The Theory of Evolution by Natural Selection A little more than seven years after his voyage on the Beagle, Darwin put together a workable theory on the origin of species by natural selection. In a letter to Hooker he said, “At last gleams of light have come, and I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable…. I think I have found out…the simple way by which species become exquisitely adapted to various ends.” In 1844, Darwin gave his wife the 213-page manuscript in which he had outlined his theory, so that it might be published if he were to die suddenly. Aware that his ideas would be controversial, Darwin continued to gather evidence and look for flaws in his reasoning rather than make the theory public. By June 1858, he had written a quarter of a million words toward a major treatise when he received a letter from Alfred Russell Wallace, a brilliant naturalist working in Malaysia. Wallace had independently arrived at the same conclusions as Darwin and described his own theory in the letter. It was an odd moment in history. Wallace, with much less supporting evidence, was anxious to present the new theory he had written in just two days, while Darwin, having spent 20 years amassing evidence in support of the same theory, had not been willing to publish. Learning of the situation, Lyell and Hooker urged Darwin to submit a paper along with Wallace’s at the next meeting of the Linnaean Society. Their theory was presented jointly on July 1, 1858. A year and a half later, Darwin published a condensed version of his treatise, entitled On the Origin of Species by Means of Natural Selection or The Preservation of Favoured Races in the Struggle for Life. It sold out on the first day. The response in society was as dramatic as predicted (Figure 1).

EXPLORE an issue

Question: Are science and scientists subject to societal influences?

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Research the life of one of these individuals. You will role-play this person at the gathering described above. In the role-play, include a summary of his or her ideas, the circumstances that most influenced his or her life, and the ways in which he or she influenced others’ thinking, particularly Darwin’s. As you

Figure 1 Darwin was often vilified by the press.

Decision-Making Skills Define the Issue Defend the Position

Role-Play: Science in Society The scientists in this chapter were influenced in their thinking by their social upbringing and by public opinion, as well as by their peers in the scientific community. Imagine a gathering at the home of Charles and Emma Darwin of the following individuals (some of whom would have been long dead by Darwin’s time): Leonardo da Vinci, Georges Cuvier, Charles Lyell, Georges Buffon, Jean Lamarck, William Thomson Kelvin, Erasmus Darwin, Captain Robert FitzRoy, Thomas Huxley, Samuel Wilberforce, Frederick Temple, Alfred Russell Wallace, and Emma Darwin.

11.6

Analyze the Issue Identify Alternatives

Research Evaluate

research print and electronic sources, consider social beliefs and customs and their influence on scientific thinking.

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In a group, briefly outline the life story of the individual you will be role-playing. Describe this person’s beliefs about the origin of life, Earth’s history, and the idea of evolution—indicating whether this individual’s beliefs changed over time. Was this person’s thinking similar to or different from others’ in the scientific community? How did his or her views compare with public opinion? In what specific way(s) did this person or his or her ideas influence Darwin in his development of the theory of evolution? Use this information to explain your answer to the question and for your role-play.

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Natural Selection Darwin summarized natural selection, the mechanism for his theory of evolution, in these words: “…can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations I call Natural Selection.” Darwin’s theory of natural selection, based on some basic observations and inferences, is outlined in Table 1. Table 1 The Theory of Evolution by Natural Selection Observation 1

Individuals within a species vary in many ways.

Observation 2

Some of this variability can be inherited.

Observation 3

Every generation produces far more offspring than can survive and pass on their variations.

Observation 4

Populations of species tend to remain stable in size.

Inference 1

Members of the same species compete with each other for survival.

Inference 2

Individuals with more favourable variations are more likely to survive and pass them on. Survival is not random.

Inference 3

As these individuals contribute proportionately more offspring to succeeding generations, the favourable variations will become more common. (This is natural selection.)

