What theory proposes that evolution occurs steadily in tiny changes over long periods of time?

Darwin proposed that small evolutionary changes occur/appear continuously over long periods of time. These were termed variations by Darwin. De Vries explained that variations arise due to mutation . In neo D it was proposed that new variations accumulate in gene pool over time, thus populations slowly evolve.

Darwin was greatly influenced by theory of uniformitarianism first proposed by James Hutton and popularised later by Charles Lyell, that geological features change uniformly but slowly over long periods of time through out the world in same manner. Darwin's theory is a parallel idea which proposes evolution via gradualism .

A contrasting theory was proposed by palaeontologists, which is the theory of punctuated equilibrium . This theory says that sudden evolutionary changes occur relatively rapidly in small isolated populations, followed by long periods of equilibrium/stability.

Darwin's theory predicted that many transitional fossils would be found illustrating the slow changes in biological systems. The fossil record however does not support this prediction. Punctuated equilibrium was proposed to explain lack of multiple transitional fossils.

The theory that proposes evolution occurs steadily in tiny changes over long periods of time is called gradualism.