Devolution (biology)
Devolution, de-evolution, or backward evolution is the notion that a species can change into a more "primitive" form over time. Devolution presumes that there is a preferred hierarchy of structure and function, and that evolution must mean "progress" to "more advanced" organisms. This may include the idea that some modern species that have lost functions or complexity accordingly must be degenerate forms of their ancestors. However, according to the definition of evolution, and particularly of the modern evolutionary synthesis in which natural selection leads to evolutionary adaptation, phenomena represented as instances of devolution are in every sense evolutionary.
Read more about Devolution (biology): Concepts Underlying Ideas of Devolution, History of Devolution, Dollo's Law, Degeneration Theory, Creationist Use, Streamlining Evolution