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Teacher candidates studying for the CSET Single Subject Science exam will often purchase ACE the CSET Multiple Subjects to help them prepare for their exam.
Another great resource for the CSET Multiple Subjects subtest 2 Science section, and the CSET Single Subject Science exam is HowStuffWorks.com
First you will read the information in the blue box, then you will be asked a question below. The answer key will follow at the end of this article.
I recommend that you grab a piece of paper and write down the letter of the answer you think is correct and check your answers using the answer key at the end of this article.
Darwin's Theory of Evolution Scientists at the beginning of the 1800s know of some kinds of fossils, and they were very aware of homologous and vestigial structures. Many scientists suspected that some kind of evolution had given rise to living things around them. However, they had no unifying theory to explain how evolution might have occurred. Two scientists led the way in the search for a mechanism of evolution. The first was Jean Lamarck. The second was one of the greatest figures in biology, Charles Darwin. Evolutionary Theory Before Darwin The first systematic presentation of evolution was put forth by the French scientist Jean Baptiste de Lamarck (1774-1829) in 1809. Lamarck described a mechanism by which he believed evolution could occur. This mechanism was known as "the inheritance of acquired characteristics." Assume that there were salamanders living in some grasslands. Suppose, Lamarck argued, that these salamanders had a hard time walking because their short legs couldn't trample the tall grasses or reach the ground. Suppose that these salamanders began to slither on their bellies to move from place to place. Because they didn't use their legs, the leg muscles wasted away from disuse and the legs thus became small. Lamarck's theory said that the salamanders passed this acquired trait to their offspring. In time the salamander's legs were used so rarely that they disappeared. Thus, Lamarck argued, legless salamanders evolved from salamanders by inheriting the acquired characteristic of having no legs. Lamarck presented no experimental evidence or observation and his theory fell out of scientific favor. The next significant idea came from the British scientist Charles Darwin. Darwin's Background Charles Darwin (1809-1882), like many people of genius, did not at first appear to have extraordinary talents. From a young age Darwin disliked school and preferred observing birds and collecting insects to study. He was sent to medical school in Scotland when he was 16. Young Darwin found medicine "intolerably dull." He was much more interested in attending natural history lectures. Seeing that Darwin lacked enthusiasm for becoming a doctor, his father suggested he study for the clergy. Darwin was agreeable to the idea and enrolled in the university at Cambridge, England, in 1827. Here again, Darwin admitted, "My time was wasted, as far as the academic studies were concerned." However, Darwin found that his friendship with John S. Henslow, professor of botany, made life in Cambridge extremely worthwhile. Through long talks with Henslow, Darwin's knowledge of the natural world increased. Henslow encouraged Darwin in his studies of natural history. In 1831 Henslow recommended that Darwin be chosen for the position of naturalist on the ship the HMS Beagle. The Voyage of the Beagle The Beagle was chartered for a five-year mapping and collecting expedition to South America and the South Pacific. Darwin's job as ship naturalist was to collect specimens, make observations, and keep careful records of anything he observed that he thought significant. At the beginning of the voyage Darwin read a geology book given to him by Henslow. This book, Principles of Geology by Charles Lyell, spurred his interest in the study of land forms. In Chile Darwin observed the results of an earthquake: the land had been lifted by several feet. In the Andes he observed fossil shells of marine organisms in rock beds at about 4,300 m. He came to agree with Lyell that over millions of years earthquakes and other geologic processes could change the geology of the land. Because the land changed, new habitats would form. Darwin realized that animals would have to adapt to these changes. During the Beagle's five-year trip the captain often dropped Darwin off at one port and picked him up months later at another. One reason that Darwin was so eager to study life on land was that he suffered from terrible seasickness and couldn't wait to get off the Beagle. During his time on land Darwin trekked hundreds of miles through unmapped region. He observed thousands of species of organisms and collected many different types of fossils. On the long sea voyages he used his time to catalog his specimens and write his notes. Darwin in England When Darwin returned to England in October 1836, his collections from the voyage were praised by the scientific community. Darwin sent many specimens to experts for study. A bird specialist, or ornithologist, studied Darwin's bird collections from the Galapagos Islands, located about 1,000 km west of South America. He reported that Darwin had collected 13 similar but separate species of finches. Each finch species had a distinctive bill specialized for a particular food source. Other experts studied Darwin's fossils and classified them as remains of extinct mammals. The fossils included rodents the size of hippopotamuses. The similarities of the Galapagos finches led Darwin to infer that the finches shared a common ancestor. The similarities between the fossil mammals Darwin collected and modern mammals led him to believe that species change over time. In 1837 Darwin began his first notebook on evolution. For several years Darwin filled his notebooks with facts that could be used to support the theory of evolution. He found evidence from his study of the fossil record: he observed that fossils of similar relative ages are more closely related than those of widely different relative ages. Comparing homologous structures, vestigial organs, and embryological development of living species gave him additional evidence of evolution. He consulted animal and plant breeders about changes in domestic species. He ran his own breeding experiments and also did experiments on seed dispersal. Evolution by Natural Selection The central question still remained: if evolution occurred, by