Table 1 Summary of the levels of explanation for large-scale evolutionary trends with hypothetical examples of each

Trend

A pattern of large-scale change in a parameter in a given direction, especially in terms of the average across multiple lineages and long periods of time.

An observed pattern in which average body size increases within several major lineages of animals over millions of years.

Dynamic

The characteristics of changes among component lineages or at smaller time scales that underlie a large-scale trend. If the dynamics occur consistently in one direction, then the trend is driven, whereas if the dynamics vary, then the trend is passive.

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.

Cause

The cause(s) of the dynamics that generate trends.

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.

Basis

The specific underlying basis (or bases) for the cause(s) of trend dynamics.

Natural selection for larger body size on the basis of larger individuals being more effective predators or acquiring better territories relative to smaller individuals, or developmental constraints related to limitations on organ function at very small sizes.