Instructor’s Manual
1
,CHAPTER 1
Adaptation by Natural Selection
Key Concepts
The purpose of this chapter is to introduce and develop Darwin’s theory of natural selection, the
concept that underlies the rest of the textbook. We present complex adaptations as a phenomenon
that requires special explanation. We return to the example of the eye throughout the chapter.
1. Before Darwin, there was no adequate biological explanation for the existence of
adaptations; such complex designs in nature were used to prove the existence of God or
support the argument that all things that appear to be designed must have a creator.
2. Darwin’s theory of natural selection states that adaptation will result when these three
postulates hold: (1) there is a struggle for existence among organisms, (2) there is variation
among organisms in their ability to survive and reproduce, and (3) this variation is
transmissible from parents to offspring.
The simplicity of Darwin’s theory of natural selection makes the concept of adaptation
more tangible. Nevertheless, there are several problem areas for students. One is population
thinking. To make this concept more clear we use Peter and Rosemary Grant’s extensive work
on Darwin’s finches as an empirical demonstration of natural selection at work in living
populations. As an instructor, you should emphasize how Darwin’s three postulates are
exemplified by the directed, adaptive change in the population of finches. You should also
contrast essentialist thinking and population thinking for students by explaining that species are
not fixed categories but populations of variable individuals. Natural selection is responsible for
both stability (stabilizing selection) and change (directional selection) within species.
A second problem area for students is understanding the biological level at which
adaptations occur. We emphasize that natural selection acts on individuals and therefore leads to
adaptations at the level of the individual organism. Since individuals make up populations, we
see changes in populations over time. It is very important to emphasize that natural selection can
and does lead to adaptations that simultaneously benefit individuals but decrease the competitive
ability of groups—that is, traits that are beneficial for the individual but deleterious for the group
or species as a whole. Merely memorizing Darwin’s postulates does not usually allow students to
2
,come naturally to this conclusion.
3. Natural selection produces complex adaptations through a series of incremental steps, each
of which is an adaptive improvement over its immediate predecessor.
Having given empirical evidence of natural selection in the wild, we return to the
problem of the evolution of complex adaptations. Some still find it incredible that natural
selection can produce complex, functionally integrated designs like the eye. The basic problem is
that an eye could not be produced by natural selection in a single jump, because the probability
of such an occurrence is astronomically low. To illustrate this, we use Richard Dawkins’s
analogy of monkeys typing randomly and producing the sentence “Methinks it is like a weasel.”
The essential point to make is that natural selection cannot produce complex adaptations in a
single generation, but natural selection can be very powerful in producing complex adaptations if
successive approximations are retained.
This insight, however, leaves us with the dilemma that each intermediate step must be
favored by selection. We use the eyes of living mollusks to illustrate potential benefits of each
small step in the evolution of the eye—from a single light-sensitive cell to the fully functional
eye.
4. Several lines of evidence suggest that natural selection can produce complex changes in
remarkably short periods of time. First, using artificial selection, people can intentionally
and unintentionally create dozens of different forms of a species in just a few hundred or a
few thousand years (for example, domesticated dogs and pigeons). Second, a recent study of
fish from the genus Poeciliopsis shows that three different types of placenta evolved in this
genus in less than 2.4 million years. Third, theoretical studies of the evolution of the eye in
an aquatic organism reveal that the complete structure of the eye can evolve from 1,829
changes of 1% each. Therefore, in an organism with a short generation time, a complex eye
could evolve in less than a million years.
5. Natural selection requires variation. Darwin’s difficulty with his theory of natural selection
occurred because the blending theory of inheritance prevalent at the time was incompatible
with the maintenance of variation.
Darwin’s difficulties concerning the maintenance of variation are solved with a modern
understanding of inheritance, which is dealt with in the next chapter. Once students have a basic
3
, understanding of genetics, natural selection and modern genetics are wed in Chapter 3.
Answers to Study Questions
1. It is sometimes observed that offspring do not resemble their parents for a particular
character, even though the character varies in the population. Suppose this were the case for
beak depth in the medium ground finch. (a) What would the plot of offspring beak depth
against parental beak depth look like? (b) Plot the mean depth in the population among (i)
adults before a drought, (ii) the adults that survived a year of drought, and (iii) the offspring
of the survivors.
(a) Students should plot parents (x axis) and offspring (y axis) on the graph. It may be
useful to first show students graphic examples of positive and negative correlations. Then
students should graph the hypothetical situation, where no adaptation-based correlation exists
between parents and offspring. The scatter plot should be spread out randomly over the graph.
