In this chapter we consider the factors that control the size and the rate of growth of populations, including the biological controls on populations that operate independently of any conscious planning. The ecological principles that we discuss include models of population growth, limits to growth, and some of the consequences of excessive growth. Although we focus mainly on human populations, these ecological principles apply equally well to populations of other organisms.

As with many of the other issues in this book, population growth cannot be looked at as a purely biological issue. There are political, religious, and ethical dimensions to population growth and its control, and so we consider some of these factors as well. In addition, even in the face of a global population increase, there has also been increasing research on the biology of infertility, in part because an increasing number of couples who are not able to conceive their own children seek these reproductive therapies. Both global population growth and individual fertility and infertility raise ethical questions. We have seen in Chapter 1 that the boundaries between the individual good and the social good are one of the subjects of ethics. Biology can inform ethical debate by assessing, for different scenarios, the biological risks to the individual and to society.

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 9:

  1. How does exponential growth differ from logistic growth?  How does K-selection differ from r-selection, and what characterizes the species that exhibit either one?  What predictions did Malthus make, and on what did he base his predictions?  (If you cannot explain these things, or if you are not sure, then you need to reread Section 9.1.)
  2. What determines sex in mammals?  What determines sexual development?  How is the menstrual cycle controlled, and which hormones are influential at each stage?  What are at least three methods by which reproductive problems can be overcome or bypassed?   (If you cannot explain this, or if you are not sure, then you need to reread Section 9.2.)
  3. How are population pyramids constructed?  What does the age distribution of a population reveal about its likely rate of increase?  What is a demographic transition?  What is demographic momentum, and how can it be detected at the population level?   (If you cannot explain these things, or if you are not sure, then you need to reread Section 9.1.)
  4. Describe at least five methods of birth control that act before fertilization.  How does each method work, and what costs, risks, and benefits are associated with each?  Describe at least three methods acting after fertilization, and the costs, risks, and benefits of each.  What do studies show about the impact of the education of women on population control, including the costs and benefits?  (If you cannot explain these things, or if you are not sure, then you need to reread Section 9.3.)

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:

HPV      Myometrium             Positive checks           Mifepristone

ESSAYS:

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

1. Explain the concept of a demographic transition, including descriptions of its several stages.

2. Describe at least five methods of birth control, including the positive and negative features of each.

3. Explain the interactions of FSH, LH, LTH, estrogen, and progesterone, and their effects on the lining of the uterus.

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Chapter 10
Stem Cells, Cell Division and Cancer