The two central elements of neo-darwinian evolution1,2 are small random variations and natural selection. In Wright's view, these lead to random drift of mean population characters in a fixed, multiply peaked 'adaptive landscape', with long periods spent near fitness peaks3,4. Using recent theoretical results5, we show here that transitions between peaks are rapid and unidirectional even though (indeed, because) random variations are small and transitions initially require movement against selection. Thus, punctuated equilibrium6,7, the palaeontological pattern8,9 of rapid transitions between morphological equlibria, is a natural manifestation of the standard wrightian evolutionary theory and requires no special developmental, genetic or ecological mechanisms 10-13.
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