Modern evolutionary theory continues to develop. Darwin's theory of evolution by natural selection, with its tree-like model of branching common descent, has become the unifying theory of the life sciences. The theory explains the diversity of living organisms and their adaptation to the environment. It makes sense of the geologic record, biogeography, parallels in embryonic development, biological homologies, vestigiality, cladistics, phylogenetics and other fields, with unrivalled explanatory power; it has also become essential to applied sciences such as medicine and agriculture. Despite the scientific consensus, a religion-based political controversy has developed over how evolution is taught in schools, especially in the United States.