Darwin's Theory of Evolution



Darwin's Theory of Evolution

Scientists at the beginning of the 1800s 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, the scientific community praised his collections from the voyage. 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 and on seed dispersal experiments.

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, Darwin believed evolution by natural selection occured.

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 on.

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.

Darwin's Theory of Evolution

Scientists at the beginning of the 1800s 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 _______________ 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 __________________________________."

Assume that there were salamanders living in some ____________________. Suppose, Lamarck argued, that these salamanders had a hard time walking because their ______________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 _______________________ from disuse and the legs thus became small. Lamarck's theory said that the salamanders passed this ________________________ to their offspring. In time the salamander's legs were used so rarely that they _____________. 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 ____________________________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 _______________ school and preferred observing birds and ____________ insects to study. He was sent to medical school in _________when he was 16. Young Darwin found medicine "intolerably dull." He was much more interested in attending ______________________ 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 ______________. 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 ___________________________.

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 _________________ 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 ___________ 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 ___________ 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 ______________ seasickness and couldn't wait to get off the __________. During his time on land Darwin trekked ___________________________ through unmapped region. He observed thousands of species of organisms and collected many different types of _________________. On the long sea ___________ he used his time to catalog his specimens and write his notes.

Darwin in England

When Darwin returned to England in October 1836, the scientific community ______________ his collections from the voyage. Darwin sent many specimens to experts for _________. A bird specialist, or ______________________, studied Darwin's bird collections from the ___________________________, located about 1,000 km west of South America. He reported that Darwin had collected 13 similar but separate species of ____________. 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 ________________ mammals. The fossils included rodents the size of __________________________. The similarities of the Galapagos finches led Darwin to infer that the finches shared a common ______________. The similarities between the fossil mammals Darwin collected and modern mammals led him to believe that species _____________________________________.

In 1837 Darwin began his first notebook on evolution. For several years Darwin filled his _______________________ with facts that could be used to support the theory of evolution. He found ________________ from his study of the fossil record: he observed that fossils of ________________ relative ages are more closely related than those of widely different relative ages. Comparing ___________________ 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 and on seed dispersal experiments.

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 ______________ every 25 years. Resources such as food, air and water cannot __________________ 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 _____________________ he needed.

Combining the idea of competition with his other _______________________, Darwin explained how evolution could occur. First, he stated that variation exists among individuals of a species. Second, he stated that _______________ of resources in a burgeoning population would lead to competition between individuals of the same _______________ because all use the same limited resources. Such ___________________________ would lead to the death of some individuals, while others would survive. From this reasoning Darwin concluded that individuals having _________________________________ are more likely to survive and reproduce than those without the advantageous variations.

Darwin coined the term ____________________________ to describe the process by which organisms with favorable variations survive and __________________________________________. An inherited variation that increases an organism's chance of survival in a ____________________________ is called an adaptation. Over many generations, an adaptation could spread throughout the entire species. In this way, Darwin believed evolution by natural selection occured.

As an example Darwin noted that the ptarmigan turns white in winter. This color change, he inferred, helped __________________________________________, which would have a hard time spo-tting 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 _____________________________ on.

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.

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