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statistics, modern theory of evolution, extinct organisms, study of fossils, characteristics of living organisms

Deeper web pages:

>  Genetic basis of evolution

>  Natural selection in populations

>  Genetic drift

>  Origin of new species

>  Patterns of descent

>  The study of evolution

>  Development of evolutionary theory

>  Human inpact

>  Religious debate

>  Common misconceptions

Evolution, in biology, complex process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. The science of evolution seeks to understand the biological forces that caused ancient organisms to develop into the tremendous and ever-changing variety of life seen on Earth today. It addresses how, over the course of time, various plant and animal species branch off to become entirely new species, and how different species are related through complicated family trees that span millions of years.

Evolution provides an essential framework for studying the ongoing history of life on Earth. A central, and historically controversial, component of evolutionary theory is that all living organisms, from microscopic bacteria to plants, insects, birds, and mammals, share a common ancestor. Species that are closely related share a recent common ancestor, while distantly related species have a common ancestor further in the past. The animal most closely related to humans, for example, is the chimpanzee. The common ancestor of humans and chimpanzees is believed to have lived approximately 6 million to 7 million years ago. On the other hand, an ancestor common to humans and reptiles lived some 300 million years ago. And the common ancestor to even more distantly related forms lived even further in the past. Evolutionary biologists attempt to determine the history of lineages as they diverge and how differences in characteristics developed over time.

Throughout history, philosophers, religious thinkers, and scientists have attempted to explain the history and variety of life on Earth. During the rise of modern science in western Europe in the 17th and 18th centuries, a predominant view held that God created every organism on Earth more or less as it now exists. But in that time of burgeoning interest in the study of fossils and natural history, the beginnings of a modern evolutionary theory began to take shape. Early evolutionary theorists proposed that all of life on Earth evolved gradually from simple organisms. Their knowledge of science was incomplete, however, and their theories left too many questions unanswered. Most prominent scientists of the day remained convinced that the variety of life on Earth could only result from an act of divine creation.

In the mid-19th century a modern theory of evolution took hold, thanks to British naturalist Charles Darwin. In his book On the Origin of Species by Means of Natural Selection, published in 1859, Darwin described the evolution of life as a process of natural selection. Life, he suggested, is a competitive struggle to survive, often in the face of limited resources. Living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment, such as changes in climate. Darwin offered that, within a given population in a given environment, certain individuals possess characteristics that make them more likely to survive and reproduce. These individuals will pass these critical characteristics on to their offspring. The number of organisms with these traits increases as each generation passes on the advantageous combination of traits. Outmatched, individuals lacking the beneficial traits gradually decrease in number. Slowly, Darwin argued, natural selection tips the balance in a population toward those with the combination of traits, or adaptations, best suited to their environment.

While On the Origin of Species was an instant sensation and best-seller, Darwin’s theories faced hostile reception by critics who railed against his blasphemous ideas. Other critics pointed to questions left unresolved by Darwin’s careful arguments. For instance, Darwin could not explain the mechanism that caused life forms to change from generation to generation.

Hostility gave way to acclaim as scientists vigorously debated, explored, and built on Darwin’s theory of natural selection. As the 20th century unfolded, scientific advances revealed the detailed mechanisms missing from Darwin’s theory. Study of the complex chemistry of all organisms unveiled the structure of genes as well as how they are duplicated, altered, and passed from generation to generation. New statistical methods helped explain how genes in specific populations change over generations. These new methods provided insight into how populations remain adaptable to changing environmental circumstances and broadened our understanding of the genetic structure of populations. Advances in techniques used to determine the age of fossils provided clues about when extinct organisms existed and details about the circumstances surrounding their extinction. And new molecular biology techniques compare the genetic structures of different species, enabling scientists to determine heretofore undetectable evolutionary relationships between species. Today, evolution is recognized as the cornerstone of modern biology. Uniting such diverse scientific fields as cell biology, genetics, paleontology, and even geology and statistics, the study of evolution reveals an exquisitely complex interaction of the forces that act upon every life form on Earth.

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