In biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a process that can result in the emergence of new species. The similarities among species suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.

The basis of evolution is the genes that are passed on from generation to generation; these produce an organism's inherited traits. These traits vary within populations, with organisms showing heritable differences (variation) in their traits. Evolution itself is the product of two opposing forces: processes that constantly introduce variation, and processes that make variants either become more common or rare. New variation arises in two main ways: either from mutations in genes, or from the transfer of genes between populations and between species. New combinations of genes are also produced by genetic recombination, which can increase variation between organisms.

Two major mechanisms determine which variants will become more common or rare in a population. One is natural selection, a process that causes helpful traits (those that increase the chance of survival and reproduction) to become more common in a population and causes harmful traits to become more rare. This occurs because individuals with advantageous traits are more likely to reproduce, meaning that more individuals in the next generation will inherit these traits. Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of the variants best-suited for their environment. The other major mechanism driving evolution is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role that chance plays in whether a given trait will be passed on as individuals survive and reproduce.

Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species changed over time. However, the mechanism driving these changes remained unclear until the theories of natural selection were independently proposed by Charles Darwin and Alfred Wallace. Darwin's landmark 1859 work On the Origin of Species brought the new theories of evolution by natural selection to a wide audience, leading to the overwhelming acceptance of evolution among scientists. In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, which connected the units of evolution (genes) and the mechanism of evolution (natural selection). This powerful explanatory and predictive theory has become the central organizing principle of modern biology, directing research and providing a unifying explanation for the diversity of life on Earth.

Evolution is change in populations of organisms over generations. Offspring differ from their parents in various ways. When these differences are helpful, the offspring have a greater chance of surviving and reproducing, making the differences more common in the next generation. In this way, differences can accumulate over time, leading to major changes in a population.

The scientific theory of evolution—the explanation for how evolution occurs—states that all living things are descended, at some point in the distant past, from a single common ancestor. This is called common descent. Since the beginning of life, evolution has transformed the first species into more and more different species as life has found a variety of ways to survive and flourish. This has resulted in the many diverse forms of life that exist today.

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• The antagonism between science and religion, about which we hear so much, appears to me to be purely factitious–fabricated, on the one hand, by short-sighted religious people who confound a certain branch of science, theology, with religion; and, on the other, by equally short-sighted scientific people who forget that science takes for its province only that which is susceptible of clear intellectual comprehension; and that, outside the boundaries of that province, they must be content with imagination, with hope, and with ignorance.
  • Thomas Huxley, The interpreters of Genesis and the interpreters of Nature (1885)

• Anthropological, biological, and genetic evidence all put the origin of modern humans at between 200,000 and 100,000 years ago, probably in Africa. There is also much data that show an outburst of cultural behavior occurring around 50,000-40,000 years ago in Europe. That's when archaeologists date the oldest evidence of burial ceremonies, body ornaments, and cave paintings.
  • William J. Cromie, Facing up to Modern Man; Harvard Gazette

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Fri Dec 4 04:32:15 2009