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evolution The fossil record indicates that organisms have changed through time, and these changes are best explained by the theory of evolution. Organic evolution is the cumulative and irreversible change of organisms through time, and results of this process explain the distribution and diversity of life throughout Earth history.
Species are a group of interbreeding populations reproductively isolated from other groups. An adaptation of a species is a change that occurs in one species to make the organism better able to cope with its environment. Genetic mutations can result in positive or negative changes to species, but negative changes tend to cause extinction, whereas positive changes tend to help species survive. The theory of evolution explains how these changes occur, the role of the environment, and how these changes help the organism to survive. To understand evolution and the history of theories of evolution, it is necessary to also understand the differences between ontogeny, the development of an individual organism from young to old age (as from a tadpole to a frog), and phylogeny, the evolutionary history of an organism.
In his 1809 treatise Philosophie Zoologique Jean-Baptiste Lamarck (1744-1829), a French naturalist, soldier, and academic, proposed a theory of evolution, stating that the fundamental aspect of nature is change, and "life is a stream of gradual complication." Lamarck was one of the first scientists to suggest that the environment can give rise to change in animals, driving evolution. He suggested that if the environment changes, this would force habits of organisms to change, and if these habits persist they would give rise to new characteristics. The most famous example he used to illustrate his case was the giraffe, which, he suggested, evolved from a horse, the descendants of whose necks became gradually stretched to reach leaves high on trees. Lamarck believed that the natural system was constantly replenished by spontaneous generation from inorganic material, and that the youngest organisms were the simplest, and the oldest were the most complex. Lamarckian evolution is not believed by scientists today, since no experimental or other evidence was ever found to support this type of cause-and-effect evolution. But Lamarck is remembered for his contribution of the idea of the inheritance of acquired characteristics.
Charles Robert Darwin (1809-82) was an English naturalist widely regarded as the father of the modern theory of evolution. Some of his most important work stems from his voyage on the research vessel HMS Beagle from 1831 to 1836, when he traveled around the world studying and collecting plants, animals, rocks and minerals. In his studies of modern and ancient biological assemblages Darwin noted that the number of individuals in a given population remains relatively constant and that predator-prey relationships within the environment ensure that only those individuals best suited for life in that environment survive. Many individuals within a population are born with inherited characteristics that prevent them from surviving in that environment, and they die. Darwin called this process natural selection, and noted that with time the process makes the entire population of that species better suited for its environment. Darwin noted that as the environment changes, "only the fittest will survive." However, as with Lamarckian evolution, the mechanism for Darwinian evolution was lacking, and no explanation for acquiring advantageous adoptions was known.
In 1869 the Austrian scientist and priest Gregor Johan Mendel (1822-84) described a system of genes, or heritable units, by which characteristics are transmitted from parents to offspring. Mendel became known as the father of genetics, and led to the study of heredity by 1910, after a period of 40 years in which the significance of his work was not appreciated. After this delay it was realized that new characteristics, or mutations, arise completely at random, and are related to chemical changes in DNA. These were thought to be caused by one of the three following mechanisms:
• imperfect replication of DNA during cell division
• physical alteration of segments of DNA by twisting, breaking, or reversed strands
• alteration by an external force such as a virus or radiation
Many mutations turn out to be harmful, causing death, while others cause very small, almost imperceptible changes.
The next major step in understanding evolution was made in 1869 by Ernst Haeckel (1834-1919), a German biologist, naturalist, physician, and professor who documented a direct relationship between the development of an embryo and the history of the group to which it belonged. Haeckel suggested that the "ontogeny [development of the individual] is a short history of the phylogeny [history of the race]." Haeckel further postulated that evolution proceeded by adding stages to the end of an individual's life. But scientists now know that evolutionary stages can be added at any time during ontogeny but are more typically added during the early stages.
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