Evolution is a fact—a hypothesis that is so thoroughly supported that it is extremely unlikely to be false. The theory of evolution is not a speculation, but rather a complex set of well-supported hypotheses that explain how evolution happens.
There is a great range of views on whether or not religion and evolution—or religion and science generally—are compatible. Especially in the United States, many reject evolution and instead accept divine creation because they think evolution conflicts with their religious beliefs. The positions taken by creationists on issues such as the age of Earth and of life vary.
Science is tentative; it accepts hypotheses provisionally and changes them in the face of convincing new evidence. It is concerned only with testable hypotheses; it depends on empirical studies that are subject to peer scrutiny and that can be verified and repeated by others. Creationism has none of the features of science, so it has no claim to be taught in science classes.
The evidence for evolution comes from all realms of biology and geology, including comparative studies of morphology, development, life histories, and other features, as well as molecular biology, genomics, paleontology, and biogeography. Evolutionary principles can explain features of organisms that would not be expected of a beneficent intelligent designer, such as imperfect adaptation, useless or vestigial features, extinction, selfish DNA, sexually selected characteristics, conflicts among genes within the genome, and infanticide. Furthermore, all the proposed mechanisms of evolution have been thoroughly documented, and evolution has been observed.
It is important to understand evolution not only because it has broad implications for how we think about nature and humanity, but also because it has many practical ramifications. Evolutionary science contributes to many aspects of medicine and public health, agriculture and natural resource management, pest management, and conservation.
One of the most difficult and controversial challenges is to join biological and social science in order to understand how the distinctively human cognitive and behavioral characteristics evolved, the extent to which human behaviors have an evolved genetic foundation, how that foundation interacts with cultural and other environmental factors to shape individual behavior, and how genes and culture have coevolved.