Given the extensive examples from Darwin’s observations of natural populations, as well as his experimentation and the experience of plant and animal breeders, Darwin’s theory seemed very difficult to refute. Clearly species did exhibit inherited variations, which were obvious even to casual observers. The fact that all species have a high rate of reproduction was also self-evident. Although the idea of evolution was not new, there had been no accepted explanation for how evolution occurred until Darwin and Wallace introduced the mechanism of natural selection. Darwin offered detailed evidence and examples in support of his theory and, although his ideas were hotly debated in public after the release of his book, many scientists were swayed by his compelling arguments. He had explained his theory so simply and clearly that his friend Thomas Huxley, after reading it, exclaimed, “How extremely stupid not to have thought of it myself!”

Practice Understanding Concepts 1. Propose an evolutionary scenario in which a species of ancient chameleons could

evolve into a species with an unusually long tongue (Figure 2). Keep in mind the two key elements required for natural selection: inherited variation and an environment that favours certain traits over others.

Figure 2

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Section 11.6

Unanswered Questions Even with substantive scientific evidence in support, the theory of evolution by natural selection had many opponents. Some genuine difficulties still needed to be overcome. For natural selection to result in the evolution of entirely new species with new adaptations and structures, a great length of time was required. Not everyone shared the view that Earth was many millions of years old. And despite the unearthing of many deep fossil beds containing extinct life forms that were suggestive of an ancient past, the limited fossil pelvic record in Darwin’s and Lyell’s day held many gaps. Further, as patella girdle opponents pointed out, the fossil record showed no transitional femur forms from ancient to more modern species. Not until radiometric dating technology was available could scientists provide fibula conclusive evidence that life on Earth began at least 3.5 billion tibia years ago—ample time for Darwinian natural selection to work. Fossil records now are much more extensive and many of the gaps have been filled, in particular for intermediate species of vertetoes Hind limb of Basilosaurus brates. In recent years, palaeontologists have unearthed 50-millionyear-old fossils of whale ancestors with fully functioning large Figure 3 hind limbs and 38-million-year-old ancestors that retain tiny Basilosaurus, an ancient relative of nonfunctional hind limbs (Figure 3). the modern whale, had rudimentary The role of environmental change, too, presented scientists with opportunities for hind limbs. investigation of variations in species. In England in the early 1800s, light-coloured lichen covered many surfaces, providing camouflage for light-coloured forms of the peppered moth (Biston betularia). But the growing Industrial Revolution changed the English countryside in many ways; the production of pollutants blanketed some areas in soot, which offered new opportunities for some species to camouflage themselves from predators. In 1848, near Manchester, the first appearance of a melanic (almost black) form of peppered moth was recorded. By the late 1800s, much of the lichen was dead and the trees, rocks, and other hard surfaces were covered in dark soot. By the 1920s, the moth population was almost entirely of melanic individuals. Scientists suspected that the melanic moths had gained a selective advantage. INVESTIGATION 11.6.1 A more daunting challenge to the theory proposed by Darwin and Wallace was the Industrial Melanism and the puzzle of variation. Many scientists were persuaded by the evidence that evolution was Peppered Moth (p. 536) How might camouflage offer species occurring but doubted the mechanism that these two scientists proposed. Although they a selective advantage? In this invesadmitted that species demonstrated subtle variations, many questioned how apparently tigation, you can conduct simulainsignificant variations could ever lead to the production of an entirely new structure, tions to model and investigate the such as an eye or a wing, where none had existed before. At the time, it was widely effects of camouflage on predator believed that inherited traits from each parent were somehow blended in offspring and, success. as a result, it was thought that unusual or rare variations would become diluted over time rather than becoming prominent. Even Darwin accepted these assumptions and openly admitted that this was the greatest weakness in his theory. Darwin and his supporters could offer no explanation for the source of new variations. As it turned out, a contemporary scientist had the beginnings of the answer. Just six years after Darwin’s publication of On the Origin of Species, Gregor Mendel presented and, in 1866, published his investigations of inheritance in garden peas. Unfortunately, his work was largely ignored until 1900. Although Mendel and others provided a foundation for an understanding of inheritance, discoveries in modern molecular genetics have revealed that mutation and recombination provide the source for new inheritable variations. As you will learn in Chapter 12, they also offer the richest and most compelling evidence for biological evolution of life on Earth. NEL

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TRY THIS activity

Evolutionary Crunch

Figure 4 presents a simplified model of a hypothetical evolutionary scenario for the evolution of larger claws in crabs over time. The questions below model Darwin’s thinking.

