The origin of higher taxa: macroevolutionary processes, and the case of the mammals

The origin of a new higher taxon is characterized by a long-term phylogenetic trend, involving evolutionary changes in a large number of characters. At this phylogenetic level, the conflict between internal integration of the phenotype and its evolvability can be resolved by the correlated progression model, in which many disparate traits evolve by a sequence of small increments in loose correlation with one another, rather than by the modularity model. The trend leading to the new higher taxon implies the existence of a long ridge in an adaptive landscape. An evolutionary lineage tracking it requires adaptive changes in broad biological characteristics, involving many traits. Species selection is a possible additional driver of the trend. These conclusions are tested against the synapsid fossil record of the origin of mammals. The reconstructed sequence of acquisition of mammalian traits supports the correlated progression model. The adaptive ridge involved is postulated to have been a sequence of overlapping niches requiring increasing ability to remain active in daily and seasonally fluctuating environments by means of increasing internal regulation. An inferred speciation bias in favour of relatively small, relatively more progressive carnivores indicates that species selection was also involved in driving the trend. Palaeoenvironmental evidence indicates that ecological opportunity probably played a role at certain points along the lineage.