In a passive trend, the number of A's descendants (asterisks) that are more complex than A is roughly equal to the number of descendants that are less complex than A, but the average will still increase because there is a lower limit on complexity.
In b, there is a passive trend at the global scale (because of an expansion in variance with a limit at the low end), even though within each component lineage the trend is driven (because the driven trends at local scales occur in both directions).
If this limitation on change occurs primarily in one direction, for example if there is a lower boundary on complexity in which a reduction becomes maladaptive, then selection would prevent decreases in the minimum within a distribution such that any increase in diversity (which may, of course, represent adaptive change) would be in one direction and a passive trend would be the result.
If the forces are homogeneous (i.e., they operate mostly in the same direction), then the trend is driven (b). Figure from McShea (1994), reproduced by permission of Blackwell 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.
If the minimum does not change, then this is suggestive of a passive trend, although it must be noted that some locally driven trends do not involve increases in the minimum (e.g., Alroy 1998).
A driven trend caused by natural selection operating among individuals, or a passive trend resulting from a developmental constraint that limits change in one direction.
In a passive trend resulting from an expansion of variance that is bounded by an immovable minimum (a "left wall" in Fig. 9a), the overall distribution will be skewed, but a subsampling within clades that are not next to this wall would be expected to exhibit a more normal (bell-shaped) distribution.
A driven trend in which descendant species consistently have larger bodies on average than their ancestors, or a passive trend in which lineages begin at small size such that a bounded increase in variance results in an increase in average size.
Accordingly, internal, staggered DSBs may contribute to a passive trend toward increased length and sequence diversity in nascent introns.
On the other hand, the most obvious explanation for passive trends involving one-sided expansions in variance is some form of constraint, perhaps internal and nonadaptive in nature.