4
1
,CHAPTER 1
Adaptation by Natural Selection
Key Concepts
The purpose of this chapter is to introduce and develop Darwin’s theory of natural selection, the
concept that underlies the rest of the textbook. We present complex adaptations as a phenomenon
that requires special explanation. We return to the example of the eye throughout the chapter.
1. Before Darwin, there was no adequate biological explanation for the existence of
adaptations; such complex designs in nature were used to prove the existence of God or
support the argument that all things that appear to be designed must have a creator.
2. Darwin’s theory of natural selection states that adaptation will result when these three
postulates hold: (1) there is a struggle for existence among organisms, (2) there is variation
among organisms in their ability to survive and reproduce, and (3) this variation is
transmissible from parents to offspring.
The simplicity of Darwin’s theory of natural selection makes the concept of adaptation
more tangible. Nevertheless, there are several problem areas for students. One is population
thinking. To make this concept more clear we use Peter and Rosemary Grant’s extensive work
on Darwin’s finches as an empirical demonstration of natural selection at work in living
populations. As an instructor, you should emphasize how Darwin’s three postulates are
exemplified by the directed, adaptive change in the population of finches. You should also
contrast essentialist thinking and population thinking for students by explaining that species are
not fixed categories but populations of variable individuals. Natural selection is responsible for
both stability (stabilizing selection) and change (directional selection) within species.
A second problem area for students is understanding the biological level at which
adaptations occur. We emphasize that natural selection acts on individuals and therefore leads to
adaptations at the level of the individual organism. Since individuals make up populations, we
see changes in populations over time. It is very important to emphasize that natural selection can
and does lead to adaptations that simultaneously benefit individuals but decrease the competitive
ability of groups—that is, traits that are beneficial for the individual but deleterious for the group
or species as a whole. Merely memorizing Darwin’s postulates does not usually allow students to
2
,come naturally to this conclusion.
3. Natural selection produces complex adaptations through a series of incremental steps, each
of which is an adaptive improvement over its immediate predecessor.
Having given empirical evidence of natural selection in the wild, we return to the
problem of the evolution of complex adaptations. Some still find it incredible that natural
selection can produce complex, functionally integrated designs like the eye. The basic problem is
that an eye could not be produced by natural selection in a single jump, because the probability
of such an occurrence is astronomically low. To illustrate this, we use Richard Dawkins’s
analogy of monkeys typing randomly and producing the sentence “Methinks it is like a weasel.”
The essential point to make is that natural selection cannot produce complex adaptations in a
single generation, but natural selection can be very powerful in producing complex adaptations if
successive approximations are retained.
This insight, however, leaves us with the dilemma that each intermediate step must be
favored by selection. We use the eyes of living mollusks to illustrate potential benefits of each
small step in the evolution of the eye—from a single light-sensitive cell to the fully functional
eye.
4. Several lines of evidence suggest that natural selection can produce complex changes in
remarkably short periods of time. First, using artificial selection, people can intentionally
and unintentionally create dozens of different forms of a species in just a few hundred or a
few thousand years (for example, domesticated dogs and pigeons). Second, a recent study of
fish from the genus Poeciliopsis shows that three different types of placenta evolved in this
genus in less than 2.4 million years. Third, theoretical studies of the evolution of the eye in
an aquatic organism reveal that the complete structure of the eye can evolve from 1,829
changes of 1% each. Therefore, in an organism with a short generation time, a complex eye
could evolve in less than a million years.
5. Natural selection requires variation. Darwin’s difficulty with his theory of natural selection
occurred because the blending theory of inheritance prevalent at the time was incompatible
with the maintenance of variation.
Darwin’s difficulties concerning the maintenance of variation are solved with a modern
understanding of inheritance, which is dealt with in the next chapter. Once students have a basic
3
, understanding of genetics, natural selection and modern genetics are wed in Chapter 3.
Answers to Study Questions
1. It is sometimes observed that offspring do not resemble their parents for a particular
character, even though the character varies in the population. Suppose this were the case for
beak depth in the medium ground finch. (a) What would the plot of offspring beak depth
against parental beak depth look like? (b) Plot the mean depth in the population among (i)
adults before a drought, (ii) the adults that survived a year of drought, and (iii) the offspring
of the survivors.
(a) Students should plot parents (x axis) and offspring (y axis) on the graph. It may be
useful to first show students graphic examples of positive and negative correlations. Then
students should graph the hypothetical situation, where no adaptation-based correlation exists
between parents and offspring. The scatter plot should be spread out randomly over the graph.
4
