http://www.blackwellpublishing.com/ridley/classictexts/eldredge.asp
Punctuated equilibria: an alternative to phyletic gradualism (1972) pp 82-115 in "Models in paleobiology", edited by Schopf, TJM Freeman, Cooper & Co, San Francisco.
Eldredge, N. & Gould, S.J.
The famous original essay which criticizes the theory of phyletic gradualism and argues that evolution proceeds in a pattern of punctuated equilibrium.