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Examine Figure 4. Assume that any observed variations can be inherited. (a) Describe the original crab population as shown in Figure 4(a). What trait shows variation? (b) Crabs produce many offspring. Considering the population size from Figures 4(a)–(d), what must be happening to many of the offspring?

(c) Are the crabs competing? Explain. (d) From your observations of Figures 4(a) and (b), how does claw size influence competition? (e) What overall change is happening to the population, as depicted in Figure 4(c)? Is your answer an inference or an observation? Explain. (f) What do you notice about the claw sizes in Figure 4(d)? This was a problem for Darwin’s theory, as he had no satisfactory explanation for the appearance of new traits. Where do you think this new variation came from?

(a)

(b)

(c)

(d) Figure 4 The evolution of large claws in crabs

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Section 11.6

SUMMARY

Natural Selection as the Mechanism for Evolution

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Darwin and Wallace independently developed the theory that the mechanism causing evolution was natural selection.

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According to their theory, all species exhibit inheritable variations that are selected through the struggle by individuals for survival in competition within their population. Individuals with more favourable traits produce more offspring that survive than others and pass along those favourable traits. Over many generations, this process results in a change in the inheritable traits of the population.

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When natural selection was first proposed, scientists had no way to date rocks or fossils with precision, little fossil evidence for species in transition from ancient to modern, and no understanding of the genetic basis of inheritance and variation.

Section 11.6 Questions Understanding Concepts

Applying Inquiry Skills

1. Assess Darwin’s contribution to the development of scien-

tific ideas about the origin of life. What aspects of the controversy did his theory and evidence not satisfy?

8. Develop reasoning to explain how an ancestor with rela-

tively short legs and neck might have evolved into the present-day great blue heron (Figure 6).

2. It is evident by his comments that Thomas Huxley did not

find Darwin’s new theory difficult to understand. Explain how the theory, although of great significance, is a rather simple one. 3. Describe the concept of “blended inheritance,” which

Darwin himself believed to be valid. Provide examples that appear to demonstrate the blending of inherited features. 4. According to Darwinian evolution, there must be variation

and selection. In the evolution of large claws in lobsters, (a) what trait(s) might have been variable? (b) what factors might have resulted in members of the population being selected? 5. Speculate about why predatory cats such as the lion and

Figure 6 Great blue heron

the leopard have not evolved to be as fast as the cheetah. 6. The elephant has evolved to be a great size, while the mouse

has evolved to be relatively small. Explain how natural selection might favour a different size in each mammal species. 7. Study Figure 5. The peppered moth caterpillar avoids

detection by extending its body out from a branch and remaining still. Explain how mimicry also contributes to natural selection.

9. Cheetahs have so little genetic variation from individual to

individual that even skin grafts among unrelated individuals from the South African subspecies are not rejected. Based on the theory developed by Darwin and Wallace, how might this influence the evolution of this species? Making Connections 10. Find out through print and electronic sources the many

contributions to science of Alfred Russell Wallace. Describe some of the personal, social, and financial challenges he faced in conducting his investigations.

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11. In an 1844 letter to his friend Hooker, Darwin noted that

Figure 5 The peppered moth caterpillar

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suggesting species are not immutable was like “confessing a murder.” Why would Darwin have felt this way? Relate your response to public reaction to On the Origin of Species.

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