I introduce a new statistical method, analysis of skewness, for quantifying large-scale evolutionary trends as a combination of both passive and driven trends. My approach is based on the skewness of subclades within a parent clade. I partition the total skewness of the parent clade into three components: (1) skewness between subclades; (2) skewness within subclades; and (3) skewness due to changes in variance among subclades. The third component corresponds to a new type of passive trend, in which overall skewness of a parent clade is due to greater variability in subclades to the right of the mean. Using this partitioning, I decompose an observed trend into two components: a driven portion and a passive portion, thus quantifying the effect of small-scale dynamics on large-scale behavior of clades. Applications are given to Miocene-Pliocene rodent size and Ordovician brachiopod muscle geometry.
Corresponding Editor: M. Zelditch