The book addresses the rarely discussed question of how new higher taxa such as orders, classes, and phyla evolve. There are two focal issues: how the tight functional integration amongst the parts of successive phenotypes is maintained in a lineage undergoing large evolutionary changes in many of the parts, and what drives the lineage in the particular and more or less consistent direction it takes to culminate in the new higher taxon. Evidence from the nature of organismal structure, palaeobiology, developmental biology, and ecology are considered, along with a review of those parts of the f ... More

The book addresses the rarely discussed question of how new higher taxa such as orders, classes, and phyla evolve. There are two focal issues: how the tight functional integration amongst the parts of successive phenotypes is maintained in a lineage undergoing large evolutionary changes in many of the parts, and what drives the lineage in the particular and more or less consistent direction it takes to culminate in the new higher taxon. Evidence from the nature of organismal structure, palaeobiology, developmental biology, and ecology are considered, along with a review of those parts of the fossil record—notably early metazoans, mammals, tetrapods, birds, turtles, and whales—that illustrate something of the pattern of acquisition of derived characters in lineages leading to actual higher taxa and the environmental conditions under which it occurred. Feedback interactions amongst developmental processes, modularity of structure, and phenotypic plasticity all play a part in the maintenance of phenotypic integration over short evolutionary distances and low taxonomic levels. Over the greater distances and longer timescales culminating in new higher taxa, correlated progression of small changes in many functionally linked parts, driven by selection acting on the whole organism, is the principal mechanism. A lineage that culminates in a new higher taxon tracks a complex ecological gradient consisting of numerous parameters. Such compound, persistent gradients are uncommon in nature, explaining why so few lineages became higher taxa compared to the huge number that generated lower taxa.