Fig. 6

A passive trend can result when the variance in some feature expands within a group of species over time after beginning at a low value close to a lower limit. In this hypothetical example, an ancestral species begins with a small body size and subsequent speciation events include both increases and decreases in size. However, there is a physical limit to how small species in this group can become, which means that the distribution is free to expand in only one direction, i.e., toward increases. Moreover, increases tend to be of a greater magnitude than decreases in this diagram, especially once species reach larger sizes. In this scenario, larger size does not need to be directly advantageous among species for the average to increase because this passive “diffusion” process alone can generate a large-scale trend. Figure from Stanley (1973), reproduced by permission of Blackwell