The Evolution of Evolution: Changing Dynamics in Macroevolution

The term macroevolution encompasses four logically and empirically distinct explanatory schemes for large-scale patterns in the fossil record: 1) the persistence of microevolutionary processes over long time periods, the default position adopted by Simpson and other participants in the modern synthesis; 2) alternative modes for generating variation; 3) differential sorting, and possibly selection, between species and clades to generate evolutionary trends and other patterns; and 4) changes in the evolutionary process itself over time, creating the potential for new forms of biological organization and ecological structure. Although empirical support for each of these alternative explanations varies considerably, research should focus on exploring their relative significance, and specifically whether different modes of macroevolutionary change are favored under different conditions and at different times. Of particular interest is whether these different explanatory modes reflect alternative mechanisms for various patterns of evolutionary diversification. The recent advent of comparative studies of the evolution of development suggest that some macroevolutionary changes may be non-uniformitarian (option 4), associated with the structuring of developmental gene regulatory networks.  Documenting the circumstances and consequences of different macroevolutionary modes remains a significant challenge for paleobiologists.