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Charles Darwin, M.A.,
Fellow of the Royal, Geological, Linnaan, etc. societies; Author of Journal of researches during H. M. S. Beagle's Voyage round the world. London: John Murray, Albemarle Street, 1859

Slow process of change from one form to another, as in the evolution of the universe from its formation in the Big Bang to its present state, or in the evolution of life on Earth. Some Christians and Muslims deny the theory of evolution as conflicting with the belief that God created all things. English naturalist Charles Darwin assigned the main role in evolutionary change to natural selection acting on randomly occurring variations (now known to be produced by spontaneous changes or mutations in the genetic material of organisms).
Organic evolution traces the development of simple unicellular forms to more complex forms, ultimately to the flowering plants and vertebrate animals, including man. The Earth contains an immense diversity of living organisms: about a million different species of animals and half a million species of plants have so far been described. There is overwhelming evidence that this vast array arose by a gradual process of evolutionary divergence and not by individual acts of divine creation as described in the Book of Genesis. There are several lines of evidence: the fossil record, the existence of similarities or homologies between different groups of organisms, embryology, and geographical distribution.
The idea of continuous evolution in the living world can be traced as far back as Lucretius in the 1st century BC, but it did not gain wide acceptance until the 19th century, following the work of Scottish geologist Charles Lyell, French naturalist Jean Baptiste Lamarck, English naturalist Charles Darwin, and English biologist Thomas Henry Huxley. Darwin assigned the major role in evolutionary change to natural selection acting on randomly occurring variations. Natural selection occurs because those individuals better adapted to their particular environments reproduce more effectively, thus contributing their characteristics to future generations. The current theory of evolution, called neo-Darwinism , combines Darwin's theory with Austrian biologist Gregor Mendel's theories on genetics and Hugo de Vries's discovery of genetic mutation. Although neither the general concept of evolution nor the importance of natural selection is doubted by the vast majority of biologists, there remains dispute over other possible processes involved in evolutionary change. Besides natural selection and sexual selection , chance may play a large part in deciding which genes become characteristic of a population, a phenomenon called `genetic drift'. It is now also clear that evolutionary change does not always occur at a constant rate, but that the process can have long periods of relative stability interspersed with periods of rapid change. This has led to new theories, such as punctuated equilibrium model . See also adaptive radiation .
Although the broad outlines of the evolutionary sequence are known, much research is still necessary to fill in the details and to discover the mechanisms of evolutionary change. Evolution depends on the presence of heritable variations in a population which confer a selective advantage on the individuals displaying them. The phrase `survival of the fittest' is misleading since it implies the death of the `unfit' individuals. From an evolutionary point of view, fertility is much more important than survival since if one type regularly leaves more offspring than another, the frequency of the more fertile type in the population is bound to increase. Fertility depends on many things including general vigor, the length of the reproductive period and the ability to mate successfully. Heritable changes arise from genetic mutations which occur spontaneously in all organisms. Many investigations, which are currently being made into the genetic structures of living plant and animal populations, show the relative importance of mutations and isolation in the origin of new species. It is believed that the processes now occurring on a very small scale are the same as those which have caused the evolution of the major groups over a vast period of geological time. These studies will therefore throw light on the mechanism of evolution.
Evolution is a diagram by Charles Darwin of Galapagos finches, drawn during the voyage of the Beagle. On the Galapagos Islands, Darwin found a colony of finches that contained at least 14 distinct species, none of which existed on the continental mainland. He proposed that, in the isolated environment of the islands, the finches had evolved over many generations and were embroiled in a `survival of the fittest'.