I've just had an e-mail from Emma, who writes: I'm getting really confused about Punctuated Equilibrium and Gradualism… do both operate at once? Or do some scientists argue for one and some argue for the other?
Gradualism is the 'standard' model for evolution: evolution occurs through the gradual accumulation of small changes & so is a slow process when viewed in the context of geological time. This contrasts with the concept of punctuated equilibrium: the idea that evolution can progress through bursts of evolutionary novelty separated by periods of stasis – this would look like rapid change in a fairly short period of (geological) time.
That point about time is interesting. Campbell & Reece (2005) comment, Suppose that a speices survived for 5 million years, but most of its morphological alterations occurred during the first 50,000 years of its existence – just 1% of its total lifetime. Because time periods this short often cannot be distinguished in fossil strate, the species would seem to have appeared suddenly and then lingered with little or no change before becoming extinct.
They go on to say that Darwin himself predicted this when he said "Although each species must have passed through numerous transitional stages, it is probable that the periods during which each uinderwent modification, though many and long as measured by years, have been short in comparison with the periods during which each remained in an unchanged condition."
As to scientific debate about the two models – I don't think it's a case of 'either/or'. The evolution of some lineages, eg Foraminifera, may be best explained by gradualism, while punctuated equilibrium may be a better explanation in other circumstances eg the relatively rapid expansion of taxa that follows mass extinction events in the fossil record.
Good luck for the exam, everyone!
References: N.A. Campbell & J.B. Reece (2005) Biology (7th edn), Pearson.