Thomas Malthus ( ) was an English economist. He wrote an essay titled On Population.

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1 THEORY OF EVOLUTION History of Evolutionary Thought The Idea of Evolution Evolution is the process of change in the inherited characteristics within populations over generations such that new types of organisms develop from preexisting types. Evolutionary Relationships Between Whales and Hoofed Mammals Ideas of Darwin s Time o Scientific understanding of evolution began to develop in the 17th and 18th centuries as geologists and naturalists compared geologic processes and living and fossil organisms around the world. Thomas Malthus ( ) was an English economist. He wrote an essay titled On Population. o o o In the essay, Malthus argued that human populations grow faster than the resources they depend on. When populations become too large, famine and disease break out. In the end, this keeps populations in check by killing off the weakest members. Ideas about Geology Cuvier promoted the idea of catastrophism Doctrine that explains the differences in fossil forms encountered in successive stratigraphic levels as being the product of repeated cataclysmic occurrences and repeated new creations. Lyell promoted the idea of uniformitarianism. Doctrine that existing processes acting in the same manner and with essentially the same intensity as at present are sufficient to account for all geologic change

2 Lamarck s Ideas on Evolution Among naturalists, Lamarck proposed the inheritance of acquired characteristics as a mechanism for evolution. Animals, in responding to different environments, adopted new habits. Their new habits caused them to use some organs more and some organs less, which resulted in the strengthening of the former and the weakening of the latter. New characters thus acquired by organisms over the course of their lives were passed on to the next generation. Darwin Darwin s Voyage

3 After Darwin returned to England in 1836 he filled notebooks with his ideas about species diversity and the process that he would later call evolution. He did not rush to publish his ideas because they disagreed with the fundamental scientific beliefs of his day. He asked his wife to publish his ideas when he died. However, Alfred Russel Wallace lived at about the same time as Darwin. He also traveled to distant places to study nature. He developed basically the same theory of evolution. While working in distant lands, Wallace sent Darwin a paper he had written. In the paper, Wallace explained his evolutionary theory. This served to confirm what Darwin already thought. Darwin s Ideas Descent with Modification Darwin wrote On the Origin of Species, in which he argued that descent with modification occurs, that all species descended from common ancestors, and that natural selection is the mechanism for evolution. Natural Selection Organisms in a population adapt to their environment as the proportion of individuals with genes for favorable traits increases. Those individuals that pass on more genes are considered to have greater fitness. Evidence of Evolution The Fossil Record Evidence of evolution can be found by comparing several kinds of data, including the fossil record, biogeography, anatomy and development, and biological molecules.

4 Evolutionary theories are supported when several kinds of evidence support similar conclusions. The Age of Fossils Geologic evidence supports theories about the age and development of Earth. The Distribution of Fossils The fossil record shows that the types and distribution of organisms on Earth have changed over time. Transitional Species Fossils of transitional species show evidence of descent with modification. Evidence of Whale Evolution Biogeography Biogeography, the study of the locations of organisms around the world, provides evidence of descent with modification. Anatomy and Physiology In organisms, analogous structures are similar in function but have different evolutionary origins.

5 Homologous structures have a common evolutionary origin. Forelimbs of Vertebrates A species with a vestigial structure probably shares ancestry with a species that has a functional form of the structure. Related species show similarities in embryological development. Biological Molecules Similarity in the subunit sequences of biological molecules such as RNA, DNA, and proteins indicates a common evolutionary history. Hemoglobin Comparison Developing Theory Modern scientists integrate Darwin s theory with other advances in biological knowledge. Theories and hypotheses about evolution continue to be proposed and investigated. Evolution in Action Convergent evolution

6 In convergent evolution, organisms that are not closely related resemble each other because they have responded to similar environment. Divergence and Radiation In divergent evolution, related populations become less similar as they respond to different environments. Adaptive radiation is the divergent evolution of a single group of organisms in a new environment. Artificial Selection The great variety of dog breeds is an example of artificial selection. Coevolution The increasing occurrence of antibiotic resistance among bacteria is an example of coevolution in progress. Another example is the relationship between some plants and insects