CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
13. Evolution and Natural Selection
13.2. The Development of Evolutionary Thought
For centuries, people believed that the various species of plants and animals were unchanged from the time of their creation. Although today we know this is not true, we can understand why people may have thought it was true. Because they knew nothing about DNA, meiosis, genetics, or population genetics, they did not have the tools to examine the genetic nature of species. Furthermore, the process of evolution is so slow that the results of evolution are usually not recognized during a human lifetime. It is even difficult for modern scientists to recognize this slow change in many kinds of organisms.
Early Thinking About Evolution
In the mid-1700s, Georges-Louis Buffon, a French naturalist, wondered if animals underwent change (evolved) over time. After all, if animals didn’t change, they would stay the same, and it was becoming clear from the study of fossils that changes had occurred. However, Buffon didn’t come up with any suggestions on how such changes might come about. In 1809, Jean-Baptiste de Lamarck, a student of Buffon’s, suggested a process by which evolution might occur. He proposed that acquired characteristics were transmitted to offspring. Acquired characteristics are traits gained during an organism’s life and not determined genetically. For example, he proposed that giraffes originally had short necks but because they constantly stretched their necks to get food, their necks got slightly longer (figure 13.2). When these giraffes reproduced, their offspring acquired their parents’ longer necks. Because the offspring also stretched to eat, the third generation ended up with even longer necks. And so Lamarck’s story was thought to explain why the giraffes we see today have long necks. Although we now know Lamarck’s theory was wrong (because acquired characteristics are not inherited), it stimulated further thought as to how evolution might occur. From the mid-1700s to the mid-1800s, lively arguments continued about the possibility of evolutionary change. Some, like Lamarck and others, thought that change did take place; many others said that it was not even possible. It was the thinking of two English scientists that finally provided a mechanism for explaining how evolution occurs.
FIGURE 13.2. The Contrasting Ideas of Lamarck and the Darwin-Wallace Theory
(a) Lamarck thought that acquired characteristics could be passed on to the next generation. Therefore, he postulated that, as giraffes stretched their necks to get food, their necks got slightly longer. This characteristic was passed on to the next generation, which would have longer necks. (b) The Darwin-Wallace theory states that there is variation within the population and that those with longer necks are more likely to survive and reproduce, passing on their genes for long necks to the next generation.
The Theory of Natural Selection
In 1858, Charles Darwin and Alfred Wallace suggested the theory of natural selection as a mechanism for evolution. The theory of natural selection is the idea that some individuals whose genetic combinations favor life in their surroundings are more likely to survive, reproduce, and pass on their genes to the next generation than are individuals who have unfavorable genetic combinations. This theory was clearly set forth in 1859 by Darwin in his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life (How Science Works 13.1).
The theory of natural selection is based on the following assumptions about the nature of living things:
1. All organisms produce more offspring than can survive.
2. No two organisms are exactly alike.
3. Among organisms, there is a constant struggle for survival.
4. Individuals that possess favorable characteristics for their environment have a higher rate of survival and produce more offspring.
5. Favorable characteristics become more common in the species, and unfavorable characteristics are lost.
Using these assumptions, the Darwin-Wallace theory of evolution by natural selection offers a different explanation for the development of long necks in giraffes (figure 13.2b):
1. In each generation, more giraffes would be born than the food supply could support.
2. In each generation, some giraffes would inherit longer necks, and some would inherit shorter necks.
3. All giraffes would compete for the same food sources.
4. Giraffes with longer necks would obtain more food, have a higher survival rate, and produce more offspring.
5. As a result, succeeding generations would show an increase in the number of individuals with longer necks.
Modern Interpretations of Natural Selection
The logic of the Darwin-Wallace theory of evolution by natural selection seems simple and obvious today, but at the time Darwin and Wallace proposed their theory, the processes of meiosis and fertilization were poorly understood, and the concept of the gene was only beginning to be discussed. Nearly 50 years after Darwin and Wallace suggested their theory, the rediscovery of the work a monk, Gregor Mendel (see chapter 10) provided an explanation for how characteristics could be transmitted from one generation to the next. Mendel’s concepts of the gene explained how traits could be passed from one generation to the next. It also provided the first step in understanding mutations, gene flow, and the significance of reproductive isolation. All of these ideas are interwoven into the modern concept of evolution. If we update the five basic ideas from the thinking of Darwin and Wallace, they might look something like the following:
1. An organism’s ability to overreproduce results in surplus organisms.
2. Because of mutation, new, genetically determined traits enter the gene pool. Because of sexual reproduction, involving meiosis and fertilization, new genetic combinations are present in every generation. These processes are so powerful that each individual in a sexually reproducing population is genetically unique. The genetic information present is expressed in the phenotype of the organism.
