The term was introduced and discussed by George Gaylord Simpson, the palaeontologist who contributed to the modern evolutionary synthesis. Others prefer not to use the term. Robert L Carroll prefers to use the term "major evolutionary transitions", though it turns out that all or most of these could also be described as adaptive radiations. Others use terms like macroevolution, or even megaevolution, as if the processes are different from those which occur below species level. It is part of evolutionary theory that all processes take place at the level of populations. All agree, though, that the speed of evolution does change, however it is measured.