The Lines of Evolution of the Bacillariophyta: II. The Centric Series

Evolution is concentrated in very rapid events of speciation. . . Most species during their geological history either do not change in any appreciable way, or else they fluctuate mildly with no apparent direction.’ (Gould & Eldredge 1977) The evolution of the centric series of diatoms from simple Ur-diatoms is discussed. Evidence from ecology and fossil studies which may help is briefly evaluated together with a discussion of phylogenetic considerations and of ‘punctuated equilibria’. The evolution of centric morphology, origin and development of areolae, occluding plates, rimoportulae, fultoportulae, ocelli and pseudocelli are discussed, and the possible lines of development of these is documented.