3. Resources, such as food, soil nutrients, water, mates, and nest materials, are in short supply, so some individuals do without. Other environmental factors, such as disease organisms, predators, and helpful partnerships with other species, also affect survival. All the specific environmental factors that affect survival by favoring certain characteristics are called selecting agents.
4. Selecting agents favor individuals with the best combination of alleles—that is, those individuals are more likely to survive and reproduce, passing on more of their genes to the next generation. An organism is selected against if it has fewer offspring than other individuals that have a more favorable combination of alleles. The organism does not need to die to be selected against.
5. Therefore, alleles or allele combinations that produce characteristics favorable to survival become more common in the population and, on the average, the members of the species will be better adapted to their environment.
Evolution results when there are changes in allele frequency in a population. Recall that individual organisms cannot evolve—only populations can. Although evolution is a population process, the mechanisms that bring it about operate at the level of the individual.
Recall that a theory is a well-established generalization supported by many kinds of evidence. The theory of natural selection was first proposed by Charles Darwin and Alfred Wallace. Since the time it was first proposed, the theory of natural selection has been subjected to countless tests yet remains the core concept for explaining how evolution occurs.
HOW SCIENCE WORKS 13.1
The Voyage of HMS Beagle, 1831-1836
Probably the most significant event in Charles Darwin's life was his opportunity to sail on the British survey ship Beagle.
Surveys were common at that time; they helped refine maps and chart hazards to shipping. Darwin was 22 years old and probably would not have had the opportunity, had his uncle not persuaded Darwin's father to allow him to take the voyage. Darwin was to be a gentleman naturalist and companion to the ship's captain, Robert Fitzroy. When the official naturalist left the ship and returned to England, Darwin replaced him and became the official naturalist for the voyage. The appointment was not a paid position.
The voyage of the Beagle lasted nearly 5 years. During the trip, the ship visited South America, the Galapagos Islands, Australia, and many Pacific Islands. The Beagle's entire route is shown on the accompanying map. Darwin suffered greatly from seasickness and, perhaps because of it, made extensive journeys by mule and on foot some distance inland from wherever the Beagle happened to be at anchor. These inland trips gave Darwin the opportunity to make many of his observations. His experience was unique for a man so young and very difficult to duplicate because of the slow methods of travel used at that time.
Charles Darwin set forth on a sailing vessel similar to this, the HMS Beagle, in 1831 at the age of 22.
Although many people had seen the places that Darwin visited, never before had a student of nature collected volumes of information on them. Also, most other people who had visited these faraway places were military men or adventurers who did not recognize the significance of what they saw. Darwin's notebooks included information on plants, animals, rocks, geography, climate, and the native peoples he encountered. The natural history notes he took during the voyage served as a vast storehouse of information, which he used in his writings for the rest of his life. Because Darwin was wealthy, he did not need to work to earn a living and could devote a good deal of his time to the further study of natural history and the analysis of his notes. He was a semiinvalid during much of his later life. Many people think his ill health was caused by a tropical disease he contracted during the voyage of the Beagle. As a result of his experiences, he wrote several volumes detailing the events of the voyage, which were first published in 1839 in conjunction with other information related to the voyage. His volumes were revised several times and eventually were entitled The Voyage of the Beagle. He also wrote books on barnacles, the formation of coral reefs, how volcanoes might have been involved in reef formation, and finally On the Origin of Species. This last book, written 23 years after his return from the voyage, changed biological thinking for all time.
13.2. CONCEPT REVIEW
2. Why has Lamarck’s theory been rejected?
3. List five assumptions about the nature of living things that support the concept of evolution by natural selection.