Macroevolutionary theory incorporates issues of scale and hierarchy, and thus includes the origin and fates of evolutionary novelties, the evolutionary role of rare events ranging from the internal redeployment of gene regulatory networks to externally driven mass extinctions, and the potential for emergent properties or dynamics at different hierarchical levels to shape large-scale patterns. Developmental factors impose non-linear relationships between magnitudes of genetic change and their phenotypic expression, an uneven probability distribution of evolutionary changes around any given phenotypic starting point, and the potential for coordinated changes among traits that can accommodate change via epigenetic mechanisms. Large-scale sorting of these biased inputs – clade dynamics – are often shaped by differential origination and extinction, including strict-sense species selection, in which rate differentials are governed by emergent, species-level traits such as geographic range size, and effect macroevolution, in which rates are governed by organism-level traits such as body size. Both processes can create hitchhiking effects, indirectly causing proliferation or decline of other traits. The nonlinear, sometimes temporally discordant, relationships among macroevolutionary currencies (taxonomic, morphologic, functional) are crucial for understanding the nature of evolutionary diversification; e.g. taxonomic diversifications can lag behind, occur in concert with, or precede, increases in disparity.
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