SCALE AND HIERARCHY IN MACROEVOLUTION

Scale and hierarchy must be incorporated into any conceptual framework for the study of macroevolution, i.e. evolution above the species level. Expansion of temporal and spatial scales reveals evolutionary patterns and processes that are virtually inaccessible to, and unpredictable from, short-term, localized observations. These larger-scale phe- nomena range from evolutionary stasis at the species level and the mosaic assembly of complex morphologies in ances- tral forms to the non-random distribution in time and space of the origin of major evolutionary novelties, as exemplified by the Cambrian explosion and post-extinction recoveries of metazoans, and the preferential origin of major marine groups in onshore environments and tropical waters. Virtu- ally all of these phenomena probably involve both ecological and developmental factors, but the integration of these com- ponents with macroevolutionary theory has only just begun. Differential survival and reproduction of units can occur at several levels within a biological hierarchy that includes DNA sequences, organisms, species and clades. Evolution by nat- ural selection can occur at any level where there is heritable variation that affects birth and death of units by virtue of interaction with the environment. This dynamic can occur when selfish DNA sequences replicate disproportionately within genomes, when organisms enjoy fitness advantages within populations (classical Darwinian selection), when dif- ferential speciation or extinction occurs within clades owing to organismic properties (effect macroevolution), and when differential speciation or extinction occurs within clades owing to emergent, species-level properties (in the strict sense species selection). Operationally, emergent species-level properties such as geographical range can be recognized by testing whether their macroevolutionary effects are similar regardless of the different lower-level factors that produce them. Large-scale evolutionary trends can be driven by trans- formation of species, preferential production of species in a given direction, differential origination or extinction, or any combination of these; the potential for organismic traits to hitch-hike on other factors that promote speciation or damp extinction is high. Additional key attributes of macroevolu- tionary dynamics within biological hierarchies are that (1) hierarchical levels are linked by upward and downward caus- ation, so that emergent properties at a focal level do not impart complete independence; (2) hierarchical effects are asymmetrical, so that dynamics at a given focal level need not propagate upwards, but will always cascade downwards; and (3) rates are generally, although not always, faster at lower hierarchical levels. Temporal and spatial patterns in the origin of major novelties and higher taxa are significantly discordant from those at the species and genus levels, sug- gesting complex hierarchical effects that remain poorly understood. Not only are many of the features promoting survivorship during background times ineffective during mass extinctions, but also they are replaced in at least some cases by higher-level, irreducible attributes such as clade-level geographical range. The incorporation of processes that oper- ate across hierarchical levels and a range of temporal and spatial scales has expanded and enriched our understanding of evolution.