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Transcript
Variety Is the Spice of Life Imagine a field full of mice. Some mice are brown, and some are white. Which mice would most likely be killed by a hungry hawk flying overhead? The white mice would be easier to see, and would therefore be more likely to be caught and eaten. If this scenario repeated itself many times, and the white mice were killed before they even had a chance to reproduce, nearly the entire population of mice would likely be brown after just a few generations. This is an example of what biologists call "natural selection." Natural Selection and Evolution English biologist Charles Darwin was the first to describe natural selection. In his book, The Origin of Species, Darwin proposed that natural selection results in evolution. Charles Darwin is widely regarded Darwin spent 4 years traveling around the world on a as the father of evolution. small sailing ship, the HMS Beagle. Over the course of his voyage, he saw numerous unusual plants and animals. He also saw seashells in rocks from high mountain peaks and documented numerous other sights and events. Although he was religious, Darwin could not reconcile his spiritual beliefs with his experience of nature. He sought a scientific explanation for his observations. Darwin had several influences when as he developed his ideas and thoughts. Sir Charles Lyell greatly influenced Darwin’s ideas. Lyell and Darwin shared the same idea about geological processes. Lyell’s theories helped Darwin validate his own ideas. Over many years and through painstaking research, Darwin developed a revolutionary new theory. According to his idea, species of plants and animals evolved, or changed, over time, and with enough time such changes could result in new species. While Darwin privately worked on his ideas, he consulted with Alfred Wallace. Wallace shared his thoughts with Darwin and aided Darwin by contributing species for study. Wallace sought Darwin’s help in publishing his own theories. To Darwin’s surprise, the theories that he had developed were very much like those of Wallace. It was through work with Lyell and Wallace that Darwin was encouraged to publish his works. Discovery Education Science © Discovery Communications, LLC Variety Is the Spice of Life Darwin was not the first to propose such a thing. Other scientists had suggested similar ideas. In fact, thinkers as far back as the ancient Romans and Greeks had proposed that species changed from one form into another. Jean Lamarck also had his own ideas about how organisms evolved. Lamarck felt that organisms evolved to better themselves or to better adapt to their lifestyles. They developed the traits that they needed and they lost the ones that they did not use. But before Darwin, no one had proposed how organisms evolved. Darwin’s process of natural selection provided a mechanism for evolution. Darwin had three key insights into how natural selection occurs. First, he recognized that not all individuals within the same species are identical. Such variation is easily observable in plants and animals. Darwin’s second insight was that plants and animals pass on their specific features or traits. If two parents both have brown hair, the chances are that their children have brown hair. Hair color is an example of a heritable trait—a trait that can be inherited. Not all traits are heritable. For example, when people get a tan, they do not pass the tan on to their children. The change in skin color resulting from a tan is not heritable. Modern biologists know that genes are responsible for passing on traits. Darwin did not know about genes, but he recognized that some traits were heritable. Darwin’s third insight was that natural selection affects survival. An individual can only pass on its traits if it survives to reproduce. If it does not survive, it will not reproduce and its traits will not be passed on. A colleague of Darwin’s coined the phrase “survival of the fittest” to describe this principle. Darwin’s insights explain how natural selection results in evolution. Trait variation causes some individuals to reproduce more successfully. Since the variation is heritable, individuals that reproduce successfully pass on their traits to offspring. Discovery Education Science © Discovery Communications, LLC Variety Is the Spice of Life Man-Made Evolution? Almost all scientists today accept Darwin’s theory of evolution. It is the simplest way to explain Earth’s variety of life. Evolution also explains why life is changing as humans alter the environment. For example, many insects and weeds are now resistant to formerly effective pesticides. Antibiotics that used to quickly clear up an infection sometimes fail to cure the disease. Aphids, such as these infesting the Pesticides prevent reproduction of most target underside of a leaf, are among the organisms, but some individuals survive. These many pests resistant to pesticides. individuals pass on their ability to resist the toxins. Over time, the resistance genes spread through the population and the pest becomes resistant due to selection imposed by the pesticide. The same principle is at work when bacteria develop resistance to antibiotics. Evolution may take place gradually over long periods of time, or it can occur relatively quickly. In the southwestern United States deserts and in Mexico, such evolution can be seen in the rock pocket mouse (chaetodipus intermedius). The mouse comes in two color variations: light and dark. The rock pocket mouse that lives in the lighter colored desert is lighter colored. The rock pocket mouse that lives in rocky outcroppings will usually be a darker color. The darker mice that live in the lighter desert are easier for predators to locate visually so they can be eaten. The lighter mice that live near the darker rocks are also easier for predators to locate visually so they can be eaten. As a result of this, the lighter-colored mice are plentiful and likely to reproduce in the lighter-colored desert. The darker-colored mice are more plentiful in the rockier and darker–colored areas of the desert. Because of this, scientists can observe evolution over a shorter period of time. Discovery Education Science © Discovery Communications, LLC Variety Is the Spice of Life This process might seem obvious today, but in Darwin’s time many did not accept his ideas. He was overturning hundreds of years of doctrine about how life originated. From the outset, his ideas were controversial. Publication of The Origin of Species stimulated a flurry of research activity. Scientists across the world worked to gather evidence to support or refute Darwin’s theory. Over time, researchers accumulated facts and Natural selection is the best way to explain evolution of species such as the horses and observations. Scientists have documented their relatives. thousands of examples of evolution. The fossil record shows how horses evolved from small dog-sized ancestors into the beautiful creatures we see today. Researchers in the Galapagos Islands have documented changes in the size of bird beaks. The observed changes only make sense if they have been brought about by natural selection. Humans deliberately change animals and plants through selection. Darwin used the artificial selection by breeders as an analogy for natural selection. Humans created hundreds of dog breeds in a few thousand years. What could natural selection do over millions of years? The Root of Ongoing Controversy Why is it that even though most scientists accept the reality of evolution, Darwin’s ideas remain controversial today? The controversy results partly from confusion over the meaning of the word “theory.” It might mean a tentative explanation, or it could mean a well-supported body of knowledge that explains some feature of the natural world, such as the theory of gravity. No one seriously doubts that gravity exists. In the same way, when scientists talk about the theory of evolution they are using the word in the latter sense. Controversy over evolution outside the scientific community also exists partly because life is so extraordinarily complex. Scientists do not have all the answers about how a particular trait evolved, and they do not fully understand all the genetic Discovery Education Science © Discovery Communications, LLC Variety Is the Spice of Life mechanisms. Nor do they have a time machine to go back and investigate the unique circumstances that result in new species arising. However, Darwin’s idea of natural selection is the only scientific way to make sense of the variety and beauty of life. Discovery Education Science © Discovery Communications, LLC Variety Is the Spice of Life Comprehension Questions 1. Identify a trait that is heritable and one that is not heritable. 2. Cite one example over evolution over a long period of time and once example of evolution over a short period of time. 3. How did Darwin’s ideas contribute to the history of science? Discovery Education Science © Discovery Communications, LLC