The Science Channel

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.

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.

The Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe.

Every high energy physics experiment carried out since the mid-20th century has eventually yielded findings consistent with the Standard Model. Still, the Standard Model falls short of being a complete theory of fundamental interactions because it does not include gravitation, dark matter, or dark energy. It is not quite a complete description of leptons either, because it does not describe nonzero neutrino masses, although simple natural extensions do.