The question of the pattern of stasis (no change) versus gradualism (persistent and continuous change) in the fossil record continues. A new paper in the Proceedings of the National Academy of Sciences by Uyeda et al. comprises a huge survey and analysis of morphological change in animals (mammals, birds, squamate reptiles, and primates) over ten million years. The purpose was to determine whether change between species in one trait—body size—accumulates gradually with the passage of time since species diverged, or whether that change is more episodic. What they did was take a tremendous amount of data from three sources: current field studies of the rate of evolutionary change, fossil data showing change (again, this is all body size) through time, and estimates of rate of body-size divergence from living organisms whose divergence times can be estimated from molecular data.
Uyeda et al. then plotted the divergence in body size between related species (measured as proportional change, thus requiring a log scale) versus the divergence time. The graph below tells the tale: they see what they call a “blunderbuss” pattern, with not much change accumulating between species until they’ve diverged for about a million years, and then change occurring more rapidly and cumulatively after a million years.