The scientific theory intended to explain the origin of species through natural selection, which can be summed up in a tautology of perhaps surprising explanatory power: whatever life forms or traits tend to survive and reproduce tend to survive and reproduce. This theory provides an explanation of the origin of species through adaptation to different and changing environments which select for - i.e., favor - those life forms or traits best adapted to their environments, and weed out those life forms or traits less well adapted. The term “best adapted” implies nothing more than the simple fact that the particular life form or trait survived and reproduced. This central constraint allows evolutionary theory to account for a great diversity of adaptations, such as the variety of ways in which flowering plants distribute seeds or animals sense patterns of light.
The theory of evolution is understood to make reference to any supernatural causes or agents unnecessary. Supernatural causes are not so much denied by this theory as they are ignored, given that the general assumption of the scientific world is that science studies nature, not the supernatural.
Evolutionary theory, in its general outlines, is accepted as virtually indisputable by the vast majority of the scientific community as well as most of those religionists who in some way disidentify with the fundamentalist world view. This latter group includes agnostics, most liberals, and certain religionists, e.g. Roman Catholics or Orthodox Jews, who do not share fundamentalist assumptions about the reliability of a literal reading of scripture, and who tend to see science and religion as approaching the understanding of life in different but complementary ways.
Since this theory does not deal with the supernatural, and claims to explain the origin of species through purely natural causes, fundamentalist religionists consider it (quite accurately, though also pejoratively) as godless and hence a threat to the wider dissemination of salvific Biblical truth throughout society, especially in the public schools.
See the Creationism and Intelligent Design entries for more on religious objections to evolutionary theory.