Figure 1. The contrasting idealized evolutionary patterns of phyletic gradualism (a) and punctuated equilibrium (b). The top of each evolutionary tree represents the most recent time depicted, such as the present day. The horizontal distance between lineages (lines of descent) indicates in a general way the degree of morphological difference. In punctuated equilibrium (b), all morphological change is concentrated in branching events, with stasis thereafter. Note that, for comparative purposes, the two patterns have been drawn with a similar number of branching points. In addition, if the patterns are superimposed on each other, corresponding lineages have the same morphology at the latest occurrence (i.e. at their extinction or at the top of the diagram), but the morphological routes by which lineages reach those points are very different in each pattern. Fine-scale zig-zags in morphology will occur in both phyletic gradualism and punctuated equilibrium, but are not shown in these simplified depictions.

Figure 2. The Plus ça change model, which combines aspects of punctuated equilibrium and phyletic gradualism. It proposes that, over geological time scales (e.g. a million years), gradualism (right) is characteristic of narrowly fluctuating, relatively stable environments such as a tropical rainforest and the deep sea. By contrast, punctuated equilibrium (left) is expected to prevail in the physically more unstable environments that dominate the fossil record, especially shallow seas. In a widely fluctuating environment (left), a threshold might occur not only when some environmental variable exceeds wide reflecting boundaries, but also when it contracts to become narrowly fluctuating (dotted line). ‘Environment’ indicates some long-term physical aspect of environment, e.g. mean temperature, sea level or type of sediment forming the sea floor. ‘Morphology’ indicates some aspect, or aspects, of morphology. High-frequency environmental oscillations such as annual cycles are not shown. From Sheldon (1996), modified after Sheldon(1990).