But the central core of evolution is generally defined as changes in trait or gene frequency in a population of organisms from one generation to the next. This has been dubbed the standard genetic definition of evolution. Natural selection is only one of several mechanisms in the theory of evolutionary change that explains how organisms historically adapt to changing environments. The principles of heredity were re-discovered in 1900, after Darwin's death, in Gregor Mendel's research on the inheritance of simple trait variations in peas. Subsequent work into genetics, mutation, paleontology, and developmental biology expanded the applicability and scope of Darwin's original theory.