Complex sentence: Gould's greatest scientific contribution, co-developed with Niles Eldredge, in 1972, was the theory of punctuated equilibrium, which proposes that most evolution is marked by long periods of evolutionary stability, punctuated by rare instances of cladogenesis, branching evolution.
Split into simpler sentences: Gould's greatest contribution to science was the theory of punctuated equilibrium, which he developed with Niles Eldredge in 1972. The theory proposes that most evolution is marked by long periods of evolutionary stability, which is punctuated by rare instances of branching evolution.