what means did it occur? In 1838 Darwin read a book called Essay on the Principle of Population by a British economist, Thomas Malthus (1776-1834). Malthus stated that a human population growing unchecked would double every 25 years. Resources such as food, air and water cannot increase at the same rate, Malthus argued. Thus human beings are involved in an intense "struggle for existence," competing for the limited resources. This idea helped Darwin uncover the mechanism he needed. Combining the idea of competition with his other observations, Darwin explained how evolution could occur. First, he stated that variation exists among individuals of a species. Second, he stated that scarcity of resources in a burgeoning population would lead to competition between individuals of the same species because all use the same limited resources. Such competition would lead to the death of some individuals, while others would survive. From this reasoning Darwin concluded that individuals having advantageous variations are more likely to survive and reproduce than those without the advantageous variations. Darwin coined the term natural selection to describe the process by which organisms with favorable variations survive and reproduce at a higher rate. An inherited variation that increases an organism's chance of survival in a particular environment is called an adaptation. Over many generations, an adaptation could spread throughout the entire species. In this way, according to Darwin, evolution by natural selection would occur. As an example Darwin noted that the ptarmigan turns white in winter. This color change, he inferred, helped protect it from predators, which would have a hard time spotting the bird in snow. Ptarmigans that didn't change color in winter would be spotted easily and eaten. In this way, Darwin implied, ptarmigans that turned white in winter would be more likely to survive, reproduce, and pass this adaptation to future generations. The Origin of Species Darwin compiled evidence for evolution by natural selection for about 20 years. Between 1842 and 1844 he wrote a 230-page essay summarizing his theory and the evidence for it. In the 1850s he began working on a detailed, multivolume book to present his theory to the scientific community. Darwin might never have completed the book if another British scientist, Alfred Russel Wallace (1823-1913), had not come up with the same idea in 1858. While living in the Malay Archipelago in the Pacific Ocean, Wallace formulated his theory and wrote it in an essay, which he sent to Darwin. Darwin's fellow scientists persuaded him to let them present his theory and Wallace's essay jointly at a scientific meeting. The presentation excited very little attention, according to the modest Darwin. However, the publication of Darwin's book The Origin of Species in 1859 changed biology forever. The first printing of the book sold out in one day. Darwin clearly and logically presented the idea that natural selection is the mechanism of evolution. In Darwin's own lifetime many scientists became convinced that evolution occurs by means of natural selection. Today this theory is the unifying one for all biology. Patterns of Evolution Natural selection can ultimately lead to the formation of new species. Sometimes many species evolve from a single ancestral species. Similarities in skeletal and muscular structure of Hawaiian honeycreepers led scientists to conclude that the 23 species of honeycreepers evolved from one ancestral species. Such an evolutionary pattern, in which many related species evolved from a single ancestral species, is called adaptive radiation. Adaptive radiation most commonly occurs when a species of organisms successfully invades an isolated region where few competing species exist. If new habitats are available, new species will evolve. Divergent and Convergent Evolution Adaptive radiation is one example of divergent evolution. Divergent evolution is the process of two or more related species becoming more and more dissimilar. The red fox and the kit fox provide and example of two species that have undergone divergent evolution. The red fox lives in mixed farmlands and forests, where its red color helps it blend in with surrounding trees. The kit fox lives on the plains and in the deserts, where its sandy color helps conceal it from prey and predators. The ears of the kit fox are larger than those of the red fox. The kit fox's large ears are an adaptation to its desert environment. The enlarged surface area of its ears helps the fox get rid of excess body heat. Similarities in structure indicate that the red fox and the kit fox had a common ancestor. As they adapted to different environments, the appearance of the two species diverged. In convergent evolution, on the other hand, unrelated species become more and more similar in appearance as they adapt to the same kind of environment. The two unrelated types of plants in the picture above have adapted to desert environments. Notice the resemblance of the cactus, which grows in the American desert, to the euphorbia, which grows in the African deserts. Both have fleshy stems armed with spines. These adaptations help the plants store water and ward off predators. Coevolution Coevolution is the joint change of two or more species in close interaction. Predators and their prey sometimes coevolve; parasites and their hosts often coevolve; plant-eating animals and the plants upon which they feed also coevolve. One example of coevolution is between plants and the animals that pollinate them. In tropical regions bats visiting flowers to eat nectar. The fur on the bat's face and neck picks up pollen, which the bat transfers to the next flower it visits. Bats that feed at flowers have a slender muzzle and a long tongue with a brushed tip. These adaptations aid the bat in feeding. Flowers that have coevolved with bats are light in color. Therefore, bats, which are active at night, can easily locate them. The flowers also have a fruity odor attractive to bats. Divergent and convergent evolution and coevolution are different ways organisms adapt to the environment. These are examples of how the diversity of life on earth is due to the ever-changing interaction between a species and its environment. Sources: Special thanks to BioWeb for distributing knowledge to all the world. Visit this incredible web site at: http://bioweb.cs.earlham.edu
A. artificial selection
B. natural selection
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